Genesis of the germ layers
From the 17th day (stage 6) the primitive streak forms an entry location for the epiblast cells to invade the underlying mesoblast and to proliferate there.
Histological methods have shown that during their migration along the primitive groove, the epiblast cells form pseudopodia. They lose contact with each other thereby. This phenomenon of the inflow of cells to form the third embryonic layer is termed epithelio-mesenchymal transition (stage 6-7) (gastrulation in the lower vertebrates) .
- 1
- Primitive groove
- 2
- Primitive pit
- 3
- Primitive node
- 4
- Oropharyngeal membrane
- 5
- Cardiogenic plate
- 6
- Sectional edge of amniotic membrane
- 7
- Mesoderm
- 8
- Endoderm
- 9
- Future cloacal membrane
- NB
- 1+2+3 primitive streak
Oval-shaped embryonic disk (dorsal view).
The red arrows show schematically the migration directions of the epiblast cells to their points of final destination
Depending on their origin and the moment of their flowing in, the epiblast cells migrate away from the primitive streak in various directions.
The first cells that enter through the node and the primitive groove replace the hypoblast layer and form the definitive endoblast (origin of the future intestine and its derivates).
At the same time, due to the migration of cells through the primitive node in the cranial direction, two further structures are formed:
- the prechordal plate (stage 6), which is located cranial to the primitive node
- the notochordal process (stage 8) the development of which will be treated in the next section
level of the primitive groove
- Primitive groove
- Epiblast
- Extraembryonic mesoblast
- Definitive endoblast
- Invading epiblastic cells forming
the intraembryonic mesoblast - Hypoblast
Transverse section at the level of the primitive groove with the immigration of epiblast cells, which form the future mesoblast, as well as the endoblast, which replaces the hypoblast.
The largest proportion of these immigrated cells form a third germinal layer, the intraembryonic mesoblast (stage 6). The mesoblast cells wander in all directions: laterally, cranially and caudally. This middle germinal layer lies between the definitive endoblast and epiblast. Exceptions are the cloacal membrane as well as the pharyngeal membrane, where the ectoderm and endoderm lie directly opposite each other (stage 6).
Cranial to the prechordal plate, mesenchyma cells of the embryonic disk will form the pericardium as well as the septum transversum (stage 9) . At the caudal extremity the cloacal membrane forms the primordium of the future opening of the uro-genital tract and the rectum.
- Ectoblast/-derm
- Mesoblast/-derm
- Definitive endoblast/-derm
The cellular adhesion molecules (or CAM)
Starting with gastrulation the epiblast cells secrete hyaluronic acid, which gets deposited in the intercellular spaces between the epiblast and the hypoblast. This molecule is able to bind a large amount of water to itself (up to 1000 times its own weight), is often associated with cell migration, and plays an anti-aggregational role for the mesoblast cells. The presence of hyaluronic acid, however, does not suffice to explain the migration of the epiblast cells away from the primitive streak.
In vertebrate embryos it is assumed that the migration also depends on the presence of fibronectins that are found on the basal lamina under the epiblast. Fibronectin is an extracellular glycoprotein.
The integrins
The integrins belong to the 4 families of the Cell Adhesion Molecules (CAM): integrin, cadherin, selectin, and the large family of the immunoglobulins. They are responsible for the recognition and binding of two cells with each other or a cell and components of the extracellular matrix. The kind of the connection can be homophil (two identical molecules) or heterophil (two differing molecules bound to each other). Being transmembranal glycoproteins, integrins insure the cohesion (aggregation) and influence the migration of cells. They are also responsible for the organization of the cells in the tissue and the tissue in the organs. The integrins arrange contacts with the collagen filaments in the basal membrane and the extracellular matrix. Laminin is a ligand of the basal membrane. Fibronectin, on the other hand, is a ligand of the intercellular substance of e.g. the mesoblast and plays a role in cell migration.