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Summary ZOL2602 Assignments & Exam notes
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ZOL2602 UNISA Embryology pdf, exam notes and Assignment 1 and 2 from 2023 first term.
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Extra-Embryonic structuresDefinition: Cellular structures that have embryonic tissues and are
found outside of the embryo which aids in protection and giving
metabolic/nutritional needs.
All membranes support embryo as it is in shell, no connection to the
outside environment.
1. Summarise in tabular form the extra-embryonic membranes that occur
in the bird's egg and name the function(s) of each. If there is any
change of function, indicate that as well.
Membranes Functions
1. Yolk Sac Provides nutrients to embryo via
vitelline blood vessels
2. Amnion sac Filled with amniotic fluid to
provide protection of embryo
against shock, indentations of
sac wall and temperature
fluctuations.
3. Chorion sac Develops from yolk sac and
provides nutrients; provides
chorionic fluid which aids in
embryo protection. *involved
with gas and nutrient exchange
due to becoming highly
vascularised after mesoderm
fusion
4. Allantois For collection of metabolic waste
*Allantois fuses with chorion mesoderm to form allantochorion = functions for
gas exchange between blood of embryo and atmosphere.
, 2. Explain the origin of the Heuser membrane in the mammalian foetus.
From the cytotrophoblast, flat cells create the Heuser membrane which
lies against the cytotrophoblast inner wall. It surrounds the primary
yolk sac. It is a combination of extracellular matrix and hypoblast cells
(from which the cytotrophoblast originates from). The cytotrophoblast
originates from when the trophoblast cells makes contact with the
uterine epithelium and penetrates the lining through proteolytic
enzymes. This divides the trophoblast into two layers, the inner
cytotrophoblast which contains cell membranes and the outer
synctiotrophoblast (contains no cells).
3. Explain how the placenta of the mammalian foetus develops. Describe
all the stages, in the correct sequence. *the placenta has a maternal
part (endometrium) and foetal part (made up of the chorion)
1. The pre-lacunar stage
- The trophectoderm attaches to the endometrium wall and fuses to
form primary syncytium.
2. The lacunar stage
- the primary syncytium invades the surface epithelium and moves into
the endometrium and changes into the decidua. Fluid filled spaces
appear within the syncytial mass which increase in size and merge. The
syncytium disintegrates the decidual glands.
3. The villous stage
- the primary villi are formed by the trophoblasts cell disintegrating and
forming branches that push through the syncytium. These arms
continue growing and the lacunae becomes the intervillous space.
- Primary villi are transformed into secondary villi due to mesodermal
cells from the chorionic plate entering the nuclear regions of the
primary villi.
- The secondary villi go deeper into the primary
syncytium/synctiotrophoblast and reach the endometrium. A
cytotrophoblast capsule is formed between the villi and decidua and
covers the whole trophoblast, dividing the decidua into three layers.
- The mesodermal cells in the secondary villi become tertiary villi due to
the formation of blood cells and vessels. The villi continue branching
from the chronic plate to form tree like formations.
- The cytotrophoblast cells leave the shell to invade the decidua as extra
villous trophoblasts. The space between the uterus lessens and the
decidua layers (capsularis and parietalis) fuse which form the full
placenta.
Steps showing the development of the extra embryonic structures
1. The ovum is released by the graafian follicle and moves through the
oviduct and is fertilised by the sperm. It undergoes cleavage and
forms a blastocyst which is divided into the trophoblast and inner
embryoblast cells.
, 2. The trophoblast cells cover the embryoblast and touch, bond and
penetrate the uterine epithelium all done through the help of
proteolytic enzymes.
3. The trophoblast is divided into two layer, the synctiotrophoblast
which form roots that penetrate the uterine stroma and
cytotrophoblast.
4. Lacunae (fluid filled spaces) are formed inside the
synctiotrophoblast due to the cytotrophoblast penetration and its
process of breaking down the uterine tissue which released tissue
fluid.
5. As the blastocyst is covered completely by the uterine tissue the
uterine epithelium recovers over the penetration point.
6. The lacunae begin to form a network space in the
synctiotrophoblast.
7. The synctiotrophbalst continues to penetrate the uterine stroma and
encounters the capillary blood vessels which contain sinusoids
(blood congestions).
8. The synctiotrophoblast cells disintegrate the uterine wall and
sinusoid linings which connects the lacunae and sinusoids and
maternal blood flows between the two. The pressure of the arterial
and ventral blood capillaries give rise to the uteroplacental
circulatory system.
9. The inner cytotrophoblast cells give rise to extra embryonic
mesoderm which fills the space between the cytotrophoblast and
the amniotic sac and inside yolk sac.
10. The space is taken over by the forming chorion which develops
into the chorionic plate. *
11. The cytotrophoblast begin to develop primary villi which grow in
the trophoblast lacunae and penetrate the intervillous spaces.
12. The secondary yolk sac begins to develop.
13. Primary villi are transformed into secondary villi due to
mesodermal cells from the chorionic plate entering the nuclear
regions of the primary villi.
14. The secondary villi go deeper into the synctiotrophoblast and
reach the endometrium. A cytotrophoblast capsule is formed and
covers the whole trophoblast, anchoring to the chorionic sac in the
endometrium.
15. The mesodermal cells in the secondary villi become tertiary villi
through the forming blood cells and vessels. The vessels created in
the tertiary villi join the blood vessels in the chorionic plates and the
embryonic stalk aka the umbilical cord which links to the embryo.
This completes the vascular system between the embryo and the
developing placenta.
16. The tertiary villi from the attachment site of the umbilicus to the
uterus wall creates the chorion frondosum (rough/bushy). The
chorion plate villi form into the chorion leave (smooth).
17. Three regions of the uterus are now seen: Decidua basalis,
capsularis and parietalis.
18. Decidua basalis forms decidual septa and spread in the villi. This
divides the uterus into cotyledons.
19. The space between the uterus lessens and the decidua
capsularis and parietalis fuse which form the full placenta.
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