Transcript Theories of Human Development

PREGNANCY & HUMAN
DEVELOPMENT
Mutiara Budi Azhar
Faculty of Medicine Sriwijaya University
Embryology
Embryology is a science that studies the
normal development as well as birth defects
of a human being in the maternal uterus.
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Historical gleanings
Hippocrates, Father of Medicine, first
recorded embryological studies; bird
embryo can be likened to that of man.
Aristotle: studied chick and other
embryos, which resulted from union
of semen with menstrual blood.
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From Egg to Fetus

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Pregnancy – events that occur from fertilization
(conception, conceptio) until the infant is born.
Conceptus – the developing offspring.
Gestation period – from the last menstrual period
until birth.
Preembryo – conceptus from fertilization until it is
two weeks old.
Embryo – conceptus during the third through the
eighth week.
Fetus – conceptus from the ninth week through birth
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Relative Size of Human Conceptus
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Gametes and Gametogenesis
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Gamete
A gamete (from Ancient Greek γαμετης; translated
gamete = wife, gametes = husband) is a cell that
fuses with another gamete during fertilization
(conception) in organism that reproduce sexually.
 In species which produce two morphologically
distinct types of gametes, and in which each
individual produces only one type, a female is any
individual which produces the larger type of
gamete—called an ovum (or egg)—and a male
produces the smaller tadpole-like type—called a
sperm.

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Gamete. Cont’
 Anisogamy
or heterogamy: the condition
wherein females and males produce
gametes of different sizes: in humans, the
human ovum is approximately 20 times
larger than the human sperm cell.
 Isogamy is the state of gametes from both
sexes being the same size.
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Gamete. Cont’

The name gamete was introduced by the Austrian
biologist biologist Gregor Mendel
 Gametes carry half the genetic information of an
individual, one chromosome of each type.
 In human an ovum can only carry X chromosom (of
the X and Y chromosom)
 As a sperm can carry either an X or a Y, males have
the control of the gender of any resulting zygote as
the genotype of the sex-determining chromosomes of
a male must be XY and a female XX.
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The Human
Life Cycle

Haploid: pertaining
to a single set of
unpaired
chromosomes or an
organism or cell that
comprises of a single
set of chromosomes
 Diploid: Contain
two homologous sets
of chromosomes
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Gametogenesis
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Spermatogenesis
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Ooogenesis
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Spermatogenesis
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Oogenesis
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Sperm Meiosis
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Spermatozoa
Spermatozoa
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Oocyte
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hypothalamus
GnRH
anterioir pituitary
FSH
LH
Mid-cycle peak of LH (triggers ovulation)
hypothalamus
Blood levels of
FSH (purple) and
LH (lavender)
anterior lobe
of pituitary
gland
FSH
LH
growth of follicle
estrogens
LH
ovulation
corpus luteum
progesterone, estrogen
Blood levels of estrogens
(light blue) and
progesterone (dark blue)
endometrium
of uterus
estrogens
progesterone, estrogen
menstruation
Days of one menstrual cycle (using
28 days as the average duration)
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FOLLICULAR PHASE OF
MENSTRUAL CYCLE
LUTEAL PHASE OF
MENSTRUAL CYCLE
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Fertilization
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Accomplishing Fertilization

The oocyte is viable for 12 to 24 hours
 Sperm is viable 24 to 72 hours
 For fertilization to occur, coitus must occur no
more than:
– Three days before ovulation
– 24 hours after ovulation
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Fertilization – when a sperm fuses with an egg
to form a zygote
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Sperm Transport and Capacitation
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Fates of ejaculated sperm
–
–
–
–
Leak out of the vagina immediately after deposition
Destroyed by the acidic vaginal environment
Fail to make it through the cervix
Dispersed in the uterine cavity or destroyed by phagocytic
leukocytes
– Reach the uterine tubes

Sperm must undergo capacitation before they can
penetrate the oocyte
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Acrosomal Reaction and Sperm Penetration
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An ovulated oocyte is encapsulated by:
– The corona radiata and zona pellucida
– Extracellular matrix

Sperm binds to the zona pellucida and
undergoes the acrosomal reaction
– Enzymes are released near the oocyte
– Hundreds of acrosomes release their enzymes to
digest the zona pellucida
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Acrosomal Reaction and Sperm Penetration. Cont.’
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Once a sperm makes contact with the oocyte’s
membrane:
– Beta protein finds and binds to receptors on the
oocyte membrane
– Alpha protein causes it to insert into the membrane
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Acrosomal Reaction and Sperm Penetration. Cont.’
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Blocks to Polyspermy

Only one sperm is allowed to penetrate the oocyte
 Two mechanisms ensure monospermy
– Fast block to polyspermy – membrane
depolarization prevents sperm from fusing with the
oocyte membrane
– Slow block to polyspermy – zonal inhibiting
proteins (ZIPs):
 Destroy sperm receptors
 Cause sperm already bound to receptors to detach
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Completion of Meiosis II and Fertilization
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Upon entry of sperm, the secondary oocyte:
– Completes meiosis II
– Casts out the second polar body

The ovum nucleus swells, and the two nuclei
approach each other
 When fully swollen, the two nuclei are called
pronuclei
 Fertilization – when the pronuclei come
together
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Events Immediately
Following Sperm
Penetration
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Preembryonic Development
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Preembryonic Development

The first cleavage produces two daughter cells
called blastomeres
 Morula – the 16 or more cell stage (72 hours
old)
 By the fourth or fifth day the preembryo
consists of 100 or so cells (blastocyst)
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Preembryonic Development. Cont.’
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Blastocyst – a fluid-filled hollow sphere
composed of:
– A single layer of trophoblasts
– An inner cell mass
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Trophoblasts take part in placenta formation
 The inner cell mass becomes the embryonic
disc
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The Period of the Zygote
Zygote 
blastocyst
 Cell
differentiation
 % of
blastocysts that
fail to implant?

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The Period of the Zygote. Cont.’
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Outer layer of blastocyst develops into:
–
–
–
–
Amnion
Chorion
Placenta
Umbilical cord
**note that these structures actually develop during
the period of the embryo
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Implantation
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Implantation

Begins six to seven days after ovulation when
the trophoblasts adhere to a properly prepared
endometrium
 The trophoblasts then proliferate and form two
distinct layers
– Cytotrophoblast – cells of the inner layer that
retain their cell boundaries
– Syncytiotrophoblast – cells in the outer layer that
lose their plasma membranes and invade the
endometrium
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Implantation. Cont.’
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The implanted blastocyst is covered over by
endometrial cells
 Implantation is completed by the fourteenth
day after ovulation
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Implantation of the Blastocyst
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Implantation of the Blastocyst. Cont.’
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Implantation of the Blastocyst
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Implantation of the Blastocyst. Cont.’
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Implantation of the Blastocyst. Cont.’
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Implantation of the Blastocyst. Cont.’

Viability of the corpus luteum is maintained by
human chorionic gonadotropin (hCG) secreted by the
trophoblasts
 hCG prompts the corpus luteum to continue to secrete
progesterone and estrogen
 Chorion – developed from trophoblasts after
implantation, continues this hormonal stimulus
 Between the second and third month, the placenta:
– Assumes the role of progesterone and estrogen production
– Is providing nutrients and removing wastes
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Hormonal
Changes During
Pregnancy
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Placentation
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Placental
Development
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Placentation

Formation of the placenta from:
– Embryonic trophoblastic tissues
– Maternal endometrial tissues
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Placentation. Cont.’
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The chorion develops fingerlike villi, which:
– Become vascularized
– Extend to the embryo as umbilical arteries and
veins
– Lie immersed in maternal blood

Decidua basalis – part of the endometrium
that lies between the chorionic villi and the
stratum basalis
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Placentation. Cont.’
capsularis – part of the
endometrium surrounding the uterine
cavity face of the implanted embryo.
 The placenta is fully formed and functional
by the end of the third month.
 Embryonic placental barriers include:
 Decidua
– The chorionic villi
– The endothelium of embryonic capillaries
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Placenta
• Permits the exchange of materials between the
bloodstream of the fetus and that of the mother
• Produces progesteron; gradually take over the
role of corpus luteum.
 The
placenta also secretes other hormones
– human placental lactogen, human
chorionic thyrotropin, and relaxin.
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Placentation. Cont.’
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Figure 28.7a-c
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Placentation. Cont.’
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Figure51
28.7d
Placentation. Cont.’
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Figure 28.7f
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Germ Layers
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Gastrulation
During the 3rd week, the two-layered
embryonic disc becomes a three-layered
embryo
 The primary germ layers are ectoderm,
mesoderm, and endoderm
 Primitive streak – raised dorsal groove that
establishes the longitudinal axis of the embryo

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Germ Layers
• The blastocyst develops into a gastrula with
three primary germ layers: ectoderm,
endoderm, and mesoderm.
• Before becoming three-layered, the inner cell
mass subdivides into the upper epiblast and
lower hypoblast: these layers form two of the
four embryonic membranes
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Gastrulation. Cont.’
• As cells begin to migrate:
– The first cells that enter the groove form the
endoderm
– The cells that follow push laterally between the
cells forming the mesoderm
– The cells that remain on the embryo’s dorsal
surface form the ectoderm
• Notochord – rod of mesodermal cells that
serves as axial support
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Three Germ Layers*
• Endoderm – formed from migrating cells
that replace the hypoblast
• Mesoderm – formed between epiblast and
endoderm
• Ectoderm – formed from epiblast cells that
stay on dorsal surface
*All layers derive from epiblast cells!
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Primary Germ Layers

Serve as primitive tissues from which all body
organs will derive.
 Ectoderm – nervous system, skin, hair, sensory
receptors.
 Endoderm – digestive system, lungs, urinary
tract, other internal organs.
 Mesoderm – muscles, bones, circulatory system,
reproductive system, excretory system.
 Endoderm and ectoderm are securely joined and
are considered epithelia.
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Derivatives of Germ Layers
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Embryonic Membranes

Amnion – epiblast cells form a transparent
membrane filled with amniotic fluid
– Provides a buoyant environment that protects the
embryo
– Helps maintain a constant homeostatic temperature
– Amniotic fluid comes from maternal blood, and
later, fetal urine
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Embryonic Membranes. Cont.’
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Yolk sac – hypoblast cells that form a sac on
the ventral surface of the embryo
– Forms part of the digestive tube
– Produces earliest blood cells and vessels
– Is the source of primordial germ cells
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Embryonic Membranes. Cont.’

Allantois – a small outpocketing at the caudal
end of the yolk sac
– Structural base for the umbilical cord
– Becomes part of the urinary bladder

Chorion – helps form the placenta
– Encloses the embryonic body and all other
membranes
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Embryonic Period
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Embryonic Period
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Embryo at 4 Weeks
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Face Development
from 5½ to 8 Weeks
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The Period of the Embryo. Cont.’
By 4th week, heart has formed and begun to
beat
 Becomes more human in appearance during
2nd month
 Sexual differentiation during 7th – 8th weeks

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The Period of the Fetus
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The Period of the Fetus

Final 7 months of pregnancy
 Fetus
 Digestive and excretory systems functional
 Sex detected by ultrasound by end of 3rd month
 Kicks and movements strong enough to be felt
 Organ systems mature rapidly during final 3
months
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The Period of the Fetus. Cont.’
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Fetus at 9 Weeks
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Fetus at 11 Weeks
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Fetus at 16 Weeks
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Fetus at 18 Weeks
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Fetus at 20 Weeks
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Fetus at 28 Weeks
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Fetal development
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Twins
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The Period of the Fetus. Cont.’

Viable between 22-28 weeks
 Receives antibodies
 Assume upside-down position in final weeks
– Weight of head
– Shape of uterus
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A Brief View of Prenatal Development
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A Brief View of Prenatal Development. Cont.’
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Developmental Events of the Fetal Period
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Developmental Events of the Fetal Period
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Table 3.1 (2 of 3)
Developmental Events of the Fetal Period
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Table 3.1 (3 of 3)
Trends in Development

Cephalocaudal Trend

Proximodistal Trend
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Teratogens
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Susceptible period
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Teratogens
Root word means “formation of monsters”
 Harm isn’t always simple or straightforward
 Amount and length of exposure
 Genetic makeup of mother/baby
 Presence of several negative factors

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Teratogens. Cont.’
Same defect  different teratogens
 One teratogen  different defects
 Some effects seen later
 Age of baby
 Sensitive period

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Teratogens. Cont.’

Paternal influences often overlooked
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Direct and indirect effects
Second hand smoke
Chemicals
Cocaine can “hitchhike”
Smoking, alcohol, drug use  sperm
Diets low in vitamin C
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Other Factors
Teratogens. Cont.’

Exercise
 Nutrition
 Emotional stress
 Rh blood incompatibility
 Maternal/Paternal age
– Older  less fertile, more risk of disorders
– Younger  increased risk of prematurity, infant
death
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Other Factors
Teratogens. Cont.’

Infectious diseases
 Prenatal care
 Previous births
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Congenital malformations
(Birth defects)
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Chemical: nitrite, benzol; lead, arsenic, cadmium,
mercury, etc.
Drugs: thalidomide (amelia and meromelia),
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Drugs: aminopterin (anencephaly, hydrocephalus,
cleft lip); streptomycin (deafness).
Hormones: estrogens, progestins
Social drugs: cigarettes (small babies); alcohol (fetal
alcohol syndrome).
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Perinatal Environment

Environment surrounding birth
 Stages of childbirth
– Contractions
– Delivery
– Afterbirth

Newborn appearance
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Birth

Labor

Oxytocin

Uterine
contractions
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Perinatal Environment

Assessing the Newborn
– Apgar scale
– 1 minute and 5 minutes after birth
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The Apgar Test
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Perinatal Environment

Assessing the Newborn
– Neonatal Behavioral Assessment Scale (NBAS)
 Several days after birth
 20 inborn reflexes
 Reactions to comforting and social stimuli
 Unresponsiveness may indicate neurological
problems
 Can be a parent teaching tool
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Perinatal Environment

Complications
– Anoxia
 Severe  cerebral palsy, mental retardation
 Mild  irritability, motor/cognitive delays
 Chances have been reduced with fetal monitoring
 Respiratory distress syndrome
– Complicated delivery
 Forceps
 Cesarean
 Medications
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Preterm and Small-for-Date

Preterm = born more than 3 weeks early, but
appropriate weight for time in womb
 Small-for-Date = underweight due to slow
fetal growth
– Greater risk than preterm
– Causes include smoking, drug use, stress, lack of
prenatal care, multiple births, social support

Postterm = born after 42 weeks
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Thank you very much for your kind attention
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