Mechanisms, mechanics and function of epithelial–mesenchymal transitions in early development
The cellular origin of the components of the basal lamina of an epithelium, the chicken epiblast, has been investigated in a model system resulting from the transplantation of metabolically labeled quail hypoblast (an epithelium without basal lamina), associated or not with individual mesoblast cells, into an unlabeled chicken blastoderm deprived of its own hypoblast. The ability to discriminate chicken from quail cells after nuclear staining, combined with autoradiographic labeling of the basal lamina components, made it possible to determine the origin of the cells and labeled compounds in tissue sections of the chimeras. The transplantation of 3H-glucosamine or 3H-fucose-labeled quail hypoblast into a chicken host embryo, and subsequent culture of the chimeric embryo for 5 h, led to the transfer of labeled macromolecules from the quail graft to the chicken basal lamina. Pre-treatment of sections with several glycosaminoglycan-degrading enzymes with different substrate specificities suggested that the 3H-glucosamine-labeled compounds that are deposited in the basal lamina are glycoproteins and/or heparan sulfate proteoglycan. However, in view of our current knowledge of the cellular origin of the latter compound, this molecule probably originates from the epithelium itself. Transfer of 3H-proline and 3H-hydroxyproline-containing molecules (mainly collagens) from the graft to the host basal lamina was not observed. Chasing the labeled compounds with unlabeled precursor during culture of the chimeras did not influence the final autoradiographic pattern. It is concluded that the basal lamina of the epiblast has a dual epithelial origin, resulting from the interaction of epiblast-derived materials and non-collagenous glycoproteins synthesized by the hypoblast. Evidence supporting the case of a non-epithelial, mesoblastic origin of non-collagenous glycoproteins was not found. An extensive review of literature on the epithelial vs non-epithelial origin of basement membrane components, mainly in mammalian species, is also provided.