the process by which a bilaminar germ disc is formed within the second week of development. second week is a week of two's. development and clinical implications or correlates. the formation of the 2 fluid cavities

1. BILAMINAR DISC
(SECOND WEEK)
Presentation by
Mudoogo Edgar

2. Objectives
• Definition
• Development
• Clinical correlates
Mudoogo Edgar

3. Defination of
bilaminar
disc
• This refers to the
epiblast and hypoblast
evolved from the
embryoblast.
• These two layers are
sandwiched between
two cavities(balloons)
i.e. primitive York sac
and amniotic cavity.
Mudoogo Edgar

4. Development
 Zygote Morula Blastocyst
Trophoblast
 Embryoblast (Formative mass- 60 cells)
Embryoblast transforms into 2 distinct epithelial
layers just before implantation occurs;
 epiblast – outer layer , has columnar cells
Hypobalst- inner layer, has cuboidal cellsMudoogo Edgar

5. The second week of
development is
significant for:
1. The formation of
the bilaminar
disc (two-layers)
– this will give
rise to all the
tissues and
organs of the
body
2. The completion
of implantation
Mudoogo Edgar

6. Syncytiotrophoblast
cells displace the
endometrium
around the
implantation site
The endometrium
has undergone
changes.
Cells have become
filled with
glycogen and
lipids
The nutrients spill
into the
connective tissue
This is called the
decidua reaction
Mudoogo Edgar

7. The syncytiotrophoblast is responsible for hormone
production. hCG maintains the corpus luteum in the
ovary, allowing it to continue to produce P+E.
Produces P+ E to
maintain the pregnancy
Ovary
Uterus
hCG
Mudoogo Edgar

8. The cells of the
embryoblast will also
differentiate into 2
layers:
1. The epiblast- a layer
of high, columnar
cells adjacent to the
amniotic cavity.
2. The hypoblast- A
layer of small
cuboidal cells adjacent
to the blastocyst
cavity.
Together these layers
form a flat disc.
Mudoogo Edgar

9. Amnioblasts (derived
from the epiblast)
separate and form
the lining of the
amniotic cavity.
Cells from the
hypoblast form a
membrane that lines
the inner surface of
the cytotrophoblast.
This forms the
exocoelomic cavity
or primitive yolk sac
Mudoogo Edgar

10.  The cavities allow
movement of the
disc
 The primordial
uteroplacental
circulation is
established
 The yolk sac
contains no yolk-
the embryo is
nourished from
the lacunar
networks- but
may have a role in
selective transfer
of nutrients
Mudoogo Edgar

11.  New cells appear
between the yolk
sac and the
cytotrophoblast
 They form a layer
of loose
connective tissue:
extraembryonic
mesoderm.
 Cavities or spaces
appear in the
extra-embryonic
mesoderm
Mudoogo Edgar

12.  The cavities form a
new space- the
chorionic cavity
 The primitive yolk sac
is pinched off- a
secondary or
definitive yolk sac is
formed
 The cavity divides the
extraembryonic
mesoderm into the
1. Extraembryonic
somatic mesoderm-
lining trophoblast and
amnion
2. Extraembryonic
splanchnic
mesoderm- lines the
yolk sac
Mudoogo Edgar

13. Mudoogo Edgar

14. The chorion is formed
by
1. Extraembryonic
somatic mesoderm
2. Cytotrophoblast
3. Syncytiotropho-
blast
The chorion forms the
wall of the
chorionic cavity-
the amniotic cavity
and yolk sac are
suspended in the
chorionic cavity by
the connecting
stalk.
1
2
3
Mudoogo Edgar

15. By day 14 the
embryo has the
form of a flat,
bilaminar disc,
ovoid in shape.
In a localized area
of the hypoblast,
the cells become
more columnar
and form a
thickened circle
area, the
prechordal plate.
Mudoogo Edgar

16. At the end of the
second week:
Trophoblast has
had a period of
growth- greater
than the
embryoblast.
The 2 layer
bilaminar disc
is formed and
will give rise to
other tissues
and structures.
Mudoogo Edgar

17. Clinical correlates
 Human chorionic gonadotrophin (HCG) hormone
produced by syncytiotrophoblast can be detected
by medical test.
 Hydatidiform mole – little or no embryonic tissue,
secrete high levels of HCG producing beneign or
malignant tumor (choriocarcinoma). Trophoblast is
well developed.
Mudoogo Edgar

18. Clinical correlates ct’d
 Implantation sites usually posterior or anterior wall
of body of uterus
 Abnormal implantation sites – ectopic pregnancy
o abdominal cavity- pouch of Douglas
o mesentery
o ampullary region of tube
Mudoogo Edgar

19. Clinical correlates ct’d
o tubal implantation, internal os, ovarian leading to
primary ovarian pregnancy
o interstitial implantation of narrow width of uterine
tube
 Bleeding on the day due to increased blood with in
lacunae spaces.
Mudoogo Edgar

20. 2nd week of development: The week of twos
WHY?
 Trophoblast ↗cytotrophoblast
↘syncytiotrophoblast
 Embryoblast ↗hypoblast
↘epiblast
 Extraembronic mesoderm ↗somatopleure
↘splanchnopleure
 Cavities ↗amniotic
↘primitive yolk sac
Mudoogo Edgar

21. Mwebare munonga
“JUST BECAUSE YOU KNOW THE STUFF DOESN’T MEAN
YOU ARE SMART……….. YOU HAVE TO KNOW HOW TO
USE THAT INFORMATION”
-JOSH KELLER-
Mudoogo Edgar