BY MUNEEB HASAN KHAN
SECOND WEEK OF DEVELOPMENT
1.
Completion
of Implantation:
•
Occurs over a time period of days 6-10 after fertilization.
•
Mediated by progressive invasion of endometrial stroma by
syncytiotrophoblast as well as apoptosis of endometrial cells, so that
i. By day 8, conceptus is partially
embedded. ii. By day 9, the surface defect in uterine epithelium is sealed by a closing plug of fibrin coagulum.
iii.
By day 10-11, it is completely embedded with a slight bulge into
uterine lumen.
iv.
By day 12-13, the surface defect has healed by signaling from cAMP
and progesterone.
•
Slight amount of bleeding may occur from implantation site because of increased bloodflow in
lacunar spaces, which may be confused with menstruation since their chronologies are similar.
•
In response to implantation, endometrial cells undergo decidua reaction in which they become polygonal, edematous and accumulate glycogen & lipids (decidual
cells).
2.
Differentiation
of Trophoblast:
•
Produces 2 layers beginning at time of implantation and becoming
very distinct by day 8:
i. Cytotrophoblast
– inner layer of mononucleated cells
with high number of mitotic figures that divide, migrate and fuse to lose their
cell membranes & form the syncytium.
ii.
Syncytiotrophoblast
– outer multinuclear zone without
distinct cell boundries at embryonic pole that:
a)
Progressively invades endometrial stroma thereby causing further embedding of the
blastocyst (extent of invasion is checked by endometrial cells)
b)
Engulfs decidual cells providing rich source of nutrition for embryo
c)
Secretes human chorionic
gonadotrophin (hCG) which maintains
corpus luteum and its hormonal action to maintain pregnancy. It seeps into
maternal serum from lacunae; levels are high enough at end of second week to give a positive
pregnancy test
3.
Formation
of Lacunae:
•
In lacunar stage starting on day 9, small vacuoles appear in syncytiotrophoblast
that fuse to form spaces called lacunae filled with a fluid i.e. embryotroph that – o Consists of a mix of maternal blood (from eroded endometrial capillaries) and cellular debris (from eroded uterine glands).
o Provides nutrition to embryonic disc by simple diffusion.
•
By day 12, a distinct lacunar network
is formed by fusion of lacunae
especially at embryonic pole, which is primordia of intervillous
spaces of placenta – whereas vascularity
of endometrium increases under influence of estrogen and progesterone; the
capillaries become congested and dilated to form maternal
sinusoids.
o the sinusoids
are eroded by syncytium and drain into the lacunae. In this way a primordial uteroplacental circulation is established in which oxygenated maternal blood flows from spiral arteries into lacunar
networks& deoxygenated blood is removed
by endometrial veins.
4.
Formation
of Primary Chorionic Villi:
•
By end of second week (days 13-14), cytotrophoblast proliferates to
form cellular columns that project into and are covered by a layer of syncytium
i.e. chorionic villi in primary stage.
•
Induced by underlying somatic
extraembryonic mesoderm.
5.
Differentiation
of Embryoblast:
•
On 7th day, blastomeres in embryoblast acquire
epiblastic/hypoblastic fates under influence of FGFs.
Initially
they are scattered by but later on they migrate to form 2 distinct layers:
i. Epiblast – thicker layer of high columnar cells facing amniotic cavity (dorsal aspect). ii. Hypoblast – thinner layer of small cuboidal cells facing blastocoele (ventral aspect). By day 14, at cranial end, a localized portion becomes columnar and forms the circular prechordal
plate, the site of future mouth and
organizer of head region.
•
After further morphogenesis, involving formation of yolk sac and amniotic cavity, these 2 layers are present as a nearly circular bilaminar embryonic disc whose growth rate is very slow as compared to that of trophoblast.
•
Cranio-caudal axis of embryo is determined by a population of
hypoblastic cells called anterior
visceral endoderm, which
secretes NODAL
antagonists to specify cranial end of embryo,
due to which NODAL establishes primitive streak only at caudal end of embryo.
6.
Formation
of Amniotic Cavity:
•
Soon after its formation (day 8) the epiblast acquires an amniotic cavity filled with amniotic fluid.
•
As the cavity enlarges, a layer of epiblastic cells i.e. amnioblasts is displaced towards the embryonic pole and gives rise to the
epithelial covering of amnion, which is continuous peripherally with the epiblast.
7.
Formation
of Yolk Sac/Umbilical Vesicle:
•
On day 9, hypoblastic cells proliferate ventrally from the margins
of embryonic disc and become squamous.
•
These eventually form a continuous membrane (Exocoelomic/Heuser’s membrane) that covers inner surface
of cytotrophoblast and completely
lines the blastocoele (now called Exocoelomic
cavity), forming the primary umbilical vesicle.
•
Once chorionic cavity forms, on day 13 another layer of extraembryonic endodermal cells from hypoblast is formed and migrates along inside of Heuser’s
membrane to form a much smaller secondary
umbilical vesicle.
•
Afterwards, a large portion of primary yolk sac is pinched off as Exocoelomic cyst(s) that are extruded into chorionic cavity, usually at abembryonic
pole.
8.
Formation
of Extraembryonic Mesoderm & Chorionic Cavity:
•
On days 10-11, endoderm covering the umbilical vesicle produces a
new population of cells i.e.
extraembryonic
mesoderm that initially lies between inner surface of cytotrophoblast and outer surface
of Exocoelomic + amniotic cavity.
•
By day 12, large spaces appear in the mesoderm which coalesce to
form the extraembryonic
coelom or chorionic
cavity which splits the extraembryonic
mesoderm into 2 layers:
i.
Extraembryonic
somatic – covers amnion & inner aspect of
cytotrophoblast. This along with the 2 layers of trophoblast forms the
outermost fetal membrane chorion.
ii.
Extraembryonic
splanchnic -covers umbilical vesicle.
•
Chorionic cavity is continuous except in the region of connecting stalk, which is a bridge of unsplit extraembryonic mesoderm present near
embryonic pole. Amnion and yolk
sac are suspended in the chorionic
cavity & attached to chorion through it.
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