Heterogeneity of smooth muscle cells in embryonic human aorta.

Cellular composition of aortas from 5- to 12-week and 18- to 28-week-old human embryos were investigated using immunocytochemistry, scanning and transmission electron microscopy. The aorta of the 5- to 12-week-old embryos consisted of three sublayers differing in cellular composition. The inner sublayer adjacent to the endothelium contained round and ovoid cells with synthetic phenotype. In the intermediate sublayer, spindle-like cells ultrastructurally similar to smooth muscle cells were found. Cells of the outer sublayer resembled fibroblasts or poorly differentiated mesenchymal cells. There were not definite morphological borders between sublayers. In the 18- to 28-week-old embryo aorta the intima was separated from media by internal elastic lamina. Intimal and innermost medial cells had predominately stellate shape and synthetic phenotype. The outer part of media contained spindle-like cells that had well developed contractile structures. Both the 5- to 12-week-old and the 18- to 28-week-old embryo aortic cells were positively stained for alpha-actin and myosin and negatively stained for macrophage antigens. Thus, the majority of embryo aortic cells appeared smooth muscle cells, however there was a regional difference in shape and synthetic state of these cells.