Lecture on embryology

1. Cleavage, Blastocyst formation
and Implantation
By
Dr. A.U. Agu
MB,BS; MSc; PhD

3. • Is the process of
subdivision of zygote into
smaller cells.
• Once the zygote reached 2
cell stage (30hrs after
ferti), it undergoes a series
of mitotic division
increasing the number of
cells.
• The cells become smaller
with each division & form
the blastomere
Cleavage

4. • The blastomeres are seen as
loosely arranged clump of
cells until eight-cell stage.
• Thereafter, the blastomeres
maximize their contact with
each other forming a compact
ball of cells held by tight
junction.
• This process of compaction,
segregates inner cells from
outer cells
• At about 3 days after
fertilization, the compacted
cells divide again to form 16-
cell morula (mulberry)

5. • Inner cells constitute the inner cell mass
(ICM) which gives rise to embryo proper
• The surrounding cells (outer cells), composed
the outer cell mass (OCM) & gives rise to
trophoblast which later contribute to
placenta.
NOTE.
4-cell stage is formed at about 40hrs after ferti
12-16 cell stage - at about 3 days
Late morula – 4 days

6. Blastocyst formation
• About the time the morula
enters the uterine tube, some
fluid enter into the intercellular
space of inner cell mass via the
zona pellucida.
• As the fluid increases, the ICM
becomes separated from the
OCM and appears at one pole.
• A single cavity or cyst is formed
called blastocele.
• The embryo is thus called
blastocyst, ICM and OCM
become embryoblast and
Trophoblast respectively

7. • A single cavity or cyst is formed
(blastocele).
• The embryo is thus called
blastocyst, ICM and OCM become
embryoblast and trophoblast
respectively
• The trophoblast flatten & form
epithelial wall of the blastocyst
• Then zona pellucida disappears to
allow implantation
• The side of attachment of
embryoblast is called embryonic
pole while the opposite side is
abembryonic pole

8. Function of zona pellucida
• The trophoblast has the property of being able
to stick into tissues & its cell can ‘eat up’ other
cells
• Thus they can invade & burrow into tissue
they come in contact
• Zona pellucida prevents the embryo from
sticking to the uterine tube

9. Uterus at time of implantation
• The mucosa of the uterus is in secretive phase
• The uterine glands and arteries are coiled & the
tissues are succulent
• Thus 3 distinct layers of are recognized in the
endometrium;
Superficial compact layer
Intermediate sponge layer
Thin basal layer
Normally, implantation occur along the anterior
of posterior wall of the body of uterus.

10. Implantation
• The trophoblastic cells
over the embryonic pole
begin to penetrate b/w
the epithelial cells of the
uterine mucosa
• Attachment and
inversion of trophoblast
involves;
Integrins – expressed by
trophoblast
Laminin - extracellular
matrix molecule(EMM)
Fibronectin - EMM

11. • Integrin receptors for laminin promote
attachment
• Those of fibronectin stimulate migration
• These molecules also regulate trophoblast
differentiation.

12. Day 8
• Blastocyst is partially
embedded in the
endometrial stroma
• In the area over the
embryoblast, the trophoblast
differentiated into 2 layers;
 Cytotrophoblast – has
mitotic figures
 Syncytiotrophoblast
• Thus, cells of cytotrophoblast
divide & migrate into
syncytiotrophoblast where
they fuse & loose their
individual cell membranes

13. • The embryoblast also
differentiates into 2 layers:
 Hypoblast
 Epiblast
• Within the epiblast, a small
cavity appears & this enlarges
to form the amniotic cavity
• Epiblast cells line the amniotic
cavity.
• Those adjacent to
cytotrophoblast are called
amnioblast
• Endometrial stroma adjacent
to implantation site becomes
edematous & highly vascular
• The tortuous glands secrete
glycogen & mucus

14. Day 9
• Blastocyst is more
deeply embedded
• The penetration defect
in the surface
epithelium is closed by
fibrin coagulum
• Trophoblast dev further
with vacuoles appearing
in the
syncytiotrophoblast
• The vacuoles fuse to
form large lacunae &
this stage is called
lacunar stage

15. • At the abembryonic
pole, flattened cells
from hypoblast forms
exocoelomic (Heuser’s)
membrane that lines the
inner surface of
cytotrophoblast
• The hypoplast & the
membrane form the
lining of primitive yolk
sac (exocoelomic cavity)

16. Days 11 and 12
• Blastocyst is completely
embedded in the
endometrium
• Surface epith almost
entirely covers the
defect in the uterine wall
• The blastocyst produces
a slight protrusion into
the uterine lumen
• The lacunar spaces form
an intercommunicating
network

17. • The syncytiotrophoblast
penetrate deeper into the
stroma & erode the endothelial
lining of maternal capillaries
(sinusoids)
• The lacunae become
continuous with the sinusoid &
maternal blood enters the
lacunar system
• Trophoblast erodes more
sinusoids, & maternal blood
flows via the trophoblast
system, establishing
uteroplacental circulation

18. • New cell populations
(extraembryonic mesoderm)
appear b/w the inner surface
of cytotrophoblast & outer
surface of primary yolk sac.
• They are derived from
primary yolk sac.
• Large cavities develop in the
extraembryonic mesoderm
• The cavities form spaces
called chorion cavity or
extraembryonic coelom

19. • The extraembryonic
mesoderm lining the
cytotrophoblast & amion is
called extraembryonic
somatopleuric mesoderm
• Those covering the primary
yolk sac is called
extraembryonic
splanchnopleuric
mesoderm
• Decidua reaction first occur
at site of implantation but
soon occur throughout the
endometrium

20. Day 13
• The surface defect of the
endometrium is healed
• Sometimes, bleeding occur
at the implantation site due
to increased blood flow to
the lacunar spaces
• Trophoblast is characterized
by villous structures
• Hypoblast proliferate &
form a new cavity within
the primary yolk sac & is
called secondary or
definitive yolk sac

21. • During its formation, a
portion of primary yolk
sac (exocoelomic cavity)
is pinched off & is called
exocoelomic cyst
• Extraembryonic coelom
expand & form a large
cavity called chorionic
cavity
• With dev of blood
vessels, connecting
stalk becomes the
umbilical cord

22. Clinical correlates
• Synsytiotrophoblast
produces h C G, thus PT
can be positive by end of
2nd week
• Abnormal implantation
– At internal os giving
placenta previa
– Ectopic gestation
• Abnormal blastocyst
causing abortion of
hydatidform mole

23. Thanks for listening