Gastrulation converts the bilaminar (epiblast, hypoblast) embryo into the trilaminar (ectoderm, mesoderm, endoderm) embryo.
This page introduces the trilaminar embryo and the fixing of cell fates to form specific tissues within the embryo.
During week 3 within the trilaminar embryo, early steps have now begun in: axial process formation (notochord), mesoderm differentiation (somites, heart, vascular) ectoderm differentiation (neuralation).
The induction of endoderm appears to be patterned by FGF4 signals from adjacent ectoderm and mesoderm within the primitive streak. (Wells & Melton, 1999).
Cells migrate through the primitive streak to form mesodermal layer. Extraembryonic mesoderm lies adjacent to the trilaminar embryo totally enclosing the amnion, yolk sac and forming the connecting stalk.
Paraxial mesoderm accumulates under the neural plate with thinner mesoderm laterally. This forms 2 thickened streaks running the length of the embryonic disc along the rostrocaudal axis. In humans, during the 3rd week, this mesoderm begins to segment. The neural plate folds to form a neural groove and folds.
Segmentation of the paraxial mesoderm into somites continues caudally at 1 somite/90minutes and a cavity (intraembryonic coelom) forms in the lateral plate mesoderm separating somatic and splanchnic mesoderm.
Note intraembryonic coelomic cavity communicates with extraembryonic coelom through portals (holes) initially on lateral margin of embryonic disc.
Somites continue to form. The neural groove fuses dorsally to form a tube at the level of the 4th somite and "zips up cranially and caudally and the neural crest migrates into the mesoderm.
Wells JM, Melton DA. Vertebrate endoderm development. Annu Rev Cell Dev Biol. 1999;15:393-410.