Immunohistochemical evidence for a mesothelial contribution to the ventralwall of the avian aorta

We report morphological and immunohistochemical evidence for a translocatio n of cells from the coelomic mesothelium to the aortic wall between the dev elopmental stages HH16 and HH22 of the quail embryos. The coelomic mesothel ial cells closest to the aorta showed, at these stages, increased mitotic a ctivity, reduced intercellular adhesion, loss of tight junctions, and long basal cytoplasmic processes. Coinciding with these morphological traits, cy tokeratin immunoreactivity was found in the mesothelium, in cells of the ao rtic wall and throughout the ventral periaortic mesenchyme (but not in the lateral and dorsal aortic regions). Vimentin immunoreactivity colocalized w ith cytokeratin in the mesothelial cells adjacent to the aorta. In the vent ral aortic wall, cytokeratin colocalized with smooth muscle alpha-actin and with the 1E12 antigen (a smooth muscle-specific alpha-actinin isoform). We think that the morphological and immunolocalization data observed are comp atible with an epithelial-mesenchymal transition by which mesothelial-deriv ed cells contribute to the splanchnic mesoderm and aortic wall. The precise coincidence between the mesothelial contribution and the emergence of the aortic smooth muscle cells progenitors, as well as the immunolocalization d ata, suggest a potential relationship of the mesothelial-derived cells with this cell lineage. This may explain the observed ventrodorsal asymmetry in the distribution of smooth muscle cells progenitors in the aortic wall.