Transcript Ch_28_lecture_presentation

Chapter 28
The Reproductive
System
Embryology and
Human Development
Lecture Presentation by
Steven Bassett
Southeast Community College
© 2015 Pearson Education, Inc.
An Overview of Development
• Development involves:
• Differentiation of cells
• Reorganization of cells
• Development can be characterized by different
periods of time
• Prenatal development
• Embryology
• Postnatal development
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An Overview of Development
• Development can be characterized by different
periods of time
• Prenatal development
• Conception to delivery
• Involves embryology (development during the
prenatal period)
• Postnatal development
• Development from birth to maturity
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An Overview of Development
• Prenatal development can be further subdivided
• Pre-embryonic development
• Fertilization to implantation
• Embryonic development
• Implantation to the end of the eighth week of
pregnancy
• Fetal development
• Ninth week of pregnancy to birth
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Fertilization
• Fertilization is the joining of two haploid
cells to create a diploid cell
• Function of the haploid cells
• Spermatozoon
• Delivers the paternal chromosomes to the ovum
• Ovum
• Provides the maternal chromosomes
• Provides nourishment for embryonic development
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Fertilization
• Fertilization occurs in the ampulla of the uterine
tube
• 200 million sperm cells enter the vaginal canal
• Only about 10,000 make it to the uterine tubes
• Less than 100 actually contact the egg
• Only one will fertilize the egg
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Fertilization
• The Oocyte at Ovulation
• When the egg is ovulated, it is surrounded by the
corona radiata
• Protects the egg as it is being ovulated
• Numerous sperm cells release hyaluronidase
from their acrosomal cap
• Hyaluronidase decomposes the corona radiata
• A sperm cell can now penetrate the egg
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Figure 28.1a Fertilization and Preparation for Cleavage
a
A secondary oocyte surrounded by spermatozoa
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Fertilization
• Pronucleus Formation and Amphimixis
• The nuclear material in the egg is the female
pronucleus
• The nuclear material in the sperm is the male
pronucleus
• Amphimixis
• The fusion of the two pronuclei
• A zygote is now formed
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Figure 28.1b Fertilization and Preparation for Cleavage
Oocyte at Ovulation
Ovulation releases a secondary oocyte and
the first polar body; both are surrounded by
the corona radiata. The oocyte is suspended
in metaphase of meiosis II.
Corona
radiata
First polar
body
1 Fertilization and Oocyte Activation
2
Acrosomal enzymes from multiple sperm create
gaps in the corona radiata. A single sperm then
makes contact with the oocyte membrane, and
membrane fusion occurs, triggering oocyte
activation and completion of meiosis.
Fertilizing
spermatozoon
Pronucleus Formation Begins
The sperm is absorbed into the cytoplasm,
and the female pronucleus develops.
Nucleus of
fertilizing
spermatozoon
Second polar
body
Female
pronucleus
Zona
pellucida
5
Cytokinesis Begins
4
The first cleavage division nears completion
roughly 30 hours after fertilization. Further
events are diagrammed in Figure 28.2.
Amphimixis Occurs and
Cleavage Begins
Metaphase of first
cleavage division
3
Spindle Formation and
Cleavage Preparation
The male pronucleus develops, and spindle
fibers appear in preparation for the first
cleavage division.
Male
pronucleus
Blastomeres
b Events at fertilization and immediately thereafter
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Female
pronucleus
Prenatal Development
• Prenatal development is known as the gestation
period (nine months)
• Prenatal development is divided into trimesters
• First trimester
• Second trimester
• Third trimester
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Prenatal Development
• The Trimesters
• First trimester
• Rudiments of all organs appear
• Second trimester
• Fetus looks like a human
• Third trimester
• Organs become functional
• Rapid growth
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Prenatal Development
• The First Trimester
• 1 to 12 weeks
• Four events within the first trimester
• Cleavage (sequence of cell divisions)
• A blastocyst forms
• Implantation (implantation into endometrial lining)
• Placentation (formation of the placenta)
• Embryogenesis (development of the embryo)
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Prenatal Development
• Cleavage and Blastocyst Formation
• Cell division results in the formation of
blastomeres
• A solid ball of cells eventually develops—this is a
morula
• Some cells migrate to one “edge” of the morula
creating a mass of cells and a hollow cavity called
the blastocoele
• The ball of cells is now called the blastocyst
• The outer layer of the blastocyst consists of cells
called the trophoblast
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Figure 28.2 Cleavage and Blastocyst Formation
Blastomeres
Polar
bodies
2-cell
stage
DAY 1
4-cell stage
DAY 2
First cleavage
division
Early morula
DAY 3
DAY 4
Advanced
morula
Inner cell
mass
DAY 0:
Fertilization
DAY 6
Ovulation
Blastocoele
DAYS 7–10:
Implantation in
uterine wall
(See Figure 28.3)
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Trophoblast
Blastocyst
Prenatal Development
• Cleavage and Blastocyst Formation (continued)
• Trophoblast cells provide nutrients to the
developing embryo
• The inner cell mass consists of stem cells that
will develop into all the cells of the body
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Prenatal Development
• Implantation
• Upon contact with the endometrial lining, the
trophoblast cells divide rapidly
• The trophoblast cells “fuse” with the endometrial
lining forming a syncytial trophoblast
• This layer of cells releases hyaluronidase to
erode away more of the endometrial lining so the
mass can implant
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Prenatal Development
• Implantation (continued)
• Upon implantation, the inner cell mass separates
from the trophoblast area
• When the inner cell mass separates from the
trophoblast, two cavities form:
• Amnionic cavity
• Blastocoele cavity
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Figure 28.3 Stages in the Implantation Process
DAY 6
FUNCTIONAL LAYER
OF ENDOMETRIUM
UTERINE
CAVITY
Uterine
glands
Blastocyst
DAY 7
Trophoblast
Blastocoele
Inner cell
mass
DAY 8
Syncytial
trophoblast
DAY 9
Cellular
trophoblast
Hypoblast
Epiblast
Developing
primary villi
Amniotic cavity
Lacuna
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Blastodisc
Prenatal Development
• Formation of the Blastodisc
• A layer of cells forms between the amnionic cavity
and the blastocoele cavity
• The layers are called:
• Epiblast
• Hypoblast
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Figure 28.4a Blastodisc Organization and Gastrulation
Blastocoele
Syncytial trophoblast
Cellular trophoblast
Amniotic cavity
Yolk
sac
Lacunae
Epiblast of blastodisc
Hypoblast of blastodisc
DAY 10
a The blastodisc begins as two layers: the epiblast,
facing the amniotic cavity, and the hypoblast,
exposed to the blastocoele. Migration of epiblast
cells around the amniotic cavity is the first step
in the formation of the amnion. Migration of
hypoblast cells creates a sac that hangs below
the blastodisc. This is the first step in yolk sac
formation.
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Prenatal Development
• Gastrulation and Germ Layer Formation
• Eventually some cells of the epiblast move toward
the center of the blastodisc creating a primitive
streak
• This movement is called gastrulation
• As the cells move toward the primitive streak area,
they begin to migrate between the epiblast and
hypoblast layers
• This creates three distinct layers of cells
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Prenatal Development
• Gastrulation and Germ Layer Formation
• The three layers of cells are:
• Ectoderm
• Derived from the epiblast layer
• Mesoderm
• New layer between the epiblast and hypoblast
• Endoderm
• Derived from the hypoblast layer
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Figure 28.1 Fertilization and Preparation for Cleavage
a A secondary oocyte surrounded by spermatozoa
Oocyte at Ovulation
1 Fertilization and Oocyte Activation
Corona
radiata
First polar
body
2
Pronucleus Formation Begins
Acrosomal enzymes from multiple sperm create
gaps in the corona radiata. A single sperm then
makes contact with the oocyte membrane, and
membrane fusion occurs, triggering oocyte
activation and completion of meiosis.
The sperm is absorbed into the cytoplasm,
and the female pronucleus develops.
Fertilizing
spermatozoon
Nucleus of
fertilizing
spermatozoon
Ovulation releases a secondary oocyte and
the first polar body; both are surrounded by
the corona radiata. The oocyte is suspended
in metaphase of meiosis II.
Second polar
body
Female
pronucleus
Zona
pellucida
5 Cytokinesis Begins
4
The first cleavage division nears completion
roughly 30 hours after fertilization. Further
events are diagrammed in Figure 28.2.
Amphimixis Occurs and
Cleavage Begins
Metaphase of first
cleavage division
3
Spindle Formation and
Cleavage Preparation
The male pronucleus develops, and spindle
fibers appear in preparation for the first
cleavage division.
Male
pronucleus
Blastomeres
b Events at fertilization and immediately thereafter
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Female
pronucleus
Table 28.1 The Fates of the Primary Germ Layers
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Prenatal Development
• Formation of Extraembryonic Membranes
• The ectoderm, mesoderm, and endoderm are
collectively known as the germ layers
• Each layer will form specific tissues and organs of
the body
• Germ layers will also form structures involved in
embryonic survival called extraembryonic
membranes
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Prenatal Development
• Formation of Extraembryonic Membranes
• There are four major extraembryonic membranes
•
•
•
•
Yolk sac
Amnion
Allantois
Chorion
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Figure 28.5 The Embryonic Membranes and Placenta Formation (1 of 8)
Week 2
Week 3
Migration of mesoderm around the inner surface of the
trophoblast creates the chorion. Mesodermal migration around
the outside of the amniotic cavity, between the ectodermal cells
and the trophoblast, forms the amnion. Mesodermal migration
around the endodermal pouch creates the yolk sac.
The embryonic disc bulges into the amniotic cavity at the head fold.
The allantois, an endodermal extension surrounded by mesoderm,
extends toward the trophoblast.
Amniotic cavity (containing
amniotic fluid)
Extraembryonic
membranes
Amnion
Syncytial
trophoblast
Amnion
Allantois
Cellular
trophoblast
Mesoderm
Yolk sac
Blastocoele
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Yolk sac
Chorion
Chorion
Head fold of embryo
Syncytial trophoblast
Chorionic villi of placenta
Prenatal Development
• Formation of Extraembryonic Membranes
• Yolk sac
• This is a pouch that extends from the hypoblast
cells into the blastocoele
• It is the early site for blood cell formation
• Amnion
• Amniotic fluid fills the amniotic cavity, which
surrounds and cushions the embryo and fetus
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Prenatal Development
• Formation of Extraembryonic Membranes
(continued)
• Allantois
• Eventually gives rise to the urinary bladder
• Chorion
• The mesoderm and trophoblast layers together
form the chorion
• The chorion will eventually develop extensions into
the endometrium
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Prenatal Development
• Placentation
• The placenta begins to form when the chorion
produces villi (chorionic villi) that extend into the
endometrial lining
• The body stalk connects the embryo to the
chorion
• As the fetus develops and moves farther into the
uterus, it obtains its nutrients via the umbilical
cord
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Figure 28.5 The Embryonic Membranes and Placenta Formation (4 of 8)
Week 5
Week 4
The developing embryo and extraembryonic membranes bulge into the
uterine cavity. The trophoblast pushing out into the uterine lumen remains
covered by endometrium but no longer participates in nutrient absorption
and embryo support. The embryo moves away from the placenta, and the
body stalk and yolk stalk fuse to form an umbilical stalk.
The embryo now has a head fold and a tail fold. Constriction of the
connections between the embryo and the surrounding trophoblast
narrows the yolk stalk and body stalk.
Tail fold
Uterus
Body stalk
Myometrium
Yolk stalk
Decidua basalis
Yolk sac
Umbilical stalk
Embryonic gut
Placenta
Yolk sac
Chorionic villi
of placenta
Decidua capsularis
Decidua parietalis
Uterine lumen
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Embryonic
head fold
Prenatal Development
• Placental Circulation
• Blood flows from the fetus to the placenta in the
paired umbilical arteries
• Blood returns via a single umbilical vein
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Figure 28.5 The Embryonic Membranes and Placenta Formation (7 of 8)
Decidua parietalis
Decidua basalis
Umbilical cord
Placenta
Amniotic cavity
Week 10
The amnion has expanded
greatly, filling the uterine cavity.
The fetus is connected to the
placenta by an elongated
umbilical cord that contains a
portion of the allantois,
blood vessels, and the
remnants of the yolk stalk.
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Amnion
Chorion
Decidua capsularis
Figure 28.7c The First Trimester
Chorionic
villi
Amnion
Umbilical
cord
Placenta
c Fiber-optic view of human development at week 8
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Figure 28.6a A Three-Dimensional View of Placental Structure
Decidua
capsularis
Amnion
Umbilical
cord (cut)
Placenta
Chorion
Yolk
sac
Decidua
basalis
Umbilical
vein
Umbilical
arteries
Chorionic
villi
Area filled with
maternal blood
Decidua
parietalis
Myometrium
Uterine cavity
Cervical
(mucous) plug in
cervical canal
Maternal
blood vessels
External os
Cervix
Vagina
Amnion
Trophoblast (cellular
and syncytial layers)
a For clarity, the uterus is shown after the embryo has been removed and the umbilical cord
cut. Blood flows into the placenta through ruptured maternal arteries. It then flows around
chorionic villi, which contain fetal blood vessels. Fetal blood arrives through paired umbilical
arteries and leaves through a single umbilical vein. Maternal blood reenters the venous
system of the mother through the broken walls of small uterine veins. Maternal blood flow
is shown by arrows; note that no actual mixing of maternal and fetal blood occurs.
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Prenatal Development
• Embryogenesis
• Shortly after gastrulation, embryogenesis begins
• By week 4, a head fold and tail fold develop
• The embryo is separated from the blastodisc area
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Figure 28.5 The Embryonic Membranes and Placenta Formation
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Prenatal Development
• Embryogenesis
• At about 12 weeks, organs begin to form
• This is called organogenesis
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Figure 28.7b The First Trimester
Medulla
Ear
Pharyngeal
arches
Forebrain
Eye
Heart
Somites
Body
stalk
Arm
bud
Tail
Leg
bud
b Fiber-optic view of human development
at week 4.
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Prenatal Development
• Second Trimester and Third Trimesters
• Second trimester
• Fetus is covered by the amnion
• Fetus grows faster than the placenta
• Third Trimester
• All fetal organs become functional
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Figure 28.8a The Second and Third Trimesters
a A four-month fetus seen through
a fiber-optic endoscope
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Prenatal Development
• Changes in the Uterus during Gestation
•
•
•
•
Uterus will increase in length from 7.5 cm to 30 cm
Contains almost 5 L of fluid
The uterus and contents weigh about 22 pounds
Abdominal organs are pushed out of their normal
positions
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Figure 28.9a The Growth of the Uterus and Fetus
Placenta
Uterus
Umbilical
cord
Fetus at
16 weeks
Amniotic fluid
Cervix
Vagina
a Pregnancy at four months (16 weeks) showing
the positions of the uterus, fetus, and placenta.
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Figure 28.9b The Growth of the Uterus and Fetus
9 months
8 months
7 months
6 months
5 months
After
dropping,
in preparation
for delivery
4 months
3 months
b Changes in the size of the uterus
during the second and third trimesters.
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Figure 28.9c The Growth of the Uterus and Fetus
Liver
Small intestine
Stomach
Pancreas
Transverse colon
Aorta
Fundus of uterus
Common
iliac vein
Umbilical cord
Cervical (mucus)
plug in cervical canal
Placenta
Urinary bladder
External
os
Pubic symphysis
Rectum
Vagina
Urethra
c
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Pregnancy at full term. Note the position of
the uterus and fetus and the displacement
of abdominal organs relative to part (d).
Labor and Delivery
• The goal of labor is parturition (expulsion of the
fetus)
• There are three stages of labor
• Dilation stage
• Expulsion stage
• Placental stage
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Labor and Delivery
• The dilation stage
• The fetus begins to move down the cervical canal
• The cervix dilates
• The amnion ruptures
• The expulsion stage
• The birth of the child
• The placental stage
• Ejection of the placenta
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Figure 28.10 The Stages of Labor
Umbilical
cord
Sacral
promontory
Cervical
canal
Pubic
symphysis
Cervix
1 The Dilation Stage
Vagina
Placenta
Fully developed fetus before labor begins
2 The Expulsion Stage
3 The Placental Stage
Uterus
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Ejection of the
placenta
Figure 28.10 The Stages of Labor (1 of 3)
Umbilical
cord
Sacral
promontory
Cervical
canal
Pubic
symphysis
Cervix
Vagina
Placenta
Fully developed fetus before labor begins
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1 The Dilation Stage
Figure 28.10 The Stages of Labor (2 of 3)
2 The Expulsion Stage
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Figure 28.10 The Stages of Labor (3 of 3)
3 The Placental Stage
Uterus
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Ejection of the
placenta
Labor and Delivery
• Premature Labor
• This is labor that begins before the fetus has
completed normal development
• The line between spontaneous abortion and
immature delivery is usually set at 17.6 oz.
• Prior to this weight
• Respiratory / cardiovascular / urinary systems have
not developed enough to support life
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The Neonatal Period
• This is the period from birth to one month
• Events that occur during this time are:
• Lungs fill with air
• Blood circulation changes with the closing of the
ductus arteriosus and the foramen ovale of the
heart
• Heart rate drops from 120–140 beats per minute
to about 70
• Breathing rate drops from 30 per minute to the
normal rate
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The Neonatal Period
• Events that occur from birth to one month are
(continued):
• Kidneys filter the infant’s own blood
• Digestive system becomes active
• Metabolic rate is increased to maintain warmth for
a few days after birth
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Embryology of Organ Systems
• Overview of Embryology
• The development of the:
•
•
•
•
•
•
•
•
•
Integumentary system
Skull
Vertebral column
Appendicular skeleton
Muscles
Nervous system
Spinal cord
Brain
Special sense organs
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•
•
•
•
•
•
•
•
Endocrine system
Heart
Cardiovascular system
Lymphatic system
Respiratory system
Digestive system
Urinary system
Reproductive system
Embryology of Organ Systems
• The Development of the Integumentary System
• 1 month
• Epithelium overlies mesenchyme
• 3 months
• Epithelium multiplies to form layers
• Mesenchyme becomes dermal connective tissue
• 4 months
• Epidermis grows into the dermis forming columns
of cells, which become: hair follicles, sweat glands,
and sebaceous glands
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Embryology Summary pages 764-765 (1 of 6)
The Development of the Integumentary System
Ectoderm
Mesoderm
At the start of the second month, the
superficial ectoderm is a simple epithelium
overlying loosely organized mesenchyme.
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Embryology Summary pages 764-765 (2 of 6)
The Development of the Integumentary System
Germinative
cells
Connective
tissue
Blood vessel
Epithelial
column
3 MONTHS
Over the following weeks, the epithelium
becomes stratified through repeated
divisions of the basal or germinative
cells. The underlying mesenchyme
differentiates into embryonic connective
tissue containing blood vessels that
bring nutrients to the region.
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Mesenchyme
4 MONTHS
During the third and fourth months, small
areas of epidermis undergo extensive
divisions and form cords of cells that grow
into the dermis. These are epithelial
columns. Mesenchymal cells surround
the columns as they extend deeper and
deeper into the dermis. Hair follicles,
sebaceous glands, and sweat glands
develop from these columns.
Embryology of Organ Systems
• The Development of the Integumentary System
(continued)
• Skin
• At 4 months, the epithelium continues to thicken
• Melanocytes migrate into the stratum basale layer
• Nails
• Develop from the thickening of the epidermis
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Embryology Summary pages 764-765 (3 of 6)
The Development of the Integumentary System
NAILS
Ectoderm
Nail field
SKIN
Fingertip
Melanocyte
Germinative
cell
Loose
connective tissue
As basal cell divisions continue, the
epithelial layer thickens and the basal
lamina is thrown into irregular folds.
Pigment cells called melanocytes migrate
into the area and squeeze between the
germinative cells. The epithelium now
resembles the epidermis of the adult.
4 MONTHS
Nails begin as thickenings of the epidermis
near the tips of the fingers and toes. These
thickenings settle into the dermis, and the
borderline with the general epidermis becomes
distinct. Initially, nail production involves all
of the germinative cells of the nail field.
Dermis
Dense
connective
tissue
Subcutaneous
layer
4 MONTHS
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The embryonic connective tissue
differentiates into the dermis. Fibroblasts
and other connective tissue cells form
from mesenchymal cells or migrate into
the area. The density of fibers increases.
Loose connective tissue extends into the
ridges, but a deeper, less vascular region
is dominated by a dense, irregular collagen
fiber network. Below the dermis the
embryonic connective tissue develops
into the subcutaneous layer, a layer of
loose connective tissue.
Nail root
Eponychium
Nail bed
BIRTH
By the time of birth, nail production is
restricted to the nail root.
Nail
Embryology of Organ Systems
• The Development of the Integumentary System
(continued)
• Hair follicles
• Develops as a deep column of cells surround a
papilla
• Exocrine glands
• Develops as the epithelial column elongates and
coils
• Mammary glands
• Develop from the thickening of the epidermis
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Embryology Summary pages 764-765 (4 of 6)
The Development of the Integumentary System
HAIR FOLLICLES
Hair
Sebaceous
gland
Sebaceous
gland
Hair column
Papilla
5 MONTHS
A hair follicle develops as a deep column
surrounding a papilla, a small mass of
connective tissue. Hair growth will occur
in the epithelium covering the papilla. An
outgrowth from the epithelial column
forms a sebaceous gland.
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BIRTH
At birth a hair projects from the follicle,
and the secretions of the sebaceous
gland lubricate the hair shaft.
Embryology Summary pages 764-765 (5 of 6)
The Development of the Integumentary System
EXOCRINE GLANDS
Duct of
sweat gland
Epithelial
column
Mesenchyme
5 MONTHS
A sweat gland develops as an epithelial
column elongates, coils, and becomes
hollow.
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BIRTH
At birth, sweat gland ducts carry the
secretions of the gland cells to the
skin surface.
Embryology Summary pages 764-765 (6 of 6)
The Development of the Integumentary System
MAMMARY GLANDS
Hollowing
nipple
Epidermis
Epidermal
thickening
Branching
duct
Developing
duct
Fat
5 MONTHS
Mammary glands develop in a comparable
fashion, but the epidermal thickenings are
much broader and extensive branching
occurs.
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BIRTH
At birth, the marry glands have not completed
their development. In females, further elaboration
of the duct and gland system occurs at puberty,
but functional maturity does not occur until late
in pregnancy.
Embryology of Organ Systems
• The Development of the Skull
• 5 to 8 weeks
• Cartilage begins to appear
• Cartilage within the brain area enlarges to form the
chondrocranium
• Skull walls and floor are beginning but no roof
• 9 weeks
• Endochondral ossification appears; frontal and
parietal structures appear
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Embryology Summary pages 766-767 (1 of 7)
The Development of the Skull
First pharyngeal arch
(mandibular)
Pharyngeal
cartilages
Second arch
(hyoid)
Arches
3, 4, 6
Brain
Eye
Nose
5 WEEKS
5-WEEK
EMBRYO
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After 5 weeks of development, the central nervous system
is a hollow tube that runs the length of the body. A series
of cartilages appears in the mesenchyme of the head
beneath and alongside the expanding brain and around
the developing nose, eyes, and ears. These cartilages are
shown in light blue. Five additional pairs of cartilages
develop in the walls of the pharynx. These cartilages,
shown in dark blue, are located within the pharyngeal,
or branchial, arches. (Branchial refers to gills—in fish
the caudal arches develop into skeletal supports for the
gills.) The first arch, or mandibular arch, is the largest.
Embryology Summary pages 766-767 (2 of 7)
The Development of the Skull
Chondrocranium
Brain
Eye
Nasal
capsule
Vertebrae
8 WEEKS
The cartilages associated with the brain enlarge
and fuse, forming a cartilaginous chondrocranium
(kon-drō-KRĀ-nē-um; chondros, cartilage +
cranium, skull) that cradles the brain and sense
organs. At 8 weeks its walls and floor are
incomplete, and there is no roof.
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Embryology Summary pages 766-767 (3 of 7)
The Development of the Skull
Frontal
bone
Occipital
bone
Sphenoid
Maxilla
The mandible forms as
dermal bone develops
around the inferior
portion of the
mandibular arch.
Hyoid bone
Larynx
9 WEEKS
During the ninth week, numerous centers of endochondral ossification
appear within the chondrocranium. These centers are shown in red.
Gradually, the frontal and parietal bones of the cranial roof appear as
intramembranous ossification begins in the overlying dermis. As these
centers (beige) enlarge and expand, extensive fusions occur.
© 2015 Pearson Education, Inc.
Embryology Summary
• The Development of the Skull (continued)
• 10 weeks
• Mandibular arch fuses with the chondrocranium
• Hyoid arch begins to form
• 12 weeks
• Ossification is well under way
• Birth
• Cranial roof remains incomplete
• The anterior and posterior fontanels are present
© 2015 Pearson Education, Inc.
Embryology Summary pages 766-767 (4 of 7)
The Development of the Skull
The dorsal portion of the mandibular arch fuses with the chondrocranium.
The fused cartilages do not ossify; instead, osteoblasts begin sheathing
them in dermal bone. On each side this sheath fuses with a bone developing
at the entrance to the nasal cavity, producing the two maxillae. Ossification
centers in the roof of the mouth spread to form the palatine processes and
later fuse with the maxillae.
Parietal
bone
Frontal
bone
Maxilla
Mandible
10 WEEKS
The second arch, or hyoid arch, forms near the temporal bones. Fusion of the
superior tips of the hyoid with the temporals forms the styloid processes. The
ventral portion of the hyoid arch ossifies as the hyoid bone. The third arch fuses
with the hyoid, and the fourth and sixth arches form laryngeal cartilages.
© 2015 Pearson Education, Inc.
Embryology Summary pages 766-767 (5 of 7)
The Development of the Skull
Frontal
bone
Parietal
bone
Occipital
bone
Maxilla
Zygomatic
arch
Mandible
Temporal bone
12 WEEKS
After 12 weeks ossification is well
under way in the cranium and face.
(Compare with Figure 5.5, p. 116.)
© 2015 Pearson Education, Inc.
Embryology Summary pages 766-767 (6 of 7)
The Development of the Skull
Frontal
bone
Parietal bone
Mandible
Occipital
bone
BIRTH
The skull at birth; compare with the situation at 12 weeks.
Extensive fusions have occurred, but the cranial roof remains
incomplete. (For further details, see Figure 6.18, p. 159.)
© 2015 Pearson Education, Inc.
Embryology of Organ Systems
• The Development of the Vertebral Column
• 4 weeks
• On either side of the developing spinal cord are
blocks of mesenchymal tissue called somites
• The medial side of the somites produces the
vertebral column
• 8 weeks
• Vertebral cartilage forms around the spinal cord
• The sacrum and coccyx are fused together
© 2015 Pearson Education, Inc.
Embryology Summary pages 768-769 (1 of 6)
The Development of the Vertebral Column
Pharyngeal
arches
Ear
Somites
Spinal cord
Somite
Eye
Sclerotome
Heart
Notochord
Tail
The developing spinal cord lies posterior to a
longitudinal rod, the notochord (NŌ-tō-kōrd;
noton, back + chorde, cord). In the fourth week
of development, mesoderm on either side of the
spinal cord and notochord forms a series of
mesenchymal blocks called somites (SŌ-mīts).
Mesenchyme in the medial portions of each
somite, a region known as the sclerotome
(SKLER-ō-tōme; skleros, hard), will produce the
vertebral column and contribute to the floor
of the cranium.
4-WEEK
EMBRYO
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Embryology Summary pages 768-769 (2 of 6)
The Development of the Vertebral Column
Spinal cord
Mesenchyme
of somite
Neural arch
Tubercle of rib
Head of rib
Centrum of vertebra
Cartilaginous rib
8 WEEKS
The cartilages of the vertebral centra grow around the spinal cord,
creating a model of the complete vertebra. In the cervical, thoracic,
and lumbar regions, articulations develop where adjacent
cartilaginous blocks come into contact. In the sacrum and coccyx,
the cartilages fuse together.
© 2015 Pearson Education, Inc.
Embryology Summary pages 768-769 (4 of 6)
The Development of the Vertebral Column
Sclerotome
Notochord
Intersegmental
mesenchyme
Somites
Cartilage of
vertebral body
4 WEEKS
Cells of the sclerotomal segments
migrate away from the somites
and cluster around the notochord.
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6 WEEKS
The migrating cells differentiate into
chondroblasts and produce a series
of cartilaginous blocks that surround
the notochord. These cartilages,
which will develop into the
vertebral centra, are separated by
patches of mesenchyme.
Embryology Summary pages 768-769 (5 of 6)
The Development of the Vertebral Column
Intervertebral
disc
Vertebra
Nucleus
pulposus
8 WEEKS
ADULT
Expansion of the vertebral centra eventually eliminates the
notochord, but it remains intact between adjacent vertebrae,
forming the nucleus pulposus of the intervertebral discs.
Later, surrounding mesenchymal cells differentiate into
chondroblasts and produce the fibrous cartilage of the
anulus fibrosus.
© 2015 Pearson Education, Inc.
Embryology Summary
• The Development of the Vertebral Column
(continued)
• 12 weeks
• Ribs and sternum undergo ossification
• The tips of the longer ribs do not ossify; they
remain
as cartilage thus forming the costal cartilages
© 2015 Pearson Education, Inc.
Embryology Summary pages 768-769 (2 of 6)
The Development of the Vertebral Column
Tubercle
of rib
Spinal cord in
spinal canal
Spinous process
Muscles of back
Transverse process
Ventral body
cavity
Ossification centers
12 WEEKS
About the time the ribs separate from the vertebrae, ossification
begins. Only the shortest ribs undergo complete ossification. In
the rest, the distal portions remain cartilaginous, forming the
costal cartilages. Several ossification centers appear in the
sternum, but fusion gradually reduces the number.
© 2015 Pearson Education, Inc.
Embryology Summary pages 768-769 (6 of 6)
The Development of the Vertebral Column
8 WEEKS
9 WEEKS
Rib cartilages expand away from the developing transverse processes of the vertebrae.
At first they are continuous, but by week 8 the ribs have separated from the vertebrae.
Ribs form at every vertebra, but in the cervical, lumbar, sacral, and coccygeal regions
they remain small and later fuse with the growing vertebrae. The ribs of the thoracic
vertebrae continue to enlarge, following the curvature of the body wall. When they
reach the ventral midline, they fuse with the cartilages of the sternum.
© 2015 Pearson Education, Inc.
Embryology of Organ Systems
• The Development of the Appendicular Skeleton
• 4 weeks
• Ridges appear along the length of the embryo
• An accumulation of ridges at the superior end and
the inferior end form pairs of limb buds
• 5 weeks
• Limb buds develop a core of cartilaginous tissue
• Cartilaginous models develop between 5 and 8
weeks
© 2015 Pearson Education, Inc.
Embryology of Organ Systems
• The Development of the Appendicular Skeleton
• 5.5–8 weeks
• Upper limbs
• Bends develop at the location of the shoulder and
elbow
• Lateral rotation of the apical ridge occurs to create
the correct position for the elbow
• Hands originate as paddles but eventually individual
fingers form
• Surfaces within the joints remain covered with
cartilage
© 2015 Pearson Education, Inc.
Embryology of Organ Systems
• The Development of the Appendicular Skeleton
• 5.5–8 weeks
• Lower limbs
• Bends develop at the location of the hip and knee
• Medial rotation of the apical ridge occurs to create
the correct position for the knee
• Feet originate as paddles but eventually individual
toes form
• Surfaces within the joints remain covered with
cartilage
© 2015 Pearson Education, Inc.
Embryology Summary pages 770-771 (2 of 4)
The Development of the Appendicular Skeleton
5 WEEKS
Pelvic
girdle
The formation of the pelvic girdle and
legs closely parallels that of the pectoral
complex. But as the pelvic limb bud
enlarges, the apical ridge rotates
medially rather than laterally. As a
result, the knee joint faces posteriorly,
while the elbow faces anteriorly.
Lower
limb
51/2 WEEKS
7 WEEKS
8 WEEKS
By week 8, cartilaginous models of all of the
major skeletal components are well formed, and
endochondral ossification begins in the future
limb bones. Ossification of the hip bones begins
at three separate centers that gradually enlarge.
© 2015 Pearson Education, Inc.
Embryology Summary pages 770-771 (3 of 4)
The Development of the Appendicular Skeleton
Cartilaginous
core of
scapula
Apical
ridge
Humerus
Mesenchyme of
pectoral girdle
51/2 WEEKS
As the limb bud enlarges, bends
develop at the future locations of
the shoulder and elbow joints. Two
cartilages form in the forearm, and
a lateral rotation of the apical
ridge places the elbow in its
proper orientation.
5 WEEKS
7 WEEKS
The hands originate as
paddles, but the death
of cells between the
phalangeal cartilages
produces individual
fingers
Cartilage
Humerus
Joint cavity
Ossified
bone
Scapula
Joints form where two cartilages are in
contact. The surfaces within the joint
cavity remain cartilaginous, while the
rest of the bones undergo ossification.
© 2015 Pearson Education, Inc.
Embryology of Organ Systems
• The Development of the Appendicular Skeleton
• 10 weeks
• Continued ossification occurs
• Distal bones of the carpus and tarsus remain
cartilaginous
• Birth
• Consists of extensive areas of cartilage
© 2015 Pearson Education, Inc.
Embryology Summary pages 770-771 (4 of 4)
The Development of the Appendicular Skeleton
BIRTH
The skeleton of a newborn infant. Note the
extensive areas of cartilage (blue) in the
humeral head, wrist, between the bones of
the palm and fingers, and in the hips. Notice
the appearance of the axial skeleton, with
reference to the two previous Embryology
Summaries.
© 2015 Pearson Education, Inc.
10 WEEKS
Ossification in the embryonic skeleton
after approximately 10 weeks of
development. The shafts of the limb
bones are undergoing rapid ossification,
but the distal bones of the carpus and
tarsus remain cartilaginous.
Embryology of Organ Systems
• The Development of Muscles
• 4 weeks
• Mesoderm on either side of the notochord forms
somites
• The medial portion of each somite forms skeletal
muscles; this region is called the myotome
• 6 weeks
• Myotome area develops into posterior muscles
called epaxial muscles and anterior muscles
called hypaxial muscles
© 2015 Pearson Education, Inc.
Embryology Summary pages 772-773 (1 of 4)
The Development of the Muscles
Near the head, mesoderm forms
skeletal muscle associated withy
the pharyngeal arches.
Myotome
Gut
Sclerotome
Pharyngeal
arches
Eye
Mesoderm from the parietal
portion of the lateral plate and
the adjacent myotome forms
the limb buds.
Migrating mesodermal
cells (arrows show
directions of movement)
Limb bud
Lateral plate mesoderm
(parietal layer)
Lateral plate
(visceral layer)
Heart
Coelom
Somites
Somites
Umbilical
stalk
After 4 weeks of development, mesoderm on either
side of the notochord has formed somites. The medial
portion of each somite will form skeletal muscles;
this region is called the myotome.
4 WEEKS
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The ventral mesoderm does not
form segmental masses, and it
remains as a sheet called the
lateral plate. A cavity appears
within the lateral plate of the
chest and abdomen; this cavity
is the coelom. Formation of the
coelom divides the lateral plate
into an inner visceral layer
and an outer parietal layer.
Embryology Summary pages 772-773 (2 of 4)
The Development of the Muscles
Eye
muscles
Hypaxial mesoderm in the trunk
grows around the body wall
toward the sternum in company
with the ribs. This creates a
mesodermal layer that extends
from the chin to the pelvic girdle.
Epaxial muscles
Hypaxial muscles
Extensors
Lung
Rib
Flexors
Upper
limb bud
Heart
Sternum
The hypaxial mesoderm near
the sacrum migrates caudally
to produce the muscles of
the pelvic floor.
Myotomal muscles organize around the
developing vertebral column in two
groups, one dorsal (epaxial muscles)
and the other ventral (hypaxial muscles).
6 WEEKS
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Each limb bud has a
flattened distal tip, with
a thickened apical ridge.
As cartilages appear in
the limb buds, surrounding
mesodermal cells from
the myotomes differentiate
into myoblasts.
Embryology of Organ Systems
• The Development of Muscles (continued)
• 7 weeks
• Muscles of the face, back, and abdomen develop
• 8 weeks
• Limbs enlarge and muscles continue to develop
© 2015 Pearson Education, Inc.
Embryology Summary pages 772-773 (3 of 4)
The Development of the Muscles
Muscles forming
at the pharyngeal
arches are associated
with the head and
neck. The muscles of
mastication develop
from the mesoderm
surrounding the
mandibular arch.
Eye
muscles
Mesoderm of the hyoid (second) arch
migrates over the lateral and ventral
surfaces of the neck and the surfaces of
the skull to form the muscles of facial
expression.
Epaxial muscles remain arranged in segments. These
deep muscles include the intervertebral muscles.
Superficial epaxial muscles form the major muscles of
the erector spinae group.
Intervertebral muscles
Mesoderm of the third, fourth,
and sixth pharyngeal arches
forms the pharyngeal and
intrinsic laryngeal muscles.
Erector spinae
Pharyngeal myoblasts form
a superficial layer that later
subdivides to create
the trapezius and
sternocleidomastoid
muscles.
Flexors
Extensors
Migration of myoblasts over
the dorsal surface of the
trunk creates limb extensors;
migration of ventral myoblasts
produces the flexors.
Quadratus lumborum
Transversus abdominis
Internal oblique
External oblique
Stomach
Rectus
abdominis
7 WEEKS
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The oblique, transverse, and
rectus muscle groups develop
in the hypaxial layer.
Embryology Summary pages 772-773 (4 of 4)
The Development of the Muscles
8 WEEKS
While the limb buds enlarge, additional
myoblasts invade the limb from
myotomal segments nearby. Lines
indicate the boundaries between
myotomes providing myoblasts to
the limb.
© 2015 Pearson Education, Inc.
Embryology of Organ Systems
• The Development of Muscles (continued)
• Birth
• Rotation of the upper limb and lower limb creates
the muscles of:
• Flexion
• Extension
© 2015 Pearson Education, Inc.
Embryology Summary pages 772-773 (4 of 4)
The Development of the Muscles
Flexors
Extensors
Flexors
BIRTH
Rotation of the upper limb bud and lower
limb bud produces a change in the position
of these masses relative to the body axis.
© 2015 Pearson Education, Inc.
Embryology of Organ Systems
• The Development of the Nervous System
• 20–23 days
• Somites appear on either side of the notochord
• A neural plate is formed
• A crease develops along the length of the neural
plate
• The edges of the crease are the neural folds
• The neural folds “fold” to form a tube (neural tube)
• The neural tube becomes the CNS
• Axons extending from the neural tube become the
PNS
© 2015 Pearson Education, Inc.
Embryology Summary page 774 (1 of 2)
The Development of the Nervous System
Neural
plate
Neural
plate
Notochord
After two weeks of development, somites are appearing on either
side of the notochord. The ectoderm near the midline thickens,
forming an elevated neural plate. The neural plate is largest near
the future head of the developing embryo.
Somite
20 DAYS
Neural
groove
A crease develops along the axis of the neural plate, creating the
neural groove. The edges, or neural folds, gradually move
together. They first contact one another midway along the axis
of the neural plate, near the end of the third week.
Neural
fold
Neural tube
Where the neural folds meet, they fuse to form a cylindrical
neural tube that loses its connection with the superficial
ectoderm. The process of neural tube formation is called
neurulation; it is completed in less than a week. The formation
of the axial skeleton and that of the musculature around the
developing neural tube were described on pages 768–769 and 772.
21 DAYS
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Embryology Summary page 774 (2 of 2)
The Development of the Nervous System
Neurocoel
Head
Cells at the tips of the neural folds do not participate in neural
tube formation. These cells of the neural crest at first remain
between the dorsal surface of the neural tube and the ectoderm,
but they later migrate to other locations. The neural tube
becomes the CNS. Axons from neurons within the neural tube
and the axons of neural crest cells form the PNS.
Neural
crest
Schwann cell
Sensory neurons
Somites
Neural crest
Autonomic
motor
neurons
Ependymal layer
Mantle layer
Marginal layer
CNS
neurons
23 DAYS
Ependymal cells
The first cells to appear in the
mantle differentiate into neurons,
while the last cells to arrive become
astrocytes and oligodendrocytes.
Further development of the CNS
and PNS will be found in the
Embryology Summaries later in
this chapter.
Astrocytes and
oligodendrocytes
The neural tube increases in thickness as its epithelial lining undergoes
repeated mitoses. By the middle of the fifth developmental week, there
are three distinct layers. The ependymal layer lines the enclosed cavity,
or neurocoel. The ependymal cells continue their mitotic activities, and
daughter cells create the surrounding mantle layer. Axons from
developing neurons form a superficial marginal layer.
© 2015 Pearson Education, Inc.
Embryology of Organ Systems
• The Development of the Spinal Cord
• 22–28 days
• The neural tube is closed thus producing
three layers
• Ependymal layer
• Mantle layer
• Marginal layer
• Neurons develop in the mantle layer
• Axons grow from the mantle layer to the marginal
layer
• Axons form bundles, or tracts, in the spinal cord
© 2015 Pearson Education, Inc.
Embryology Summary page 775 (1 of 2)
The Development of the Spinal Cord, Part I
Ectoderm
Neural crest
Neural tube
Neurocoel
Ependymal layer
Mantle layer
Marginal layer
22 DAYS
By the end of the fifth developmental
week, the neural tube is almost completely
closed. In the spinal cord the mantle layer
that contains developing neurons and
neuroglial cells will produce the gray
matter that surrounds the neurocoel.
As neurons develop in the mantle
layer, their axons grow toward central
or peripheral destinations. The axons
leave the mantle layer and travel toward
synaptic targets within a peripheral
marginal layer.
23 DAYS
Neuroepithelial
(ependymal)
layer
Eventually, the growing
axons will form bundles,
or tracts, in the marginal
layer, and these tracts
will crowd together in
the columns that form
the white matter of the
spinal cord.
Mantle layer
Marginal layer
28 DAYS
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Embryology of Organ Systems
• The Development of the Spinal Cord (continued)
• 7 weeks
•
•
•
•
Ventral roots and dorsal roots develop
Glial cells develop
Spinal and cranial meninges develop
Spinal nerves develop
© 2015 Pearson Education, Inc.
Embryology Summary page 776 (1 of 3)
The Development of the Spinal Cord, Part II
In addition to forming dorsal root
ganglia and associated glial cells,
neural crest cells migrate around
the central nervous system and
develop into the spinal and cranial
meninges.
Larynx
Teeth
Dorsal root ganglia
Suprarenal medulla
Meninges
Autonomc ganglia
Melanocytes
7 WEEKS
(Distribution of neural crest cells)
Neural crest cells aggregate to form autonomic ganglia near the vertebral
column and in peripheral organs. Migrating neural crest cells contribute
to the formation of teeth and form the laryngeal cartilages, melanocytes
of the skin, the skull, connective tissues around the eye, the intrinsic
muscles of the eye, Schwann cells, satellite cells, and the suprarenal medullae.
© 2015 Pearson Education, Inc.
Embryology Summary page 776 (2 of 3)
The Development of the Spinal Cord, Part II
Cranial nerves
and ganglia
Eye
Cervical plexus
Brachial plexus
Spinal nerves
Lumbosacral
plexus
7 WEEKS
(Peripheral nerve distribution)
Several spinal nerves innervate each developing limb. When
embryonic muscle cells migrate away from the myotome, the
nerves grow right along with them. If a large muscle in the
adult is derived from several myotomal blocks, connective
tissue partitions will often mark the original boundaries, and
the innervation will always involve more than one spinal nerve.
© 2015 Pearson Education, Inc.
Embryology of Organ Systems
• Developmental Abnormalities
• Spina Bifida
• The vertebral laminae fail to unite
• The neural arch is incomplete
• The spinal meninges bulge outward
• Neural Tube Defect
• A portion of the spinal cord develops as a broad
plate
© 2015 Pearson Education, Inc.
Embryology Summary page 776 (3 of 3)
The Development of the Spinal Cord, Part II
DEVELOPMENTAL
ABNORMALITIES
Spina bifida
Spina bifida (BI-fi-da) results when the
developing vertebral laminae fail to unite
due to abnormal neural tube formation at
that site. The neural arch is incomplete, and
the meninges bulge outward beneath the
skin of the back. The extent of the abnormality
determines the severity of the defects. In mild
cases, the condition may pass unnoticed;
extreme cases involve much of the length of
the vertebral column.
© 2015 Pearson Education, Inc.
Neural tube defect
A neural tube defect (NTD) is a condition
that is secondary to a developmental error in
the formation of the spinal cord. Instead of
forming a hollow tube, a portion of the spinal
cord develops as a broad plate. This is often
associated with spina bifida. Neural tube
defects affect roughly one individual in 1000;
prenatal testing can detect the existence of
these defects with an 80–85 percent success
rate.
Embryology of Organ Systems
• The Development of the Brain
• 23 days
• The cephalic extension forms three brain vesicles
• Prosencephalon
• Mesencephalon
• Rhombencephalon
© 2015 Pearson Education, Inc.
Embryology Summary page 777 (1 of 3)
The Development of the Brain, Part I
Before proceeding,
briefly review the
summaries of skull
formation, vertebral
column development,
and development of
the spinal cord in the
previous Embryology
Summaries.
Mesencephalon
Rhombencephalon
Neurocoel
Prosencephalon
Cephalic
area
Neural
tube
The initial cephalic expansion occurs
as the neurocoel enlarges, forming
three distinct brain vesicles: (1) the
prosencephalon (prōs-en-SEF-a-lon)
or “forebrain,” (2) the mesencephalon or
“midbrain,” and (3) the rhombencephalon
(rom-ben-SEF-a-lon) or “hindbrain.” The
prosencephalon and rhombencephalon
will be subdivided further as development
proceeds.
Even before neural tube formation has been completed, the cephalic portion begins to enlarge.
Major differences in brain versus spinal cord development include (1) early breakdown of mantle
(gray matter) and marginal (white matter) organization; (2) appearance of areas of neural cortex;
(3) differential growth between and within specific regions; (4) appearance of characteristic bends
and folds; and (5) loss of obvious segmental organization.
23 DAYS
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Embryology of Organ Systems
• The Development of the Brain (continued)
• 4 weeks
• The rhombencephalon subdivides to form:
• Metencephalon
• Myelencephalon
• The prosencephalon subdivides to form:
• Telencephalon
• Diencephalon
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Embryology Summary page 777 (2 of 3)
The Development of the Brain, Part I
Mesencephalon
Metencephalon
Myelencephalon
The rhombencephalon first subdivides
into the metencephalon (met-en-SEF-a-lon;
meta, after) and the myelencephalon
(mī-el-en-SEF-a-lon; myelon, spinal cord).
Diencephalon
Telencephalon
The prosencephalon forms the telecephalon (tel-en-SEF-a-lon; telos, end
+ enkephalos, brain) and the diencephalon. The telencephalon begins as a
pair of swellings near the rostral, dorsolateral border of the prosencephalon.
4 WEEKS
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Embryology Summary
• The Development of the Brain (continued)
• 5 weeks
• Mesencephalon develops
• Cranial nerves begin to develop
• 8 weeks
• Choroid plexus develops
• Cerebral hemispheres expand
• 11 weeks
• Cerebellum develops
© 2015 Pearson Education, Inc.
Embryology Summary page 777 (3 of 3)
The Development of the Brain, Part I
N III
N IV
NV
N VII
Developing ear
Cranial nerves develop as
sensory ganglia and link
peripheral receptors with
the brain, and motor fibers
grow out of developing
cranial nuclei. Special
sensory neurons of cranial
nerves I, II, and VIII develop
in association with the
developing receptors. The
somatic motor nerves III, IV,
and VI grow to the eye
muscles; the mixed nerves
(V, VII, IX, and X) innervate
the pharyngeal arches (page 766).
5 WEEKS
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Myelencephalon
N IX
Pharyngeal
arches
NX
N XI
N XII
Development of the mesencephalon
produces a small mass of neural tissue
with a constricted neurocoel, the
aqueduct of the midbrain.
As differential growth proceeds and the
position and orientation of the embryo
change, a series of bends, or flexures
(FLEK-sherz), appears along the axis of
the developing brain.
Embryology Summary page 778 (1 of 2)
The Development of the Brain, Part II
N III
Cephalic flexure
N IV
Pontine flexure
The roofs of the
diencephalon and
myelencephalon fail to
develop, leaving a thin
ependymal layer in
contact with the
developing meninges.
Blood vessels invading
these regions create
areas of the choroid
plexus.
8 WEEKS
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N XI
N XII
NI
N II
N VI
N VII
N VIII
N IX
NX
As growth continues and the
pontine flexure develops, the
brain becomes more compact.
The expanding cerebral
hemispheres now dominate
the superior and lateral
surfaces of the brain. Migrating
neuroblasts create the cerebral
cortex, and underlying masses
of gray matter develop into the
basal nuclei.
Cervical flexure
Embryology Summary page 778 (2 of 2)
The Development of the Brain, Part II
Cerebral hemisphere
(telencephalon)
Cerebral
hemisphere
Diencephalon
Mesencephalon
Cerebellum
Pons
Pons
Medulla
oblongata
Cerebellum
Medulla
oblongata
Spinal
cord
Cranial
nerve XI
After 11 weeks, the expanding cerebral hemispheres
have overgrown the diencephalon. At the
metencephalon, cortical formation and expansion
produce the cerebellum, which overlies the nuclei
and tracts of the pons.
11 WEEKS
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CHILD
Embryology of Organ Systems
• The Development of Special Sense Organs
• Vision
• 4 weeks
• Optic vesicles appear in the lateral walls of the
prosencephalon
• The bulges indent forming the optic cup
• The epidermis covering the optic cup forms the lens
© 2015 Pearson Education, Inc.
Embryology Summary page 779 (1 of 3)
The Development of Special Sense Organs, Part I
All special sense organs develop from the interaction
between the epithelia and the developing nervous
system of the embryo.
VISION
Neurocoel
Epidermis
Prosencephalon
Choroid
Retina
Optic cup
Optic vesicle
Lens
N II
Lens placode
Optic stalk
4 WEEKS
The first indication of optic
development appears as a pair
of bulges called optic vesicles
in the lateral walls of the
prosencephalon. These extend
to either side like a pair of
dumbbells, each containing a
cavity continuous with the
neurocoel.
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Sclera
Lens vesicle
These bulges become indented,
forming a pair of optic cups, which
remain connected to the diencephalon
by optic stalks. The epidermis
overlying the optic cup responds by
forming a lens placode, which
thickens and creates another vesicle.
This lens vesicle becomes the lens.
Mesoderm aggregating around
this complex forms the choroid
and scleral coats. The anterior
and posterior chambers develop
as cavities appear within the
mesoderm.
Embryology of Organ Systems
• The Development of Special Sense Organs
• Olfaction
• 5 weeks
• A pair of thickened areas in the front of the
prosencephalon form the olfactory receptors
• Olfactory receptors begin as thickened tissue called
nasal placodes
© 2015 Pearson Education, Inc.
Embryology Summary page 779 (2 of 3)
The Development of Special Sense Organs, Part I
OLFACTION
Eye
Nasal
placode
External
nares
5 WEEKS
Nasal
placode
© 2015 Pearson Education, Inc.
Olfactory receptors begin as a pair
of thickened areas in front of the
prosencephalon during the fifth
developmental week. The thickenings
are called nasal placodes.
10 WEEKS
Over time, the nasal placodes are
enfolded and protected by developing
facial structures. (Development of the
face was discussed in the previous
Embryology Summary of the Skull.)
Embryology of Organ Systems
• The Development of Special Sense Organs
• Gustation
• 5 weeks
• Taste buds develop as nerve fibers grow into the
mouth and pharynx areas
• When nerves innervate the tongue, the epithelial
cells differentiate to form gustatory cells
© 2015 Pearson Education, Inc.
Embryology Summary page 779 (3 of 3)
The Development of Special Sense Organs, Part I
GUSTATION
Epithelial
cells
Sensory
neuron
Gustatory receptors are
the least specialized of
any of the special sense
organs. Taste buds develop
as sensory fibers grow
into the developing
mouth and pharynx.
© 2015 Pearson Education, Inc.
Taste buds
When the nerve endings contact
epithelial cells, the epithelial cells
differentiate into gustatory cells.
If the sensory nerves are cut, the
taste buds degenerate; if the
sensory nerve is moved, it will
stimulate the development of new
taste buds at its new location.
Embryology of Organ Systems
• The Development of Special Sense Organs
• Equilibrium and hearing
• 3 weeks
• Otic placodes begin to appear on the lateral sides
of the rhombencephalon
• 4 weeks
• Deep pockets form in the otic regions, creating otic
vesicles
• 6–7 weeks
• The vesicles form the bony labyrinth
© 2015 Pearson Education, Inc.
Embryology Summary page 780 (1 of 2)
The Development of Special Sense Organs, Part I
Neural
groove
EQUILIBRIUM
AND HEARING
Late in the third week of
development, a pair of otic
placodes appears on either
side of the rhombencephalon.
Otic placode
Pharynx
Otic
placode
3 WEEKS
The otic placodes form deep
pockets that subsequently
lose their connection with the
epidermis, creating hollow
otic vesicles.
Neural tube
Otic vesicle
Epidermis
Tail
4 WEEKS
These vesicles gradually change
shape, forming the membranous
labyrinth. This process has
essentially been completed by
the end of the third developmental
month.
Developing
membranous
labyrinth
Ganglia of N VIII
External pharyngeal
groove
Pharyngeal pouch
6 WEEKS
© 2015 Pearson Education, Inc.
Embryology Summary page 780 (2 of 2)
The Development of Special Sense Organs, Part II
Thickened portions of the otic
vesicles differentiate into the
spiral and vestibular ganglia,
and their sensory terminals grow
toward the developing hair cells.
Developing ossicles
Vestibular ganglion
Spiral ganglion
External acoustic meatus
Cartilage
Middle ear cavity
Auditory tube
Semicircular
ducts
7 WEEKS
Auricle
Auditory ossicles
External acoustic meatus
Tympanic membrane
Middle ear cavity
Temporal
bone
FULL TERM
Cochlea
© 2015 Pearson Education, Inc.
As these developments are underway, the surrounding mesenchyme
begins to differentiate into cartilage.
This cartilage will later ossify to form
the bony labyrinth.
Embryology of Organ Systems
• The Development of the Endocrine System
• Week 5
• Several pharyngeal pouches are formed
• The pouches form:
• Thymus gland
• Parathyroid gland
• Thyroid gland
© 2015 Pearson Education, Inc.
Embryology Summary page 781 (1 of 2)
The Development of the Endocrine System, Part I
PARATHYROID GLANDS AND THYMUS
As noted in Chapter 3, all secretory
glands, whether exocrine or endocrine,
are derived from epithelia. Endocrine
organs develop from epithelia
(1) covering the outside of the embryo,
(2) lining the digestive tract, and
(3) lining the coelomic cavity.
Ectoderm
Neural
tube
First
pharyngeal
pouch
I
Pharyngeal
arches
Pharyngeal
cleft
First
pharyngeal
cleft
Pharynx
The dorsal masses of the third
and fourth pouches form the
parathyroid glands. The ventral
masses move toward the midline
and fuse to create the thymus.
II
III
IV
V–VI
Endoderm
WEEK 5
The pharyngeal region of the embryo plays
a particularly important role in endocrine
development. After 4–5 weeks of
development, the pharyngeal arches are
well formed. Human embryos develop five
or six pharyngeal arches, not all visible from
the exterior. (Arch 5 may not appear or may
form and degenerate almost immediately.)
The five major arches (I–IV, VI) are separated
by pharyngeal clefts, deep ectodermal grooves.
© 2015 Pearson Education, Inc.
Developing
ear
Cells originating in the walls of the
small fifth pouch will be incorporated
into the thyroid gland (see below), where
they will differentiate into C thyrocytes.
Pharynx
Parathyroids
Thyroid
Thymus
In sectional view, five pharyngeal pouches extend laterally toward the
pharyngeal clefts. The first pouch lies caudal to the first (mandibular) arch.
Pharyngeal pouches 5 and 6 are very small and are interconnected.
Endoderm lining the third, fourth, and fifth pairs of pharyngeal pouches
forms dorsal and ventral masses of cells that migrate beneath the
endodermal epithelium.
Embryology Summary page 781 (2 of 2)
The Development of the Endocrine System, Part I
THYROID GLAND
Endoderm
Ectoderm
Ventral
pocket
WEEK 5, Mid-sagittal section
The boundary between ectoderm and
endoderm lies along the line formed by
the circumvallate papillae of the tongue
(see Figure 18.7, p. 484). This line
roughly corresponds to the middle of the
mandibular (first) arch. The thyroid gland
forms here in the ventral midline.
© 2015 Pearson Education, Inc.
Thyroid
gland
The thyroid gland begins as a pocket in the ventral
midline. As this pocket branches slightly, its walls
thicken, and the paired masses lose their connection
with the surface.
Thyroid
C thyrocytes
As the embryo enlarges and changes
shape, the thyroid shifts caudally to
a position near the thyroid cartilage
of the larynx. On its way, the thyroid
gland incorporates C thyrocytes from
the walls of the fifth pouch.
Embryology of Organ Systems
• The Development of the Endocrine System
• Week 5
• The pituitary gland forms as a pocket in the
pharyngeal area; this ectodermal pocket breaks
away from the pharyngeal area and develops just
superior to the thyroid gland area
© 2015 Pearson Education, Inc.
Embryology Summary page 782 (1 of 2)
The Development of the Endocrine System, Part II
PITUITARY GLAND
Developing
pituitary gland
WEEK 5, Mid-sagittal section
The pituitary gland forms in the dorsal
midline above the forming thyroid gland.
© 2015 Pearson Education, Inc.
Hypothalamus
Ectodermal
pocket
The pituitary gland has a compound origin.
The first step is the formation of an
ectodermal pocket in the dorsal midline of
the pharynx. This pocket loses its
connection to the pharynx, creating a
hollow ball of cells that lies inferior to the
floor of the diencephalon posterior to the
optic chiasm.
Anterior
lobe
Posterior
lobe
As these cells undergo division, the central
chamber gradually disappears. This endocrine
mass will become the adenohypophysis
(anterior lobe) of the pituitary gland. The
neurohypophysis (posterior lobe) of the
pituitary gland begins as a depression in the
hypothalamic floor and grows toward the
developing adenohypophysis.
Embryology of Organ Systems
• The Development of the Endocrine System
• Week 5 (continued)
• After the formation of the neural tube, some neural
cells migrate away from the CNS and form an
aggregate of cells that become the suprarenal
glands
© 2015 Pearson Education, Inc.
Embryology Summary page 782 (2 of 2)
The Development of the Endocrine System, Part II
SUPRARENAL GLANDS
Neural crest
cell mass
Pharyngeal
arches
Mesothelium
Lining of
coelomic cavity
Sympathetic
preganglionic
fibers
Spinal cord
Suprarenal
medulla
Migrating neural
crest cells
Suprarenal
cortex
Dorsal root
ganglion
Sympathetic
chain ganglion
Overlying epithelial cells respond by
undergoing division, and the daughter
cells surround the neural crest cells
to form a thick suprarenal cortex.
Future suprarenal
medulla
Digestive tube
WEEK 5
Each suprarenal gland also has a compound origin. Shortly after
the formation of the neural tube, neural crest cells migrate away
from the CNS. This migration leads to the formation of the dorsal
root ganglia and autonomic ganglia. On each side of the coelomic
cavity, neural crest cells aggregate in a mass that will become a
suprarenal medulla.
For additional details concerning the development
of other endocrine organs, refer to the subsequent
Embryology Summaries on the Lymphoid, Digestive,
Urinary, and Reproductive systems.
© 2015 Pearson Education, Inc.
Embryology of Organ Systems
• The Development of the Heart
• Week 2
• The heart begins as a pair of tubes in the pharynx
area
• Week 3
•
•
•
•
The heart begins to pump
The tubes fuse forming a one-chambered heart
Two large veins bring blood to the heart
One large artery carries blood away from the heart;
the artery is called the truncus arteriosus
© 2015 Pearson Education, Inc.
Embryology Summary page 783 (1 of 3)
The Development of the Heart
Mesoderm
Pharynx
Pharynx
Future pericardial cavity
Neural groove
Heart tubes
WEEK 2
LATERAL VIEW
During the second
developmental week,
the heart consists of
a pair of thin-walled,
muscular tubes beneath
the floor of the pharynx.
© 2015 Pearson Education, Inc.
Future pericardial
cavity
The lateral plate mesoderm
in this region has already
split into parietal and visceral
layers, creating a space that
will eventually form the
pericardial cavity.
Truncus arteriosus
Future pericardial
cavity
Ventricle
Left atrial
primordium
WEEK 3
VENTRAL VIEW
By the third week, the heart is pumping and
circulating blood. The cardiac tubes have fused,
producing a heart with a single central chamber.
Two large veins bring blood to the heart, and a
single large artery, the truncus arteriosus,
carries blood to the general circulation.
Embryology Summary
• The Development of the Heart (continued)
• Week 4 to 5
• The interatrial and interventricular septa begin to
form, ultimately forming the four chambers of the
heart
• The foramen ovale and ductus arteriosum form
• 1 year
• The foramen ovale and ductus arteriosum have
closed (at birth)
• The remnant of the foramen ovale is the fossa ovalis
• The remnant of the ductus arteriosum is the ductus
ligamentum
© 2015 Pearson Education, Inc.
Embryology Summary page 783 (2 of 3)
The Development of the Heart
Right
atrium
Left
atrium
Pharynx
Aortic
arches
Future interatrial
septum
Ventricle
Opening of
sinus venosus
Atrium
Sinus
venosus
WEEK 5
Future
interventricular
septum
© 2015 Pearson Education, Inc.
Truncus
arteriosus
In week 5, the interatrial
and interventricular septa
begin to subdivide the
interior of the heart.
WEEK 4
The heart elongates as the embryo grows
larger. It curves back upon itself, forming
as S-curve that gradually becomes more
pronounced. The atrial and ventricular
regions already differ in thickness.
Embryology Summary page 783 (3 of 3)
The Development of the Heart
Foramen
ovale
Interatrial
septa
Left atrium
Fossa ovalis
Right atrium
Two interatrial septa develop,
one overlapping the other. A gap
between the two, called the
foramen ovale, permits blood
flow from the right atrium to the
left atrium. Backflow from left to
right is prevented by a flap that
acts as a one-way valve. Until
birth, this atrial short circuit
diverts blood from the
pulmonary circuit.
© 2015 Pearson Education, Inc.
Right ventricle
Left ventricle
AGE 1 YEAR
At birth, the foramen
ovale closes, separating
the pulmonary and
systemic circuits in
the heart. A shallow
depression, the fossa
ovalis, remains through
adulthood at the site
of the foramen ovale.
(Other cardiovascular
changes at birth are
detailed in Figure 22.23,
p. 606.)
Embryology of Organ Systems
• The Development of the Cardiovascular System
• The aortic arches
• Week 4
• Several aortic arches form and fuse together to
form the dorsal aorta
• Some of the aortic arches disintegrate leaving
ultimately just one aortic arch
© 2015 Pearson Education, Inc.
Embryology Summary page 784-785 (1 of 6)
The Development of the Cardiovascular System
THE AORTIC ARCHES
An aortic arch carries arterial
blood through each of the
pharyngeal arches. In the
dorsal pharyngeal wall, these
vessels fuse to create the
dorsal aorta, which distributes
blood throughout the body. The
arches are usually numbered
from I to VI, corresponding to
the pharyngeal arches.
I
II
III
IV
V
VI
Left dorsal aorta
Right dorsal aorta
VENTRAL VIEW
Dorsal aorta
Aortic arches
Yolk
sac
4 WEEKS
We will follow the development of three major vessel
complexes: the aortic arch, the venae cavae, and the hepatic
portal and umbilical systems. (In diagrams of prenatal
circulation arterias are shown in red and veins in blue
regardless of the oxygenation of the blood they carry.)
© 2015 Pearson Education, Inc.
Aortic arches
Fused dorsal aorta
Embryology Summary page 784-785 (2 of 6)
The Development of the Cardiovascular System
External carotid arteries
Internal carotid
artery
Brachiocephalic
trunk
Common carotid arteries
As development proceeds, some
of these arches disintegrate. The
ductus arteriosus provides an
external short-circuit between the
pulmonary and systemic circuits.
Most of the blood entering the right
atrium bypasses the lungs, passing
instead through the ductus arteriosus
or the foramen ovale in the heart.
Dorsal aorta
Aortic arches
Yolk sac
4 WEEKS
We will follow the development of three major vessel
complexes: the aortic arch, the venae cavae, and the hepatic
portal and umbilical systems. (In diagrams of prenatal
circulation arterias are shown in red and veins in blue
regardless of the oxygenation of the blood they carry.)
© 2015 Pearson Education, Inc.
Right common
carotid artery
Aortic arch
Right subclavian
artery
Left subclavian
artery
Ligamentum
arteriosum
Ductus arteriosus
Pulmonary artery
Left common
carotid artery
The left half of arch IV
ultimately becomes the
aortic arch, which carries
blood away from the left
ventricle.
Pulmonary artery
Descending aorta
Embryology of Organ Systems
• The Development of the Cardiovascular System
• The venae cavae
• Week 4
• Venous circulation begins with the following
vessels draining the tissues:
• Anterior cardinal veins
• Posterior cardinal veins
• Subcardinal veins
© 2015 Pearson Education, Inc.
Embryology of Organ Systems
• The Development of the Cardiovascular System
• The venae cavae
• Week 4
• The fusion and disintegration of the paired vessels
on the previous slide ultimately form
• Superior vena cava
• Inferior vena cava
© 2015 Pearson Education, Inc.
Embryology Summary page 784-785 (3 of 6)
The Development of the Cardiovascular System
THE VENAE CAVAE
Dorsal aorta
Aortic arches
Anterior
cardinal veins
Heart
Posterior
cardinal veins
Subcardinal veins
Yolk
sac
4 WEEKS
We will follow the development of three
major vessel complexes: the aortic arch,
the venae cavae, and the hepatic portal
and umbilical systems. (In diagrams of
prenatal circulation arterias are shown
in red and veins in blue regardless of
the oxygenation of the blood they carry.)
© 2015 Pearson Education, Inc.
DORSAL VIEW
The early venous circulation draining the
tissues of the body wall, limbs, and head
centers around the paired anterior cardinal
veins, posterior cardinal veins, and
subcardinal veins.
Embryology Summary page 784-785 (4 of 6)
The Development of the Cardiovascular System
Posterior
cardinal
vein
Right internal
and external
jugular veins
Superior
vena cava
Inferior
vena cava
Inferior vena
cava
Right
common iliac
vein
Interconnections form among these veins,
and a combination of fusion and
disintegration produces more-direct, largerdiameter connections to the right atrium.
Dorsal aorta
Aortic arches
Yolk
sac
4 WEEKS
We will follow the development of three major vessel
complexes: the aortic arch, the venae cavae, and the hepatic
portal and umbilical systems. (In diagrams of prenatal
circulation arterias are shown in red and veins in blue
regardless of the oxygenation of the blood they carry.)
© 2015 Pearson Education, Inc.
This process continues, ultimately
producing the superior and inferior
venae cavae.
Embryology of Organ Systems
• The Development of the Cardiovascular System
• The hepatic portal and umbilical vessels
• Week 4
• Umbilical arteries carry blood to the placenta
• Umbilical veins carry blood to the liver
• Week 12
• Right umbilical vein disintegrates
• Ductus venosus allows some blood to bypass the
liver
• Veins draining the digestive tract fuse to form the
hepatic portal vein
© 2015 Pearson Education, Inc.
Embryology of Organ Systems
• The Development of the Cardiovascular System
• The hepatic portal and umbilical vessels
• Full term
• Before birth, blood travels to the liver and then to
the IVC
• Much of the blood bypasses the lungs by traveling
through the ductus arteriosus and foramen ovale of
the heart
© 2015 Pearson Education, Inc.
Embryology of Organ Systems
• The Development of the Cardiovascular System
• The hepatic portal and umbilical vessels
• Newborn
• At birth
• Pulmonary vessels dilate
• Foramen ovale closes
• Ductus arteriosus forms the ligamentum arteriosum
© 2015 Pearson Education, Inc.
Embryology Summary page 784-785 (5 of 6)
The Development of the Cardiovascular System
Dorsal aorta
Aortic arches
We will follow the development of three
major vessel complexes: the aortic
arch, the venae cavae, and the hepatic
portal and umbilical systems.
(In diagrams of prenatal circulation
arterias are shown in red and veins in
blue regardless of the oxygenation of
the blood they carry.)
Yolk sac
4 WEEKS
THE HEPATIC PORTAL AND UMBILICAL VESSELS
Heart
Liver
Heart
Ductus venosus
Liver
Hepatic
portal vein
Umbilical veins
Left umbilical vein
Digestive tract
Right
umbilical
vein
Umbilical arteries
4 WEEKS
Paired umbilical arteries deliver blood to the placenta.
At 4 weeks, paired umbilical veins return blood to capillary
networks in the liver. Veins running along the length of the
digestive tract have extensive interconnections.
© 2015 Pearson Education, Inc.
12 WEEKS
By week 12, the right umbilical vein disintegrates, and the blood from
the placenta travels along a single umbilical vein. The ductus venosus
allows some venous blood to bypass the liver. The veins draining the
digestive tract have fused, forming the hepatic portal vein.
Embryology Summary page 784-785 (6 of 6)
The Development of the Cardiovascular System
Dorsal aorta
Aortic arches
We will follow the development of three
major vessel complexes: the aortic arch,
the venae cavae, and the hepatic portal
and umbilical systems. (In diagrams of
prenatal circulation arterias are shown in
red and veins in blue regardless of the
oxygenation of the blood they carry.)
Yolk
sac
4 WEEKS
Lung
Ductus
arteriosus
Foramen
ovale
Pulmonary
artery
Descending
aorta
Pulmonary
vein
Descending aorta
Liver
Inferior
vena cava
Hepatic
portal vein
Umbilical vein
Umbilical
arteries
FULL TERM
Shortly before birth, blood returning from the
placenta travels through the liver in the ductus
venosus to reach the inferior vena cava. Much
of the blood delivered by the venae cavae
bypasses the lungs by traveling through the
foramen ovale and the ductus arteriosus.
© 2015 Pearson Education, Inc.
NEWBORN
At birth, pressures drop in the pleural cavities as the chest expands and
the infant takes its first breath. The pulmonary vessels dilate, and blood
flow to the lungs increases. Pressure falls in the right atrium, and the
higher left atrial pressures close the valve that guards the foramen ovale.
Smooth muscles contract the ductus arteriosus, which ultimately
converts to the ligamentum arteriosum, a fibrous strand.
Embryology of Organ Systems
• The Development of the Lymphatic System
• Week 6–8
• The thymus gland forms from a pharyngeal pouch
• The cells lose their attachment and form just inferior to
the thyroid gland area
• Formation of
• Jugular lymph sacs
• Primordial lymph sacs
• Median lymph sac—will become the cisterna chyli
• Right lymphatic duct
• Thoracic duct
• Lymph nodes
• Numerous lymphatic sacs form throughout the body
© 2015 Pearson Education, Inc.
Embryology Summary page 786 (1 of 4)
The Development of the Lymphatic System
Jugular
lymph sac
The development of the lymphatic
vessels is closely tied to the formation
of blood vessels. Paired jugular lymph
sacs form from the fusion of small,
endothelium-lined pockets in the
mesoderm of the neck. By week 7,
these sacs become connected to
the venous system.
Primordial
lymph sacs
Median
lymph sac
7 WEEKS
© 2015 Pearson Education, Inc.
Primordial lymph sacs form parallel
with veins of the trunk, and a large
median lymph sac marks the future
location of the cisterna chyli.
Embryology Summary page 786 (2 of 4)
The Development of the Lymphatic System
Parathyroid
Third pharyngeal
pouch
Pharynx
Thyroid
6 WEEKS
The thymus forms from cells of the third
pharyngeal pouch. These cells lose their
connection with the epithelium and divide
repeatedly. As the embryo changes shape,
the thymic lobes are brought together
near the midline of the chest. At birth, the
thymus is relatively large, filling much of
the anterior mediastinum.
Larynx
Pharynx
Thymus
Thyroid
7 WEEKS
Larynx
Parathyroid
Thyroid
Esophagus
Thymus
Trachea
8 WEEKS
© 2015 Pearson Education, Inc.
Embryology Summary page 786 (3 of 4)
The Development of the Lymphatic System
Right lymphatic
duct
As growth continues, the isolated
lymphatic sacs fuse, forming the
thoracic duct and right lymphatic
duct. As the limb buds enlarge,
lymphatic vessels grow into the
area along with developing
arteries and veins.
Thoracic duct
Cisterna chyli
8 WEEKS
© 2015 Pearson Education, Inc.
Embryology Summary page 786 (4 of 4)
The Development of the Lymphatic System
Lymphatic
sac
Lymphocyte
cluster
Small blood vessels grow into areas
where lymphocytes cluster within
developing lymphatic sacs. Connective
tissue capsules form, and the internal
organization of a lymph node gradually
appears.
Lymph
vessel
Capsule
Lymph node
© 2015 Pearson Education, Inc.
Embryology of Organ Systems
• The Development of the Respiratory System
• Week 3
• A pulmonary groove forms in the pharynx area
• Week 4
• This groove forms deeper and then forms two blind
pouches at the end of the groove
• The groove forms a tube, which becomes the
trachea, and the two blind pouches become the
lungs
• Week 9
• The diaphragm forms
© 2015 Pearson Education, Inc.
Embryology Summary
• The Development of the Respiratory System
(continued)
• 3 months
• The lung buds continue to branch numerous times
• 6 months
• There are a million branches
• All the bronchioles are formed
• Alveoli begin to form
© 2015 Pearson Education, Inc.
Embryology Summary page 787 (1 of 2)
The Development of the Respiratory System, Part I
THE LUNGS
Pharyngeal
pouches
Pulmonary
groove
Lung buds
Heart
Yolk sac
3 WEEKS
A shallow pulmonary groove
appears in the midventral floor
of the pharynx after roughly
3 1/2 weeks of development. This
groove, which lies near the level
of the last pharyngeal arch,
gradually deepens.
© 2015 Pearson Education, Inc.
4 WEEKS
By week 4, the groove has become
a blind pocket that extends caudally,
anterior to the esophagus. This tube
will become the trachea. At its tip,
the tube branches, forming a pair
of lung buds.
Embryology Summary page 787 (2 of 2)
The Development of the Respiratory System, Part I
The lung
buds continue
to elongate
and branch
repeatedly.
Bronchioles
Alveoli
3 MONTHS
By the end of the sixth
fetal month, there are
around a million terminal
branches, and the
conducting passageways
are complete to the level
of the bronchioles.
Over the next three months, each of the bronchioles
gives rise to several hundred alveoli. This process
continues for a variable period after birth.
© 2015 Pearson Education, Inc.
Embryology Summary page 788 (1 of 3)
The Development of the Respiratory System, Part II
THE PLEURAL CAVITIES
Digestive
tube
Lung buds
Heart
4 WEEKS
© 2015 Pearson Education, Inc.
Elongation of the tube carries
it into the mediastinum, and
as the branching proceeds,
the lung buds project into the
ventral cavity dorsal to the
developing heart.
Embryology Summary page 788 (2 of 3)
The Development of the Respiratory System, Part II
Esophagus
Developing lung
Pleuropericardial
membrane
6 WEEKS
© 2015 Pearson Education, Inc.
The pericardial sac
begins forming in
week 6 as a thin
pleuropericardial
membrane forms
between the heart and
the developing lungs.
Heart
Embryology Summary page 788 (3 of 3)
The Development of the Respiratory System, Part II
By week 9, the
diaphragm completes
its formation, forming
a transverse sheet
superior to the liver.
Pleural cavity
Lung
Heart
Heart
Pericardial
cavity
Pericardium
Left lung
Diaphragm
Liver
9 WEEKS
© 2015 Pearson Education, Inc.
8 WEEKS
By week 8, the pericardial sac is complete
and the pericardial cavity is isolated from
the rest of the ventral body cavity. The
diaphragm then attaches to the pericardial
sac and tissues of the mediastinum. This
attachment separates the abdominopelvic
cavity from the pleural cavities.
Embryology of Organ Systems
• The Development of the Digestive System
• Week 3
• Yolk sac is formed
• The layer between the yolk sac and the amniotic sac
develops a hindgut and a foregut
• The hindgut and the foregut form the digestive tube
• Week 4
• Lateral to the digestive tube are cavities called the
coelomic cavities
• The digestive tube is held in position within the
midline of the coelomic cavities by the dorsal
mesentery and ventral mesentery
© 2015 Pearson Education, Inc.
Embryology Summary
• The Development of the Digestive System
• Week 4 (continued)
• The pancreas is formed within the dorsal
mesentery area
• The liver is formed within the ventral mesentery
area
• Part of the ventral mesentery becomes the
falciform ligament of the liver
• The remaining mesenteric tissue becomes the
greater omentum and lesser omentum
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Embryology Summary
• The Development of the Digestive System
(continued)
• Week 6
• The intestines elongate and coil
• The yolk sac and the body stalk fuse to form the
umbilical cord
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Embryology Summary page 789 (1 of 3)
The Development of the Digestive System, Part I
Foregut
Endoderm
Somite
Amniotic cavity
Amniotic
cavity
Hindgut
Yolk
sac
3 WEEKS
By week 3, endodermal
cells have migrated
around the inside of the
blastocyst, completing
a pouch known as the
yolk sac.
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Yolk
sac
Hindgut
Developing
coelom
Yolk
stalk
Mesoderm
As the embryo forms on the
embryonic shield, two pockets
of endoderm are created:
the foregut and hindgut.
A broad connection between
these pockets and the yolk
sac remains within the
yolk stalk.
In sectional view, the embryonic
gut is a simple endodermal tube
surrounded by mesoderm.
Cavities appearing within the
mesoderm create the coelom
(ventral body cavity).
Embryology Summary page 789 (2 of 3)
The Development of the Digestive System, Part I
Neural tube
Notochord
Dorsal mesentery
Digestive tube
Yolk stalk
Body stalk
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4 WEEKS
Coelomic cavity
Ventral mesentery
The digestive tube remains suspended in the coelom
by a dorsal mesentery and a ventral mesentery.
The ventral mesentery disintegrates everywhere
except where major vessels or visceral organs have
grown into it. It remains intact along the path of the
umbilical arteries and where the umbilical vein and
liver develop.
Embryology Summary page 789 (3 of 3)
The Development of the Digestive System, Part I
Neural
tube
Pancreas
Greater omentum
Liver
Aorta
Lesser omentum
Pancreas
Liver
As the embryo enlarges, the stomach
and liver rotate toward the right,
creating two pockets. The mesenteries
that form these pockets are the greater
omentum and the lesser omentum.
© 2015 Pearson Education, Inc.
Pancreas
Liver
Coelom
Stomach
Falciform ligament
The pancreas and liver begin as
epithelial pockets that grow away
from the digestive tract and into
the dorsal and ventral mesenteries,
respectively.
Embryology Summary page 790 (1 of 3)
The Development of the Digestive System, Part II
Liver
The intestines begin
to elongate, and with
the breakdown of the
ventral mesentery,
they push outward
into the umbilical
stalk. Further
elongation and
coiling occur outside
the body of the embryo.
Stomach
See sectional view
at 4 weeks in Part I.
6 WEEKS
Pancreas
Allantois
Cloaca
Umbilical stalk
The hindgut extends into the tail, where it forms a large
chamber, the cloaca. A tubular extension of the cloaca,
the allantois (a-LAN-tō-is; allantos, sausage), projects
away from the body and into the body stalk. Fusion of
the yolk stalk and body stalk will create the umbilical
stalk, also known as the umbilical cord.
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Embryology Summary page 790 (2 of 3)
The Development of the Digestive System, Part II
Entrance to
trachea
Esophagus
Liver
Pancreas
Small
intestine
Urogenital
sinus
Rectum
8 WEEKS
A partition grows across the cloaca,
dividing it into a posterior rectum and
an anterior urogenital sinus that
retains a connection to the allantois.
© 2015 Pearson Education, Inc.
Embryology Summary page 790 (3 of 3)
The Development of the Digestive System, Part II
Heart
Stomach
Gallbladder
Small intestine
Umbilical cord
Urinary bladder
10 WEEKS
By week 10, the intestines have begun
moving back into the coelomic cavity,
although they continue to grow longer.
© 2015 Pearson Education, Inc.
Embryology of Organ Systems
• The Development of the Urinary System
• Along the urogenital ridge are:
•
•
•
•
Pronephros
Mesonephros
Metanephros
Cloaca
• Week 3.5
• The kidneys begin as seven pairs of tubules called
pronephric tubules in the pronephros region
© 2015 Pearson Education, Inc.
Embryology Summary page 791 (1 of 3)
The Development of the Urinary System, Part I
Pronephros
Mesonephros
Metanephros
Cloaca
Urogenital
ridge
The kidneys develop in
stages along the axis of
the urogenital ridge, a
thickened area beneath
the dorsolateral wall of
the coelomic cavity.
© 2015 Pearson Education, Inc.
Kidney development proceeds
along the cranial/caudal axis
of this ridge, beginning with the
formation of the pronephros,
continuing along the mesonephros,
and ending with the development
of the metanephros.
Embryology Summary page 791 (2 of 3)
The Development of the Urinary System, Part I
Neural tube
The pronephros consists of a
series of tubules (generally 7
pairs) that appears within the
nephrotome, the narrow band
of mesoderm between the
somites and the lateral plate.
Nephrotome
Notochord
Somite
Pronephric
tubule
Pronephric
duct
Lateral plate
mesoderm
The pronephric tubules are very small and
nonfunctional, and they disintegrate almost
at once. The only significant contribution of
the pronephros is the formation of a pair of
pronephric ducts that grow caudally until
they connect to the cloaca.
31/2 WEEKS
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Embryology of Organ Systems
• The Development of the Urinary System
• Week 4
• Metanephros forms a dense mass and will
eventually become the kidneys
• Week 6
• Ureteric bud forms in the wall of the mesonephric
ducts
• Large glomeruli and nephrons are formed
• These will eventually diminish in size
© 2015 Pearson Education, Inc.
Embryology Summary page 791 (3 of 3)
The Development of the Urinary System, Part I
Pronephros
Developing
aorta
Mesonephric
duct
Mesonephros
Mesonephric
duct
Mesonephric
tubule
Metanephros
Nephrotomal mesoderm of the metanephros
forms a dense mass without a trace of
segmental organization. This will become
the functional adult kidney.
4 WEEKS
© 2015 Pearson Education, Inc.
After approximately 4 weeks of development, the
mesoderm midway along the urogenital ridge
begins organizing into the mesonephros. On
either side of the midline, approximately 70 tubules
develop within these segments. These tubules
grow toward the adjacent pronephric duct and
fuse with it. From this moment on, the duct is
called the mesonephric duct.
Embryology Summary page 792 (1 of 3)
The Development of the Urinary System, Part II
A ureteric bud, or metanephric diverticulum, forms in the
wall of each mesonephric duct, and this blind tube elongates
and branches within the adjacent metanephros. Tubules
developing within the metanephros then connect to the
terminal branches of the ureteric bud.
Glomerulus
Mesonephros
Allantois
Mesonephric
duct
Mesonephric
duct
Renal
corpuscle
Cloaca
Ureteric bud
Metanephros
Most of the metabolic wastes produced by the developing
embryo are passed across the placenta to enter the maternal
circulation. The small amount of urine produced by the
kidneys accumulates within the cloaca and the allantois, an
endoderm-lined sac that extends into the umbilical stalk.
6 WEEKS
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In each segment, a branch of the aorta grows toward
the nephrotome, and the tubules form large nephrons
with enormous glomeruli. Like the pronephros, the
mesonephros does not persist, and when the last
segments of the mesonephros are forming, the first
are already beginning to degenerate.
Embryology of Organ Systems
• The Development of the Urinary System
• Week 8
• Cloaca divides into the rectum and the urogenital
sinus
• The allantois becomes the urinary bladder
• The urethra forms
• Week 12
• The ureteric buds form the calyces and the
collecting system
• Kidneys begin producing filtrate even though there
aren’t any waste products in it
© 2015 Pearson Education, Inc.
Embryology Summary page 792 (1 of 3)
The Development of the Urinary System, Part II
A ureteric bud, or metanephric diverticulum, forms in the
wall of each mesonephric duct, and this blind tube elongates
and branches within the adjacent metanephros. Tubules
developing within the metanephros then connect to the
terminal branches of the ureteric bud.
Glomerulus
Mesonephros
Allantois
Mesonephric
duct
Mesonephric
duct
Renal
corpuscle
Cloaca
Ureteric bud
Metanephros
Most of the metabolic wastes produced by the developing
embryo are passed across the placenta to enter the maternal
circulation. The small amount of urine produced by the
kidneys accumulates within the cloaca and the allantois, an
endoderm-lined sac that extends into the umbilical stalk.
6 WEEKS
© 2015 Pearson Education, Inc.
In each segment, a branch of the aorta grows toward
the nephrotome, and the tubules form large nephrons
with enormous glomeruli. Like the pronephros, the
mesonephros does not persist, and when the last
segments of the mesonephros are forming, the first
are already beginning to degenerate.
Embryology Summary page 792 (2 of 3)
The Development of the Urinary System, Part II
Mesonephric duct
Degenerating mesonephros
Developing
metanephros
Urinary
bladder
Urogenital
sinus
Rectum
8 WEEKS
Near the end of the second developmental month, the cloaca
is subdivided into a dorsal rectum and a ventral urogenital
sinus. The proximal portions of the allantois persist as the
urinary bladder, and the connection between the bladder
and an opening on the body surface will form the urethra.
© 2015 Pearson Education, Inc.
Embryology Summary page 792 (3 of 3)
The Development of the Urinary System, Part II
Nephron
Collecting tubule
Collecting duct
Collecting
system
Major calyx
Metanephros
Ureteric
bud
Ureter
12 WEEKS
The kidneys begin producing filtrate by the third
developmental month. The filtrate does not contain
waste products, as they are excreted at the
placenta for removal and elimination by the
maternal kidneys. The sterile filtrate mixes with the
amniotic fluid and is swallowed by the fetus and
reabsorbed across the lining of the digestive tract.
© 2015 Pearson Education, Inc.
The ureteric bud branches
within the metanephros,
creating the calyces and
the collecting system. The
nephrons, which form
within the mesoderm of
the metanephros, tap into
the collecting tubules.
Embryology of Organ Systems
• The Development of the Reproductive System
• Development of the gonads
• Week 3
• Endodermal cells of the yolk sac migrate to the
genital ridges in the abdominal cavity
• Each ridge consists of columns of cells called
primary sex cords; these will further develop into
gonads
© 2015 Pearson Education, Inc.
Embryology Summary page 793 (1 of 3)
The Development of the Reproductive System
SEXUALLY INDIFFERENT STAGES
(WEEKS 3–6)
DEVELOPMENT OF THE GONADS
Aorta
Gut
Primary
sex cords
Genital
ridge
Yolk sac
Allantois
Mesonephric
duct
Paramesonephric
(Müllerian)
duct
3 WEEKS
During the third week, endodermal cells migrate
from the wall of the yolk sac near the allantois to
the dorsal wall of the abdominal cavity. These
primordial germ cells enter the genital ridges
that parallel the mesonephros.
© 2015 Pearson Education, Inc.
Each ridge has a thick epithelium continuous with columns of cells, the
primary sex cords, that extend into the center (medulla) of the ridge.
Anterior to each mesonephric duct, a duct forms that has no connection to
the kidneys. This is the paramesonephric (Müllerian) duct; it extends
along the genital ridge and continues toward the cloaca. At this sexually
indifferent stage, male embryos cannot be distinguished from female embryos.
Embryology of Organ Systems
• The Development of the Reproductive System
• Development of ducts and accessory organs
• Week 3
• If not exposed to androgens, a female will develop
• If exposed to androgens, a male will develop
• The genital ridge begins producing testosterone
after week 6
© 2015 Pearson Education, Inc.
Embryology Summary page 793 (2 of 3)
The Development of the Reproductive System
SEXUALLY INDIFFERENT STAGES
(WEEKS 3–6)
DEVELOPMENT OF DUCTS
AND ACCESSORY ORGANS
Paramesonephric
duct
Mesonephric duct
Both sexes have mesonephric and
paramesonephric ducts at this stage.
Unless exposed to androgens, the
embryo—regardless of its genetic sex—
will develop into a female. In a normal
male embryo, cells in the core (medulla)
of the genital ridge begin producing
testosterone sometime after week 6.
Testosterone triggers the changes in
the duct system and external genitalia
that are detailed on the following page.
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Gonad
Kidney
Cloacal opening
Embryology of Organ Systems
• Development of External Genitalia
• Week 4
• There are mesenchymal swellings
• Cloacal folds
• Genital tubercle (forms the glans penis and clitoris)
• Week 6
• Cloaca divides to form
• Anal fold
• Urethral fold
© 2015 Pearson Education, Inc.
Embryology Summary page 793 (3 of 3)
The Development of the Reproductive System
SEXUALLY INDIFFERENT STAGES
(WEEKS 3–6)
DEVELOPMENT OF EXTERNAL GENITALIA
Urethral fold
Genital
tubercle
Urogenital
membrane
Cloacal fold
Genital swelling
Cloacal
membrane
Anal fold
4 WEEKS
After 4 weeks of development, there are
mesenchymal swellings called cloacal
folds around the cloacal membrane
(the cloaca does not open to the exterior).
The genital tubercle forms the glans
of the penis in males and the clitoris
in females.
© 2015 Pearson Education, Inc.
6 WEEKS
Two weeks later, the cloaca has been subdivided,
separating the cloacal membrane into a posterior
anal membrane, bounded by the anal folds, and
an anterior urogenital membrane, bounded by
the urethral folds. A prominent genital swelling
forms lateral to each urethral fold.
Embryology of Organ Systems
• Development of the Male Reproductive System
• Development of the testes
• Week 7–12
• The primary sex cords proliferate and eventually
form the seminiferous tubules within the
developing testes
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Embryology Summary page 794-795 (1 of 6)
The Development of the Reproductive System
DEVELOPMENT OF THE MALE REPRODUCTIVE SYSTEM
DEVELOPMENT OF TESTES
Degenerating
mesonephric
tubule
Testis
cords
7 WEEKS
In the male, the primary sex cords
proliferate and the germ cells migrate
into the sex cords. The resulting testis
cords will form the seminiferous tubules.
© 2015 Pearson Education, Inc.
Tunica
albuginea
Rete
testis
Testis cords
(seminiferous
tubules)
12 WEEKS
Connections form between the arching
testis cords and the adjacent mesonephric
nephrons. Although these nephrons later
degenerate, the seminiferous tubules remain
connected to the mesonephric duct.
Embryology of Organ Systems
• Development of the Male Reproductive System
• Development of male ducts and accessory
organs
• 4 months
• The testis cords connect to the remnants of the
mesonephric tubules via the rete testis
• 7 months
• The testes descend into the scrotum
© 2015 Pearson Education, Inc.
Embryology Summary page 794-795 (2 of 6)
The Development of the Reproductive System
DEVELOPMENT OF THE MALE REPRODUCTIVE SYSTEM
DEVELOPMENT OF MALE DUCTS AND ACCESSORY ORGANS
Rete
testis
Developing
testis
Paramesonephric
duct
Seminal
gland
Paramesonephric
duct degenerates
Prostate
Testis
cord
Ductus
deferens
Testis cords
Mesonephric duct
(becomes ductus
deferens)
Mesonephros
Testis
Epididymis
Mesonephric
duct
Urogenital sinus
A view of the testis and ducts of the
left side as seen in frontal section.
Note the location and orientation
of the mesonephros relative to the
developing testis.
© 2015 Pearson Education, Inc.
4 MONTHS
7 MONTHS
After four months of development, the
testis cords are connected to the remnants
of the mesonephric tubules by the rete
testis. The paramesonephric (Müllerian)
duct has degenerated.
Definitive organization after the testis
has descended into the scrotum
(see Figure 27.2, p. 718). Note the
relationships between the definitive sex
organs and the embryonic structures.
Embryology of Organ Systems
• Development of the Male Reproductive System
• Development of male external genitalia
• 10 weeks
• Urethral folds move together to form the spongy
urethra
• Scrotal sac develops
© 2015 Pearson Education, Inc.
Embryology Summary page 794-795 (3 of 6)
The Development of the Reproductive System
DEVELOPMENT OF THE MALE REPRODUCTIVE SYSTEM
DEVELOPMENT OF MALE EXTERNAL GENITALIA
Urethral
folds
External urethral
orifice
Spongy
urethra
Glans of
penis
Urethral
folds
Line of
fusion
Scrotal
swelling
Scrotum
Anus
10 WEEKS
At 10 weeks, the genital tubercle has enlarged,
the tips of the urethral folds are moving together
to form the spongy urethra (see sectional views),
and paired scrotal swellings have developed
from the genital swellings.
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BIRTH
In the newborn male, the line of
fusion between the urethral folds
is quite evident.
Embryology of Organ Systems
• Development of the Female Reproductive System
• Development of the ovaries
• Weeks 7–12
• Primary sex cords degenerate
• Primordial germ cells migrate to the genital ridge
© 2015 Pearson Education, Inc.
Embryology Summary page 794-795 (4 of 6)
The Development of the Reproductive System
DEVELOPMENT OF THE FEMALE REPRODUCTIVE SYSTEM
DEVELOPMENT OF THE OVARIES
Primordial
germ cells
Uterine tube
Mesonephric
duct
Degenerating
primary sex
cords
Primary
sex cords
7 WEEKS
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Cortex
In the female embryo, the
primary sex cords
degenerate and the
primordial germ cells
migrate into the outer
region (cortex) of the
genital ridge.
12 WEEKS
Embryology of Organ Systems
• Development of the Female Reproductive System
• Development of the female ducts and
accessory organs
• Weeks 7–10
• Mesonephric tubules and ducts degenerate
• Paramesonephric duct develops into the peritoneal
cavity
• The urogenital sinus forms the uterus
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Embryology Summary page 794-795 (5 of 6)
The Development of the Reproductive System
DEVELOPMENT OF THE FEMALE REPRODUCTIVE SYSTEM
DEVELOPMENT OF FEMALE DUCTS AND ACCESSORY ORGANS
Peritoneal
opening of
uterine tube
Degenerating
mesonephric
tubules
Ovary
Mesonephric
tubule
remnants
Ovary
Cortex of
ovary
Ovarian ligament
Mesonephros
Uterine
tube (from
paramesonephric
duct)
Uterus
Paramesonephric
(Müllerian) duct
Uterus
Vagina
Urogenital sinus
7 WEEKS
10 WEEKS
BIRTH
The mesonephric tubules and duct degenerate; the paramesonephric (Müllerian) duct develops
a broad opening into the peritoneal cavity. Note the fusion of the ducts and the separation of
the common chamber, which will form the uterus, from the urogenital sinus.
© 2015 Pearson Education, Inc.
Embryology of Organ Systems
• Development of the Female Reproductive System
• Development of female external genitalia
• Week 7
•
•
•
•
Urethral folds do not fuse
Urethral folds form the labia minora
Genital swellings will form the labia majora
Genital tubercle forms the clitoris
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Embryology Summary page 794-795 (6 of 6)
The Development of the Reproductive System
DEVELOPMENT OF THE FEMALE REPRODUCTIVE SYSTEM
COMPARISON OF MALE AND
FEMALE EXTERNAL GENITALIA
Males
Penis
Corpora cavernosa
Corpus spongiosum
Proximal shaft of penis
Spongy urethra
Bulbo-urethral glands
Scrotum
DEVELOPMENT OF FEMALE EXTERNAL GENITALIA
Clitoris
Females
Clitoris
Erectile tissue
Vestibular bulbs
Labia minora
Vestibule
Greater vestibular glands
Labia majora
Genital tubercle
Labia minora
Genital swelling
Urethra
Urethral fold
Labia majora
Urogenital
membrane
Opening
to vagina
Hymen
Anus
7 WEEKS
BIRTH
In the female, the urethral folds do not fuse; they develop into the labia
minora. The genital swellings will form the labia majora. The genital
tubercle develops into the clitoris. The urethra opens to the exterior
immediately posterior to the clitoris. The hymen remains as an
elaboration of the urogenital membrane.
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