2nd week of pregnancy

1. Formation of the Bilaminar Embryonic Disc: Second Week

2. Formation of the Bilaminar Embryonic Disc: Second Week
• Implantation of the blastocyst is completed during the second
week.
• As this process occurs, morphologic changes in the embryoblast
produce a bilaminar embryonic disc composed of epiblast and
hypoblast.
• The embryonic disc gives rise to the germ layers that form all the
tissues and organs of the embryo.
• Extraembryonic structures forming during the second week are the
amniotic cavity, amnion, umbilical vesicle (yolk sac), connecting
stalk, and chorionic sac.

3. Figure : A 7.5-day human blastocyst, partially embedded in the endometrial stroma. The trophoblast consists of an
inner
layer with mononuclear cells, the cytotrophoblast, and an outer layer without distinct cell boundaries, the
syncytiotrophoblast. The embryoblast is formed by the epiblast and hypoblast layers. The amniotic cavity appears as

4. Figure: 9-day human blastocyst. The syncytiotrophoblast shows a large number of lacunae. Flat cells form the
exocoelomic membrane. The bilaminar disc consists of a layer of columnar epiblast cells and a layer of cuboidal
hypoblast

5. Figure: Human blastocyst of approximately 12 days. The trophoblastic lacunae at the embryonic pole are in open
connection with maternal sinusoids in the endometrial stroma. Extraembryonic mesoderm proliferates and fi lls the
space

6. Figure: Human blastocyst of approximately 12 days. The trophoblastic lacunae at the embryonic pole are in open
connection with maternal sinusoids in the endometrial stroma. Extraembryonic mesoderm proliferates and fi lls the
space

7. Figure: A 13-day human blastocyst. Trophoblastic lacunae are present at the embryonic as well as the abembryo
pole, and the uteroplacental circulation has begun. Note the primary villi and the extraembryonic coelom or chorio
cavity. The secondary yolk sac is entirely lined with endoderm.

8. DAY 8:
• At the eighth day of development, the blastocyst is partially embedded
in the endometrial stroma.
• In the area over the embryoblast, the trophoblast has differentiated
into two layers:
• (1) an inner layer of mononucleated cells, the cytotrophoblast, and
• (2) an outer multinucleated zone without distinct cell boundaries, the
syncytiotrophoblast
• Mitotic figures are found in the cytotrophoblast but not in the
syncytiotrophoblast.
• Thus, cells in the cytotrophoblast divide and migrate into the
syncytiotrophoblast, where they fuse and lose their individual cell
membranes.

9. Figure : A 7.5-day human blastocyst, partially embedded in the endometrial stroma. The trophoblast consists of an
inner
layer with mononuclear cells, the cytotrophoblast, and an outer layer without distinct cell boundaries, the
syncytiotrophoblast. The embryoblast is formed by the epiblast and hypoblast layers. The amniotic cavity appears as

10. • Cells of the inner cell mass or embryoblast also differentiate into two
layers:
1. a layer of small cuboidal cells adjacent to the blastocyst cavity, known
as the hypoblast layer, and
2. a layer of high columnar cells adjacent to the amniotic cavity, the
epiblast layer .
• Together, the layers form a flat disc ( bilaminar germ disc).
• At the same time, a small cavity appears within the epiblast.
• This cavity enlarges to become the amniotic cavity.
• Epiblast cells adjacent to the cytotrophoblast are called amnioblasts;
together with the rest of the epiblast, they line the amniotic cavity.
• The endometrial stroma adjacent to the implantation site is edematous
and highly vascular.
• The large, tortuous glands secrete abundant glycogen and mucus.

11. DAY 9:
• The blastocyst is more deeply embedded in the endometrium, and the
penetration defect in the surface epithelium is closed by a fibrin
coagulum.
• The trophoblast shows considerable progress in development,
particularly at the embryonic pole, where vacuoles appear in the
syncytium.
• When these vacuoles fuse, they form large lacunae, and this phase of
trophoblast development is thus known as the lacunar stage.
• At the abembryonic pole, meanwhile, flattened cells probably
originating from the hypoblast form a thin membrane, the exocoelomic
(Heuser’s) membrane that lines the inner surface of the
cytotrophoblast.
• This membrane, together with the hypoblast, forms the lining of the
exocoelomic cavity, or primitive yolk sac.

12. Figure: 9-day human blastocyst. The syncytiotrophoblast shows a large number of lacunae. Flat cells form the
exocoelomic membrane. The bilaminar disc consists of a layer of columnar epiblast cells and a layer of cuboidal
hypoblast

13. DAYS 11 AND 12:
• By the 11th to the 12th day of development, the blastocyst is completely
embedded in the endometrial stroma, and the surface epithelium almost
entirely covers the original defect in the uterine wall.
• The blastocyst now produces a slight protrusion into the lumen of the
uterus.
• The trophoblast is characterized by lacunar spaces in the syncytium that
form an intercommunicating network.
• This network is particularly evident at the embryonic pole; at the
abembryonic pole, the trophoblast still consists mainly of cytotrophoblastic
cells.
• Concurrently, cells of the syncytiotrophoblast penetrate deeper into the
stroma and erode the endothelial lining of the maternal capillaries.
• These capillaries, which are congested and dilated, are known as sinusoids.

14. Figure: Human blastocyst of approximately 12 days. The trophoblastic lacunae at the embryonic pole are in open
connection with maternal sinusoids in the endometrial stroma. Extraembryonic mesoderm proliferates and fi lls the
space

15. • The syncytial lacunae become continuous with the sinusoids, and
maternal blood enters the lacunar system.
• As the trophoblast continues to erode more and more sinusoids,
maternal blood begins to flow through the trophoblastic system,
establishing the uteroplacental circulation.
• In the meantime, a new population of cells appears between the
inner surface of the cytotrophoblast and the outer surface of the
exocoelomic cavity.
• These cells, derived from yolk sac cells, form a fine, loose connective
tissue, the extraembryonic mesoderm, which eventually fills all of
the space between the trophoblast externally and the amnion and
exocoelomic membrane internally.
• Soon, large cavities develop in the extraembryonic mesoderm, and
when these become confluent, they form a new space known as the
extraembryonic cavity, or chorionic cavity.

16. • This space surrounds the primitive yolk sac and amniotic cavity, except where
the germ disc is connected to the trophoblast by the connecting stalk.
• The extraembryonic mesoderm lining the cytotrophoblast and amnion is
called the extraembryonic somatic mesoderm; the lining covering the yolk
sac is known as the extraembryonic splanchnic mesoderm.
• Growth of the bilaminar disc is relatively slow compared with that of the
trophoblast; consequently, the disc remains very small (0.1 to 0.2 mm).
• Cells of the endometrium, meanwhile, become polyhedral and loaded with
glycogen and lipids; intercellular spaces are filled with extravasate, and the
tissue is edematous.
• These changes, known as the decidua reaction, at first are confined to the
area immediately surrounding the implantation site but soon occur
throughout the endometrium.

17. DAY 13
• By the 13th day of development, the surface defect in the
endometrium has usually healed.
• Occasionally, however, bleeding occurs at the implantation site as a
result of increased blood flow into the lacunar spaces.
• Because this bleeding occurs near the 28th day of the menstrual
cycle , it may be confused with normal bleeding and therefore, may
cause inaccuracy in demining the expected date of delivery.
• The trophoblast is characterized by the villous structure. Cells of
cytotrophoblast proliferate locally and penetrate into the
syncytiotropoblast, forming cellular columns surrounded by
syncytium.
• cellular columns with Syncytial coverings are known as Primary Villi

18. • In the meantime the hypoblasts produces additional cells
that migrate along the inside of the exocoelomic
membrane.
• These cells proliferate and gradually form a new cavity is
known as the secondary yolk sac or definitive yolk sac.
• This yolk sac is much smaller than the original exocelomic
cavity or primitive yolk sac.
During formation, large portions of exocelomic cavity are
pinched off.
These portions are represented by exocelomic cysts,
which are often found in the extraembryonic coelom or
chorionic cavity.

19. Figure: A 13-day human blastocyst. Trophoblastic lacunae are present at the embryonic as well as
the abembryonic
pole, and the uteroplacental circulation has begun. Note the primary villi and the extraembryonic
coelom or chorionic
cavity. The secondary yolk sac is entirely lined with endoderm.

20. • Meanwhile the extraembryonic coelom expands and forms a large cavity the
Chorionic cavity.
• The extraembryonic mesoderm lining the inside of the cytotrophoblast is then
known as the Chorionic plate.
• The only plate where extraembryonic mesoderm traverse the chorionic cavity
is in the connecting stalk.
• With development of blood vessels the stalk becomes the umbilical cord

21. • The 14-day embryo:
• The embryo still has the form of a flat bilaminar embryonic
disc, but the hypoblastic cells in a localized area are now
columnar and form a thickened circular area-the prechordal
plate, which indicates the future site of the mouth and an
important organizer of the head region.

22. Implantation
• Definition: the attachment of Blastocyst to the uterine endometrium is
known as implantation.
• After implantation of blastocyst the functional layer of endometrium is
called “Decidua”.
• Time: starts at the end of 1st week (on the 6th or 7th day after
fertilization) and completed by the end of the second week .
• Normal site: At Junction of fundus & posterior wall of the body of
uterus.
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24. Summary of implantation
• The zona pellucida degenerates (day 5). Its disappearance results from enlargement of the
blastocyst and degeneration caused by enzymatic lysis. The lytic enzymes are released from
the acrosomes of the sperms that surround and partially penetrate the zona pellucida.
• The blastocyst adheres to the endometrial epithelium (day 6).
• The trophoblast differentiates into two layers: the syncytiotrophoblast and cytotrophoblast
(day 7).
• The syncytiotrophoblast erodes endometrial tissues, and the blastocyst starts to embed in the
endometrium (day 8). Blood-filled lacunae appear in the syncytiotrophoblast (day 9).
• The blastocyst sinks beneath the endometrial epithelium, and the defect is filled by a closing
plug (day 10). Lacunar networks form by fusion of adjacent lacunae (days 10 and 11).
• The syncytiotrophoblast erodes endometrial blood vessels, allowing maternal blood to seep
in and out of lacunar networks, thereby establishing a uteroplacental circulation (days 11 and
12).
• The defect in the endometrial epithelium is repaired (days 12 and 13).
• Primary chorionic villi develop (days 13 and 14).
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25. • Sometimes the blastocyst is embedded in abnormal sites which may be,
Intra-uterine
Or
Extra-uterine
Intra-uterine
• This type of abnormal implantation occurs when blastocyst is
attached to the lower part of uterine cavity overlapping the
internal os.
• This condition is known as placenta praevia.
• This produce severe haemorrhage in the later part of
gestation & during parturition.
Abnormal site of implantation

26. Extra-uterine:
Ectopic pregnancy may occur in the,
1) ovary
Ovarian pregnancy is rare & probably is a factor of ovarian teratomas.
2) abdominal cavity
Implantation of blastocyst in the abdominal cavity is rare & may take place in peritoneal
lining of the pouch of douglas.
3) uterine tube
Tubal pregnancy is followed by the rupture of uterine tube, which may produce alarming
haemorrhage endangering the life of the mother.
This sort of implantation results in eventual death of embryo.

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Figure: Implantation sites of blastocysts. The usual site in the posterior wall of the uterus is indicated by an X.
The approximate order of frequency of ectopic implantations is indicated alphabetically (A, most common, H, least
common). A to F, tubal pregnancies; G, abdominal pregnancy; H, ovarian pregnancy. Tubal pregnancies are the
most common type of ectopic pregnancy. Although appropriately included with uterine pregnancy sites, a cervical
pregnancy is often considered to be an ectopic pregnancy.

28. 1. Normal Site
2. Intestine
3. Mesentery
4. Ovary
5. Cervical
6. Internal Os
1
3
4
6
5
2

29. SUMMARY OF THE SECOND WEEK
• Rapid proliferation and differentiation of the trophoblast occurs as the blastocyst
completes its implantation in the endometrium.
• The endometrial changes resulting from the adaptation of these tissues in
preparation for implantation are known as the decidual reaction. Concurrently, the
primary umbilical vesicle (yolk sac) forms and extraembryonic mesoderm develops.
The extraembryonic coelom forms from spaces that develop in the extraembryonic
mesoderm. The coelom later becomes the chorionic cavity.
• The primary umbilical vesicle becomes smaller and gradually disappears as the
secondary umbilical vesicle develops. The amniotic cavity appears as a space
between the cytotrophoblast and the embryoblast.
• The embryoblast differentiates into a bilaminar embryonic disc consisting of
epiblast, related to the amniotic cavity, and hypoblast, adjacent to the blastocyst
cavity.
• The prechordal plate develops as a localized thickening of the hypoblast, which
indicates the future cranial region of the embryo and the future site of the mouth;
the prechordal plate is also an important organizer of the head region.