Trilaminar germ disc (week 3 embryology)

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 A nus)

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.