ANHB2212 Lecture Notes - Lecture 2: Zona Pellucida, Amniotic Sac, Fallopian Tube
25 May 2018
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Early Embryology: first 4 weeks
Day 1: fertilization
- Usually in uterine tube
- Takes approximately 24 hours
Phase 1 – passage of sperm through corona radiata
Phase 2 – penetration of zona pellucida
Phase 3 – fusion of oocyte and sperm cell membranes
Following successful fertilization the following has occurred – restoration of diploid no. of
chromosomes
- determination of sex
- initiation of cleavage
Day 2-4: cleavage
- Series of mitotic divisions to increase cell numbers
Within 24 hours from fertilization
Divisions not accompanied by cell growth – zygote subdivided
into smaller daughter cells called blastomeres
By 3 days the embryo consists of 6-12 cells, by day 4 16-32 cells – known as morula.
Day 4-5: early to late blastocyst formation
- Blastocyst hatches from zona pellucida prior to implantation
- Fluid penetrates through zona pellucida and into inter-cellular space of the inner cell mass
- Occurs around time the morula enters the uterine cavity
- Resultant fluid filled space is the blastocystic cavity.
Inner cell mass is the embryoblast – becomes embryo
outer cell mass is the trophoblast – becomes placenta
Day 6: Implantation
- Zygote has passed through morula and blastocyst stages and
begins implantation in the uterine mucosa.
- Blastocyst attaches to endometrial epithelium – adjacent to
embryonic pole
Trophoblast begins rapid proliferation and differentiates into two layers
- cytotrophoblast – inner layer that divide and migrate into syncytiotrophoblast
- syncytiotrophoblast – highly invasive outer layer that allows the blastocyst to
burrow into the endometrium.
find more resources at oneclass.com
find more resources at oneclass.com
Summary of first week
Day 7-9: Differentiation of the inner cell mass
- Embryoblast splits in to two layers that make the bilaminar disc – epiblast
(upper/external layer of
columnar cells).
- hypoblast (primitive
endoderm)
- Cavity forms within epiblast and enlarges to be – amniotic cavity lined by amnioblasts
- Hypoblast in combination with Heuser’s membrane gives rise to the primitive yolk sac.
Day 11-12: extra embryonic mesoderm:
- Blastocyst completely embedded in endometrial stroma
New cells (derived from yolk sac) appear between the inner surface of the cytotrophoblast
and the outer surface of the yolk sac – extraembryonic mesoderm.
Cavities form and fuse in the mesoderm – give rise to chorionic cavity.
find more resources at oneclass.com
find more resources at oneclass.com
Cavity separates the embryo (with attached
amnion and yolk sac) from outer wall of chorion
(formerly blastocyst) – except at the
connecting stalk.
End of second week: definitive two layered embryo.
Bilaminar embryonic disc suspended in the chorionic cavity by a thick connecting stalk.
- Secondary yolk sac forms within its primary – large
portions of the primary sac compressed into
exocoelomic cells
- Ectodermal vesicle – amniotic cavity
- endodermal vesicle – secondary yolk sac
- embryonic disc – point of contact between two
vesicles
- prechordal plate develops as a localized thickening of
the hypoblast – gives rise to cranial region of embryo.
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Phase 1 passage of sperm through corona radiata. Phase 3 fusion of oocyte and sperm cell membranes. Following successful fertilization the following has occurred restoration of diploid no. of chromosomes. Series of mitotic divisions to increase cell numbers. Divisions not accompanied by cell growth zygote subdivided into smaller daughter cells called blastomeres. By 3 days the embryo consists of 6-12 cells, by day 4 16-32 cells known as morula. Blastocyst hatches from zona pellucida prior to implantation. Fluid penetrates through zona pellucida and into inter-cellular space of the inner cell mass. Occurs around time the morula enters the uterine cavity. Resultant fluid filled space is the blastocystic cavity. Inner cell mass is the embryoblast becomes embryo outer cell mass is the trophoblast becomes placenta. Zygote has passed through morula and blastocyst stages and begins implantation in the uterine mucosa. Blastocyst attaches to endometrial epithelium adjacent to embryonic pole begins rapid and. Cytotrophoblast inner layer that divide and migrate into syncytiotrophoblast.
