Laboratory Exercise 20: Embryology and Fetology
Conception occurs when a sperm cell penetrates the secondary oocyte. This normally
occurs within the oviduct within the 10-16 day of the menstrual cycle.
Once fertilized, the secondary oocyte completes its meiosis. The male and female
pronuclei unite to form the fertilized ovum or zygote. The fusion of the two nuclei
restores the full complement of genetic material. Surrounding the zygote is the zona
pellucida. This thick transparent membrane is a barrier to prevent other sperms from
entering after the first sperm enters. This is monospermy.
The first 8 prenatal weeks is the embryonic period, a time of growth and development
during which the zygote progresses to a recognizable human form. The 3rd – 9th months
of gestation are the fetal period during which there is mainly growth.
A. Human Embryonic Development
Post-fertilization (Days)
1-2 days Cleavage
Within 24-36 hours fertilization (1-2 days postfertilization) the zygote undergoes a series of rapid
mitotic divisions, referred to as cleavage. The one cell
zygote divides to 2 cells, then to 4 cells, etc.
3 days Morula
Three days post-fertilization, the morula stage is reached.
This is solid spherical mass of cells only slightly larger
than the zygote due to a progressive decrease in cell
size during cleavage.
4 days Blastocyst
Blastulation
The morula continues to divide as it descends through
the oviduct, by the 4th day post-fertilization the
blastocyst stage is reached.
The blastocyst is characterized by:
An outer layer, the trophoblast which will become
the chorion;
An inner cell mass which will become the embryo
proper; and a cavity, the blastocoele.
5 days Blastocyst
arrives in the
uterine cavity
The 5th day post-fertilization, the blastocyst arrives
in the uterine cavity.
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6-7 days
Implantation
Gastrulation –
establishes 3 layers
as the cell
rearrange
themselves
The blastocyst implants within the endometrium by the
6-7 days post-fertilization. The trophoblast erodes the
endometrial uterine layer and the area will anchor and
nourish the embryo.
In this preplacental period the endometrium and
endometrial glands supply nutrients directly to
the embryo during the first month of pregnancy.
7, 8-14 days
Embryonic
Organization
Ectoderm and
Endoderm
After implantation, between the 7th – 14th days postfertilization, the inner cell mass organizes into a
flattened embryonic disk of two layers, the outer ectoderm
and an inner endoderm.
14-15 days
Mesoderm
By the 14th – 15th days post-fertilization, a 3rd layer
develops between the ectoderm and endoderm, the
mesoderm.
The 3 primary germ layers give rise to the tissues:
Ectoderm: nervous system, epidermis, skin derivates –
hair, nails, epidermal glands
Mesoderm: muscles, blood vessels, bone and other connective tissues.
Endoderm: epithelial linings of the digestive, respiratory systems, urinary
bladder and urethra.
Extra-Embryonic Membranes and Placenta
15-30 days
During 15-30 days post-fertilization membranes develop
external to the embryo. The amnion is the innermost of
these membranes. It is derived from the ectoderm. The
amnion encloses a fluid-filled cavity to provide a shockabsorbing, protective zone for the developing individual.
The chorion is the outermost membrane. It is derived from
the trophoblast and mesoderm. The chorion and the uterine
lining create the placenta, which appears after 4 weeks and
is fully functional at 2 months post-fertilization.
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The yolk sac derived from the endoderm. It will extend
into the umbilical cord. Some of its cells migrate to the
gonads to give rise to sex cells.
The allantois derived from the endoderm. It will extend
into the umbilical cord. The blood vessels of the allantois
will serve as the umbilical arteries between mother and
fetus. The allantois will serve in the formation of the
urinary bladder.
The yolk sac and the allantois function as early sites for
blood cell formation.
B. Fetology (3-9 months post-fertilization)
As the embryo passes its 8th week, it is called a fetus. The placenta becomes fully
functional by this time. The placenta, a vascular organ formed by the chorion, the
embryonic part of the placenta and the decidua basalis, the maternal part of the placenta.
The decidua basalis is the endometrium of the uterus that lies deep to the embryo. The
placenta serves to exchange nutrients and wastes between fetal and maternal circulations.
The placenta is also an endocrine organ as it secretes estrogen and progesterone to
maintain the pregnancy during the last 6 months of the pregnancy.
The trophoblast and chorion of the embryo secrete human chorionic gonadotropin
(HCG), a LH type hormone to maintain the corpus luteum to continue secretion of
progesterone and estrogen to maintain the pregnancy until the placenta becomes
functional.
Prenatal life consists of development and growth.
Development – organs undergo change which makes them structurally and functionally
complete.
Growth – cells increase in number and size. From the 3rd month to birth the fetus will
increase its weight by more than 50 times. By the 7th month all organ systems are welldeveloped and growth occurs without further development.
The circulatory system is the exception. Final adjustments are made after birth within the
first 6 months post-natal.
Fetal circulation – Blood rich in O2 and nutrients leaves the placenta enters the fetus
through the umbilical vein.
The umbilical venous blood bypasses the fetal liver by the ductus venosus.
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The foramen ovale (FO) diverts the O2 blood away from the right ventricle and
pulmonary circulation RA
FO
LA
LV
Aorta
The ductus arteriosus (DA) shunts blood from the pulmonary artery to the aorta so as to
bypass the lungs. RA
RV
Pulmonary artery
DA
Aorta
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