Embryology | Developmental Period Week 1 &2 |First aid lectures | Lec3 #drjamillectures #drjamil
Channel: Dr. Jamil Lectures
Date: 04/03/2023

In this lecture we will discuss about developmental week 1 & 2. we will talk about cleavge,morulla,blastulation,implantation/trophoblast,embryoblast,trilaminar disk.

https://youtu.be/gXGK6F_vDJQ
https://youtu.be/U3aD-wn2zMM
https://youtu.be/r9yQe_u8keQ

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Development up until week 1.
❖ Cleavage
● Zygote undergoes repeated mitotic divisions that results in a rapid increase in the
number of cells where each cell is called a blastomere → It occurs while the zygote
passes along the uterine tube towards the uterus → Zygote is still covered by zona
pellucida that prevents from abnormal implantation i,e zona pellucida makes sure
normal implantation in uterus → While
passing along the fallopian tube towards
uterus,the zygote undergoes repeated division
under following stages
➢ 1-cell stage at day 1
➢ 2-cell stage at day 2
➢ 4-cell stage,8-cell stage and 16-cell
stage (multicells stage) at day3 →
16-cell stage zygote is called morula,
which is a hollow ball of cells.
➢ More than 32-cell stage zygote at day
4 (called advanced morula that has
now entered the uterus).
● Blastulation →At the end of day 4 the
blastomeres change shape and tightly align
themselves against each other and the advanced
morula is transformed into a compact ball of cells that
is called blastula/blastocyst → this process is called
blastulation that occurs in following steps.
➢ A group of cells align themselves around the
edge/periphery, forming the outer cell mass and
another group of cells compact together on one
side,forming the inner cell mass →It forms a
blastula with outer cell mass,inner cell mass
and a hollow, fluid-filled cavity that is still
covered by zona pellucida preventing
implantation.
● At day 5 the zona pellucida will be lost and blastula will hatch out of zona pellucida
and at day 6 blastula will attach with endometrium (implantation) but implantation is
week -long process that starts at day 5 and completes at day 12.
● Cells of blastula become more functional and differentiate into following structures.
➢ Outer cell mass will differentiate into trophoblast that will further
differentiates into two specialized layers
1) Cytotrophoblast
2) Syncytiotrophoblast
➢ Inner cell mass will differentiate into embryoblast that will further
differentiate into a bilaminar disk which is made up of 2 layers
1) Epiblast
2) Hypoblast

Development up until week 2
❖ Implantation/Trophoblast
● When the blastula is implanted in the endometrium,then at day 6 to day 8 the some
of the cells of trophoblast (outer cell mass) starts proliferating outside of the zona
pellucida that disintegrates the cell membrane →It forms a fluid-like cytoplasm which
consists of the nuclei of the cell and cells lose their well-defined margins → all of the
cytoplasm will fuse together and form a finger-like structures syncytium which is
called the syncytiotrophoblast.
● At the base of the finger-like structures of the syncytiotrophoblast there are some
cells of trophoblast with cell membrane and nucleus (well-defined margins) which are
called the cytotrophoblast → cytotrophoblast keeps proliferating into centre of actual
finger like villi (syncytiotrophoblast).
● Syncytiotrophoblast will continue to release hydrolytic enzymes → these hydrolytic
enzymes will keep making the way of syncytiotrophoblast through the uterine
lining.→ As the uterine lining has maternal blood vessels and the syncytiotrophoblast
will move deeper into the endometrium until it becomes confluent with the maternal
blood vessels → in this way the embryo can receive oxygen, nutrients, hormones
from maternal blood vessels→ so in this way trophoblast helps in implantation and
placenta formation.
● At day 24, the syncytiotrophoblast secretes the β-hCG hormone which stimulates the
corpus luteum to continue producing progesterone → increased progesterone level
prevents the endometrial lining from shedding like
in the normal menstrual cycles.
❖ Embryoblast/Bilaminar disk
● Embryoblast (inner cell mass) differentiates into a
bilaminar disk (it forms the embryo) that contains 2 sheet-like layers.
1. Top layer which is called the Epiblast
2. Bottom layer which is called the Hypoblast
Above the epiblast is the dorsal amniotic cavity and below the
hypoblast is the ventral primitive yolk sac.
● From the top view of the bilaminar disc,there is a depression in the epiblast
which is called prechordal plate.
 
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