Origin and Role of Collagen in the Embryo

The first collagen recognizable in the embryo is in the formof an incomplete basal lamina under the epiblast and hypoblast.We suggest that this collagen acts as a railroad track to guidethe migration of the primitive streak mesenchyme. The mesenchymeaggregates into chordamesoderm, a layer which is said to "induce"the overlying epiblast (now ectoderm) to develop into neuralfolds. This tissue interaction may be mediated by the formationof complete basal laminas separating the two tissues and bydeposition of sulfated mucopolysaccharides in the interveningextracellular space. At the very least, the collagenous basallamina serves to give the elongating cells of the developingneural tube a firm foothold. The fully formed neural tube andadjacent notochord are said to induce the sclerotome of thesomite to migrate medially and differentiate into cartilage.Notochord and neural tube basal lamina and collagen fibrilsmay play a role by guiding the migrating cells and stabilizingthe already existing chondrogenic bias of the cells. We wereunable to prove this hypothesis directly (by adding collagento somite cultures), because in our hands the somites died invitro even in the presence of neural tube and notochord. Wedid obtain direct evidence, however, that the basal lamina ofthe lens can promote the differentiation of the cornea in vitro.