BY:
MUNEEB HASAN KHAN
FIRST WEEK OF DEVELOPMENT
1.
Cleavage of
zygote:
• A series of rapid mitotic divisions in zygote that converts it into
progressively smaller blastomeres.
• Begins approximately 30 hours after fertilization and occurs as
zygote passes through uterine tube.
• Does not involve cell growth and hence the embryo remains confined
within zona pellucida.
• Following stages are observed: o 1st cleavage => 2
cell stage o 2nd cleavage => 4
cell stage o 3rd cleavage => 8
cell stage
• Compaction
occurs in 8 cell stage in which blastomeres maximize their contact with each
other, mediated by cell-surface-adhesive glycoproteins.
The inner cell mass acquires extensive gap junctions, a prerequisite for
segregation of the embryo into 2 layers later on.
2.
Formation of
Morula:
• It is a ball of 12-32 blastomeres formed after 3 to four cleavage
divisions, appx. on day 3.
• Corona radiata disperses but zona pellucida remains.
• Consists of 2 cell masses based on their position:
a. Inner cell mass
with extensive gap junctions formed during compaction.
b. Outer cell mass surrounding
the periphery,
3.
Formation of
Blastocyst:
• On day 4, when morula enters uterus, fluid-filled space i.e. blastocoele
appears inside morula due to entry of fluid through zona pellucida in a process
called blastogenesis.
• As blastocoele enlarges it separates blastomeres into 2 parts:
1) Embryoblast – inner
cell mass that becomes eccentric and projects into blastocoele at embryonic
pole and gives rise to tissues of embyo proper
2) Trophoblast – outer
cell mass that flattens to form epithelial wall of blastocyst and gives rise to
embryonic part of placenta&
other extra-embryonic structures.
• Early pregnancy factor
(immunosuppressant) is secreted by trophoblast and appears in maternal serum
24-48 hours after fertilization, and can be used for early pregnancy tests
during first 10 days.
4.
Partial
implantation:
• On day 5 the zona pellucida disappears
because of lytic enzymes
released from trophoblast &
acrosomes of the attached sperms
i.e. hatching of
blastocyst which allows
a. Rapid enlargement of embryo
b. Initiation of implantation
• The blastocyst floats about in uterine secretion for about 2 days.
• On day 6, it attaches to endometrial epithelium by following
interactions:
1. Trophoblast at embryonic pole expresses L-selectin
which attaches to carbohydrate receptors on
uterine epithelium for initial attachment.
2. Further embedding occurs between trophoblastic integrins and
endometrial ECM molecules fibronectin (for
attachment) & laminin (for
migration).
5.
Early
differentiation:
• Trophoblast
differentiates into 2 layers immediately upon implantation under influence of
TGFs, with fingers of syncytium invading the compact layer of endometrium.
• Embryoblast
differentiates under influence of FGFs, causing random scattered blastomeres to
acquire epiblastic and hypoblastic fates. By day 7, hyboblastic cells migrate
and delaminate to form a distinct hypoblast
facing the blastocoele& ventral to epiblast, and some of them also form anterior visceral endoderm that
secretes nodal antagonists to
specify cranial end of embryo, thus
establishing a dorsoventral and cranio-caudal axis.
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