The process of formation of trilaminar germ disc from bilaminar germ disc is known as gastrulation. It occurs in 3rd week of the embryonic development.
- Gastrulation begins with the formation of primitive streak on the surface of the epiblast.
- 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. This inward movement is known as invagination.
- Some of these cells displace the hypoblast cells and become endoderm.
- Therefore the epi-blast cells through the process of grastrulation, become the source of all germ layers.
- Because of fusion of ectoderm and endoderm buccophryngeal membrane and cloacal membrane are formed.
- Another development of the third week is formation of primitive streak on 15th day by a thickening of the axial part of the epiblast.
- Appears along the middle of the disk for about one-half of its length. It is rounded at first but becomes linear along with the elongation of germ disc.
- At its anterior end (cephalic end) - a knob-like thickening - primitive node [Hensen’s node] appear whose centre has a pit “primitive pit”
- The primitive streak cells are active till the end of the fourth week.
- Normally it under go degenerative changes and become insignificant structure in sacrococcygeal region.
- Although it disappear completely but in some cases it may persist and may give rise to tumour –teratoma.
- As this type of tumour is derived from primitive streak cells they may give rise to variuous types of tissues giving typical example of pluripotent cells.
- These types of cells are found in testes and ovaries as well.
- In Amphioxus, one of the lower chordate, it is the only skeleton.
- In human it forms the midline axis for development of vertebral column.
- Therefore it defines the axis around which the embryo develops.
- The notochord is the cellular rod formed by the migrating epiblast cells that extend from primitive streak to prochordal plate.
- The primitive pit invaginates into it and forms a lumen. Therefore it becomes a tubular column of cells.
- By 18th Day, the floor of the notochord fuses with the underlying endoderm and they disappear. The remaining cells proliferate and forms a solid cellular rod. At this stage, a temporary canal the neuro-enteric canal connects the yolk sac with amniotic cavity.
Gastrulation Animation Video