Origin and Role of Collagen in the Embryo

Abstract

The first collagen recognizable in the embryo is in the form of an incomplete basal lamina under the epiblast and hypoblast. We suggest that this collagen acts as a railroad track to guide the migration of the primitive streak mesenchyme. The mesenchyme aggregates into chordamesoderm, a layer which is said to “induce” the overlying epiblast (now ectoderm) to develop into neural folds. This tissue interaction may be mediated by the formation of complete basal laminas separating the two tissues and by deposition of sulfated mucopolysaccharides in the intervening extracellular space. At the very least, the collagenous basal lamina serves to give the elongating cells of the developing neural tube a firm foothold. The fully formed neural tube and adjacent notochord are said to induce the sclerotome of the somite to migrate medially and differentiate into cartilage. Notochord and neural tube basal lamina and collagen fibrils may play a role by guiding the migrating cells and stabilizing the already existing chondrogenic bias of the cells. We were unable to prove this hypothesis directly (by adding collagen to somite cultures), because in our hands the somites died in vitro even in the presence of neural tube and notochord. We did obtain direct evidence, however, that the basal lamina of the lens can promote the differentiation of the cornea in vitro.