Evidence for partial epithelial-to-mesenchymal transition (pEMT) and recruitment of motile blastoderm edge cells during avian epiboly.

Embryonic epiboly has become an important developmental model for studying the mechanisms underlying collective movements of epithelial cells. In the last couple of decades, most studies of epiboly have utilized Xenopus or zebrafish as genetically tractable model organisms, while the avian epiboly model has received virtually no attention. Here, we re-visit epiboly in quail embryos and characterize several molecular markers of epithelial-to-mesenchymal transition (EMT) in the inner zone of the extraembryonic Area Opaca and at the blastoderm edge. Our results show that the intermediate filament vimentin, a widely-used marker for the mesenchymal phenotype, is strongly expressed in the edge cells compared to the cells in the inner zone. Laminin, an extracellular matrix protein that is a major structural and adhesive component of the epiblast basement membrane and the inner zone of the Area Opaca, is notably absent from the blastoderm edge. While these expression profiles are consistent with a mesenchymal phenotype, several other epithelial markers, including cytokeratin, β-catenin, and E-cadherin, are present in the blastoderm edge cells. Moreover, the results of a BrDU proliferation assay strongly suggest that expansion of the edge cell population is primarily due to recruitment of cells from the inner zone, as opposed to proliferation. Taken together, our data show that the edge cells of the avian blastoderm have characteristics of both epithelial and mesenchymal cells, and that the avian epiboly model, which has been dormant for so many years, may yet again prove to be helpful as a unique developmental model for studying partial EMT in the context of collective epithelial cell migration.