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.
Primitive Streak:
  • 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.
Notochord:
  • 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

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