Immunophenotyping and spatio-temporal distribution of aortic cell clusters in the bovine embryo.

In the present study the temporal and spatial appearance of aortic cell clusters in bovine embryos is described. Aorta-associated c-kit-positive cell clusters can be observed first in 23 days post inseminationem (dpi) bovine embryos and disappear after 34 dpi. For the first time, it was shown that the immunophenotype of these aortic cluster cells changes during embryonic development. Aortic cell clusters are c-kit+/CD45-/STA-, when they are first detected in the 23 dpi embryo, and acquire a c-kit+/CD45+/STA- phenotype in 27-29 embryos and a c-kit+/CD45+/STA+ immunophenotype in 32-34-day-old specimens. Cell clusters are most prominent in the vicinity of lateral and ventral aortic branches, but rare in omphalomesenteric arteries and absent in Aa. umbilicales. Free c-kit-positive cells in an intravasal position are common, suggesting separation from the clusters in order to colonize subsequent hematopoietic organs, i.e., the liver and the mesonephros. Transmission electron microscopic analysis reveals the existence of primitive desmosomes between the clusters cells and adjacent endothelial cells as well as a fine basal lamina as a demarcation between the cluster cells and underlying mesenchymal cells. Material resembling extracellular matrix is found in large vacuoles in cluster cells of 23 dpi embryos. Immunocytochemistry reveals an intense accumulation of heparan sulfate proteoglycan and collagen IV in the aortic wall at the sites where cell clusters are attached. These observations suggest that the hematopoietic cell clusters induce the formation of a specific microenvironment within the aortic wall.