Transcript Fetal Development Chapter 5

FETAL
DEVELOPMENT
CHAPTER 5
Learning Objectives
1. Explain mitosis and meiosis and differentiate between the two.
2. Describe the processes of spermatogenesis and oogenesis, and how
they differ.
3. Explain how the sex of the conceptus is determined.
4. Describe the three developmental stages of pregnancy with regard to
beginning and ending periods and major events occurring during
each stage.
5. Describe the development of support structures during pregnancy.
6. Name four major functions of amniotic fluid.
7. Discuss three functions of the placenta.
8. List the steps in the process of the exchange of nutrients and wastes
between the maternal and fetal bloodstreams.
9. Trace the path of fetal circulation, including the three fetal shunts.
10. Name three categories of teratogens and list examples of each kind.
11. Discuss the threat to pregnancy that occurs with ectopic pregnancy.
12. Differentiate between the types of multifetal pregnancies.
Cellular Processes
• Soma cells
• Make up the organs and tissues of the
human body
• Nucleus contains 23 pairs of chromosomes
• 22 pairs of autosomes
• 1 pair of sex chromosomes
• Chromosomes composed of genes
Cellular Processes (cont.)
• Gametes (germ cells or sex cells)
• Found in the reproductive glands only
• Ovum is female gamete
• Spermatozoon (sperm) is male gamete
• Each gamete has 23 chromosomes
Types of Cellular Division
• Mitosis
• The process by which somatic (body) cells give
birth to daughter cells
• Each daughter cell contains the same number
of chromosomes as the parent cell
• Meiosis
• The process by which gametes undergo two
sequential cellular divisions of the nucleus
(gametogenesis)
Types of Cellular Division (cont.)
• Meiosis (cont.)
• Gametogenesis = formation and
development of germ cells
• Spermatogenesis
‒Begins at puberty
‒Primary spermatocytes undergo the first
meiotic division
‒Secondary spermatocytes a second
meiotic division
‒Spermatids undergo a change in form to
become mature spermatozoa
Mitotis and Gametogenisis
Types of Cellular Division (cont.)
• Meiosis (cont.)
• Gametogenesis (cont.)
• Oogenesis begins before birth in ovaries
• At birth, primary oocytes have completed the
prophase stage of the first meiotic division
• Completion of the first meiotic division occurs
before ovulation
• Results in secondary oocyte and the first polar
body
• First polar body disintegrates
• Secondary oocyte begins its second meiotic
division at ovulation but does not complete the
process unless a sperm fertilizes it
Question
In the development of a human being, a process
occurs in which cells divide to produce a germ
cell with 23 unpaired chromosomes. What is this
process called?
a. Meiosis
b. Mitosis
c. Oogenisis
d. Gametogenisis
Answer
a. Meiosis
Rationale: Meiosis is the process by which gametes
undergo two sequential cellular divisions of the
nucleus. This process reduces the number of
chromosomes in the gametes by half. Remember,
each gamete has only 23 chromosomes, which is
half, also known as the haploid number, of the
total number of chromosomes required for human
cells.
Stages of Fetal Development
• Three stages
• Pre-embryonic
• Embryonic
• Fetal
• Pre-embryonic
• Begins at fertilization
• Lasts until two weeks after fertilization
Stages of Fetal Development (cont.)
• Pre-embryonic (cont.)
• Conception usually occurs when the ovum is in the
ampulla (the outermost half) of the fallopian tube
• Zygote is formed and has diploid number of
chromosomes
• Gender determination
• Occurs at the time of fertilization
• Dependent on whether the sperm has an X or Y
chromosome
• Research indicates that there is an approximately 5050 chance of either occurrence
Stages of Fetal Development (cont.)
• Pre-embryonic (cont.)
• Cellular reproduction
• Cleavage begins
• 3 days after fertilization morula forms
• 5 days after fertilization blastocyst forms
• 14 days after fertilization the inner cell mass
becomes the embryonic disk
Stages of Fetal Development (cont.)
• Pre-embryonic (cont.)
• Implantation
• By 10th day after fertilization blastocyst is buried
in uterine lining
• Begins to produce human chorionic
gonadotropin (hCG)
• Endometrium is called the decidua from
implantation through end of pregnancy
Stages of Fetal Development (cont.)
• Embryonic stage
• Lasts from the end of the 2nd week after
fertilization until the end of the 8th week
• Developing conceptus becomes the embryo
• Differentiation begins
• 3rd week, three germ layers develop
• Ectoderm
• Mesoderm
• Endoderm
Three Germ Layers of the Embryo
• Ectoderm: outer layer of cells
• Forms skin, hair, nails, and the nervous system
• Mesoderm: middle layer
• Forms the skeletal, muscular, and circulatory
systems
• Endoderm: inner layer
• Forms the glands, lungs, and urinary and
digestive tracts
Stages of Fetal Development (cont.)
• Fetal stage
• From the beginning of the 9th week after
fertilization and continues until birth
• Additional growth and maturation of the organs
and body systems
Question
In the embryonic stage of fetal development what
occurs?
a. Implantation begins
b. Gender differentiation takes place
c. Maturation of germ cells in ovaries
d. Differentiation begins
Answer
d. Differentiation begins
Rationale: During the embryonic period, the cells
of the embryo multiply, and tissues begin to
assume specific functions, a process known as
differentiation.
Development of Supportive Structures
• Fetal membranes
• amniotic cavity begins to develop around 9 days
after conception surrounded by amnion
• amnion is a thick fibrous lining, made up of
several layers, that helps to protect the fetus,
and it forms the inner part of the sac in which
the fetus grows
• chorion is a second layer of thick fibrous tissue
that surrounds the amnion
Development of Supportive Structures
(cont.)
• Amniotic fluid
• Fills the amniotic cavity
• Serves four main functions
• Physical protection
• Temperature regulation (fetus not mature
enough to regulate own temperature)
• Provision of unrestricted movement
• Symmetrical growth
Development of Supportive Structures
(cont.)
• Placenta
• Organ that sustains and nourishes the growing pregnancy
• Three main functions
• Provide for the transfer of nutrients-from maternal blood supply
(O2, glucose, some maternal antibodies) and exchange of
substances or removal of waste-from fetal circulation (carbon
dioxide, carbon monoxide, urea, uric acid)
• To act as a barrier to certain substances (some medications and
hormones)
• To function as an endocrine gland by producing hormones to
sustain the pregnancy (hCG, hPL, estrogen, progesterone)specific functions page 108
Development of Supportive Structures (cont.)
• Umbilical cord
• Extends from the umbilicus of the fetus to the
fetal surface of the placenta
• Two arteries that bring deoxygenated blood
from the fetus to the placenta and one vein that
carries oxygenated and nourished blood from
the placenta to the fetus
• Wharton’s jelly (gives support to the cord,
helps prevent compression of the cord)
surrounds these three vessels
Fetal and Placental Circulation
• Fetal circulation-dependent upon maternal circulation
for oxygenation (does not use its lungs to oxygenate
blood)
• Low oxygen tension
• High hematocrit
• Three fetal shunts
• Ductus venosus-shunts ½ of oxygen rich blood past the liver and
flows it directly into the inferior vena cava
• Foramen ovale-majority of blood in the right atrium flows into the
left atrium via this shunt
• Ductus arteriosus-links pulmonary artery w/ the aortadeoxygenated blood
• Placental circulation
• Exchange occurs in the intervillous spaces (surrounded by the
chorionic villi)
-chorionic villi contain the fetal blood vessels
-exchange of nutrients for wastes occurs by simple diffusion or
active transport across a thin membrane that separates
fetal and maternal bloodstreams
Fetal Circulation cont’d
• At birth, fetal blood not longer flows to the placenta for
oxygenation
• Rapid physiological changes occur to allow the newborn
to oxygenate his own blood by using his lungs
• After birth the ductus venosus, ductus arteriosus, the
umbilical vein, and arteries constrict and eventually form
ligaments (present for the rest of the infant’s life)
• The ductus arteriosus can remain open, esp. in premature
infants and infants with hypoxia- known as “patent ductus
arteriosus”
-may require medication or surgery to close the duct
Fetal Circulation
Question
Tell whether the following statement is true or false.
The ductus arteriosus carries oxygenated blood from the
pulmonary artery to the aorta.
Answer
False
Rationale: The ductus arteriosus does connect the
pulmonary artery and the aorta, but it carries
deoxygenated blood.
Special Considerations of Fetal
Development
• Teratogens and the fetus
• Substances that cause birth defects
• Severity of the defect depends upon when during development the
conceptus is exposed to the teratogen (i.e., which body systems are
developing at the time of exposure) and the particular teratogenic
agent to which the fetus is exposed
Special Considerations of Fetal
Development (cont.)
• Types of teratogenic agents
• Ingested
• Prescription and over-the-counter medications, illicit drugs, and
alcohol
• Infectious
• Varicella, cytomegalovirus (CMV), and rubella
• Environmental substance
• Ionizing X-rays, radioactive substances, and certain chemicals
Common Teratogens and Associated Effects
Teratagen
Possible Effect
Ingested Agents
Dilantin
Cleft palate
Chemotherapy agents
Major congenital malformations, esp. of the CNS
Tetracycline
Damage to developing dental and osseous tissue
Alcohol
Fetal alcohol syndrome w/facial defects, low birth
weight, affects brain
Infectious Agents
Varicella
Fetal varicella syndrome, ranges in severity from
generalized multi-organ damage to isolated defects
(i.e. incomplete limb or skin scarring)
Rubella (German Measles)
Cataracts, deafness, & cardiac malformations
CMV
Hearing loss, microcephaly
Environmental Agents
Mercury
Neurologic damage, blindness
Radiation
Congenital malformations, mental retardation
Special Considerations of Fetal
Development (cont.)
• Ectopic pregnancy
• The zygote implants in places other than the uterus
• 95% occur in the fallopian tube (tubal pregnancy)
• Usually caused by blockage or scarring of the fallopian tubes either
from infection or trauma (such as tubal ligation)
• Occurrence is 19.7 of 1000 pregnancies
• If undetected-the tube will rupture, hemorrhage into the peritoneal
cavity can lead to maternal death
• Surgical removal of the affected tube and products of conception is the
most common treatment
Special Considerations of Fetal
Development (cont.)
• Multifetal pregnancy
• Monozygotic twins
• Identical twins derived from one zygote
• Share same genetic material; always the same sex
• Dizygotic twins
• Fraternal twins develop from separate egg and sperm
fertilizations
• Genetic material is not identical; may or may not be the same
sex
Special Considerations of Fetal
Development (cont.)
• Classification of twins
• Diamniotic-dichorionic twins
• Each develop in their own amniotic sac
• Placentas do not share any vessels
• Fraternal twins
• Diamniotic-monochorionic twins
• Each have their own amniotic sac but share a common chorionic
sac
• Each have a separate placenta but the placentas share some
vessels
• Risks: twin-to-twin transfusion syndrome
• Monoamniotic-monochorionic
• Have one amniotic cavity that they both share
• Risks: cord entanglement, twin-to-twin transfusion syndrome, or
become conjoined
Special Considerations of Fetal
Development (cont.)
• Factors that increase a woman’s chance of becoming
pregnant with dizygotic twins
• Family history of twins
• Recent stoppage of oral contraceptive
• Tall or large stature of the mother
• African-American heritage
• Use of fertility medications
Discussion Topic #1
A 28-year-old patient is admitted to the hospital where a
diagnosis of a ruptured ectopic pregnancy is made in her
fallopian tube.
• As the nurse caring for this patient, what are the primary
concerns with this condition? Also, identify two priority
nursing diagnoses.
• What are other health care problems that share similar
clinical symptoms?
• What nursing care measures will this patient require pre
and post operative removal of the ectopic pregnancy?
DT#1, Q#1