This powerpoint presentation describes the second week of pregnancy day by day, and provide diagrams to clear the ideas,

2.  Implantations occurs at the end of the week
 It may occur outside the uterus, in the uterine tube or in the ovary
 During the second week many developmental processes occur:
1) The Trophoblast differentiates into 2 layers: the Cytotrophoblast and
Syncytiotrophoblast
2) The embryo blast forms 2 layers: the epiblast and hypoblast
3) The extraembryonic mesoderm splits into 2 layers: the somatic and splanchnic
layers
4) Two cavities form: the amniotic and yolk sac cavities

4.  Blastocytes
partially embedded in
endometrial stroma.
 Trophoblast
differentiate into:
-cytotrophoblast
-syncytiotrophoblast
 Emberyoblast
differentiate into:
-hypoblast layer
-epiblast layer

6.  The blastocyst is more deeply embedded in the endometrium and fibtin
coagulum close the penetration defect.
 The trophoblast continues development and vacules appear in
syncytium.
 A large lacunae Is formed and this phase of trophoblast development is
called “lacunar stage”.
 Exocoelonic (Heuser) membrane is formed from flattened cells
originating from hypoblast.
 Exocoelonic (Heuser) and hypoblast membranes form the lining of the
exocoelonic cavity, or primitive cavity.

9.  The blastocyst is completely
embedded in the wall of the uterus
 The epithelium covers the defect on
the uterine wall
 The blastocyst produces a slight
protrusion in the lumen of uterus

10.  The lacunae of the
syncytiotrophoblast form an
intercommunicating network at
the embryonic pole.
 syncytiotrophoblast erodes more
sinusoids so that maternal blood
flows into the trophoblaastic
system to establish the
uteroplacental circulation

11. Extra
embryonic
mesoderm
•Cytotrophoblast externally
•Amnion and exocoelomic membrane internally
Extra
embryonic
coelom or
chorionic
cavity
•Large cavities appear in the mesoderm
•And become confluent
Extra
embryonic
somatopleuri
c mesoderm
•Extraembryonic mesoderm lining the cytotrophoblast and amnion
•Also, the lining covering the yolk sac is known as the
extraembryonic splanchnopleuric mesoderm .

12.  Endometrium:
→ The cells become more polyhedral and located with glycogen
and lipids. The intracellular spaces are filled by extravasated .
→ Thus , the tissue is edematous
Deciduous reaction

14. Implantation
 Surface defect of the
endometrium has normally healed
Bleeding at the implantation site
occasionally occurs
 Inaccuracy in learning the
expected delivery date

15. Trophoblast
The trophoblast is characterized by
villous structures.
The cytotrophoblast’s cells multiply thus
penetrating into the syncytiotrophoblast
forming cellular columns covered by
syncytium* known as “primary villi”.
*mass of cytoplasm resulting from fusion of cells

16. Embryoblast:
The hypoblast produces additional cells that migrate along inside of the
exocoelomic membrane forming a new smaller cavity (the secondary yolk
sac or definitive yolk sac) within the already present exocoelomic cavity.
Formation of exocoelomic cysts found in chorionic cavity.

17. Expansion of the chorionic cavity.
extraembryonic mesoderm
Connecting stalk (only site where extra
embryonic mesoderm traverses the chrionic
cavity)
 Connecting stalk later becomes the
umbilical cord
chrionic plate.

19. Clinical Correlates:
Abnormal Implantation
 By the second week quantities of the
human chorionic gonadotropin
hormone(hCG) produced by
syncytiotrophoblast are sufficient to be
detected by radioimmunoassay
allowing us to test pregnancy.

20. Clinical Correlates:
Abnormal implantation In the uterus
 The blastocyst is attached to the internal os (opening of the cervix) rather than
the anterior or posterior walls of the uterus
 Later, The placenta bridges the opening causing placenta previa
 Life threatening bleeding during second stage of pregnancy and delivery

21. Clinical Correlates:
Abnormal implantation In the uterus
 Are called Ectopic pregnancies
 They mostly in occur in the uterine tube specifically in the ampulla.
 Will also appear in the rectouterine cavity, pouch of Douglas, peritoneal
covering of the intestinal tract or the omentum.
 May also develop in the ovary proper causing a primary ovarian
pregnancy.
 Ectopic implantations are responsible for 9% of pregnancy-related
deaths of the mother.