summary of Trilaminar Germ disc (embryology) for health science students ... By Abdoulwahab Mahdi Ali (Student)

1. CHAPTER 4: TRILAMINAR GERM DISC
(PRESENTATION)
GROUP 4
1- ABDIWAHAB MAHDI ALI
2- FARHAN AHMED MOHAMEDD
3- ALI XUSEIN AHMED
4- ABDIKADIR ALI MUUSE
5- ABDILAHI BURHAN AHMED
6- MOHAMOUD ABDIRAHMAN XIRSI
7- ZAKARIE BASHIR AHMED
8: ABDIQANI MAXAMED SALEEBAN
Course Lecturer: Dr Rabile

2. OVERVIEW OF PREVIOUS LESSONS
• Zygote multiplies and form a morula which contains inner cell mass in the
center.
• The morula changes into blastula or blastocyst, during the same week and
the inner cell mass has shifted to one-side, and is now called Embryoblast.

3. • During second week of development the cells of embryoblast differentiate into
two layers, the epiblast which forms the floor of the amniotic cavity and
hypoblast which forms the roof of yolk sac.
• The cells of each of the germ layers form a flat circular disc and together they
are known as a bilaminar germ disc.

4. A) GASTRULATION
• Is the process by which the Embryo acquires three germ layers (ectoderm,
mesoderm, and endoderm).
• During gastrulation, the bilaminar embryonic disc is converted into a trilaminar
embryonic disc.
• Cells of the epiblast migrate toward the primitive streak. Upon arrival in the region
of the streak, they become flask-shaped, detach from the epiblast, and slip
beneath it. This inward movement is known as invagination

5. • Once the cells have invaginated, some displace the hypoblast, creating the embryonic
endoderm.
• others come to lie between the epiblast and newly created endoderm to form
mesoderm.
• Cells remaining in the epiblast then form ectoderm.
• Thus, the epiblast, through the process of gastrulation, is the source of all of the germ
layers.
• The cells in these layers will give rise to all of the tissues and organs in the embryo.

6. Each of the three germ layers gives rise to specific tissues and organs:
- Embryonic ectoderm:
Gives rise to the epidermis, central and peripheral nervous systems, the eyes, and internal
ears…
- Embryonic endoderm:
Is the source of the epithelial linings of the respiratory and alimentary (digestive) tracts,
including the glands opening into the gastrointestinal tract and the glandular cells of
associated organs such as the liver and pancreas.
- Embryonic mesoderm:
Gives rise to all skeletal muscles, blood cells and the lining of blood vessels, all visceral
smooth muscular coats, the serosal linings of all body cavities, the ducts and organs of
the reproductive and excretory systems, and most of the cardiovascular system.

7. PRIMITIVE STREAK
The first morphologic sign of gastrulation is
formation of the primitive streak on the
surface of the epiblast of the embryonic
disc.
It results from the proliferation and movement
of cells of the epiblast to the median plane
of the embryonic disc.

8. As the streak elongates by addition of cells to its caudal end, its cranial end
proliferates to form a primitive node.

9. At the same time, a narrow groove, the primitive
groove, develops in the primitive streak that is
continuous with a small depression in the
primitive node, the primitive pit.

10. • At the same time, due to the migration of cells through the primitive node
in the cranial direction two further structures are formed:
1- prechordal plate: is a thickening in the endodermal layer which
is formed by mesendodermal cells from the primitive node that migrate
cephalically along the midline between the ectoderm and endoderm layers
2- notochordal process…

11. FORMATION OF NOTOCHORD
• The cells detaching and falling from primitive pit
move straight forward and form a cord-like
process, known as notochordal process (day-17).
• Remember that notochordal process contains a
canal continuous with primitive node.
• The notochordal process extends cephalically from
the primitive node toward prechordal plate.

12. • Later on the floor of the notochordal process
fuses with the underlying endoderm (day 18).
• A small passage appears, the neuroenteric
canal temporarily communicates the amniotic
cavity with the yolk sac.
• The notochordal process forms a dome-
shaped plate grooved from below, known as
the notochordal plate.
• Then the cells forming notochordal plate
proliferate more and more till they form,
rounded dense disc called definitive
notochord

13. ALLANTOISES:
• The allantois, also called allanto-enteric diverticulum, appears on about day
16, After cloacal membrane appears.
• It is an endodermal outgrowth from the posterior wall of the secondary yolk
sac and grows into the mesoderm of the connecting stalk.
• In humans it remains rudimentary but may be involved in abnormalities of
bladder development.

14. • buccopharyngeal
membrane: membrane in an early
embryo composed of ectoderm and
endoderm and come into direct contact
with each other. (future mouth)
• Cloacal membrane: is formed at the
caudal end of the embiyonic disc.
• This membrane, which is similar in
structure to the oropharyngeal
membrane, consists of tightly adherent
ectoderm and endoderm cells with no
intervening mesoderm. (Future Anus)

15. B) FURTHER DEVELOPMENT OF TROPHOBLAST
• By the beginning of the third week, the trophoblast is characterized by
primary villi that consist of a cytotrophoblastic core covered by a syncytial
layer.

16. • After 3rd week mesodermal cells penetrate the core of primary villi forming
secondary villi.
• At the end of 3rd week mesodermal cells in the core of villi differentiate
into blood cells forming tertiary villus.