Characterization of the phenotype of smooth muscle cells in human fetal aorta on the basis of ultrastructure, immunofluorescence, and the composition of cytoskeletal and cytocontractile proteins.

The phenotype of smooth muscle cells (SMCs) in the aortic media of 7 human fetuses (14-20 weeks of gestation) was examined with transmission electron microscopy, immunofluorescence microscopy, and gel electrophoresis of the cytoskeletal and cytocontractile proteins. Ultrastructurally, virtually all medial cells were identified as SMCs having a poorly differentiated phenotype with a cytoplasm rich in rough endoplasmic reticulum and organelles, and with only a few myofilaments. All medial cells stained intensely with antibodies to vimentin, but only in a 20-week-old fetus could we find a few SMCs staining with antibodies to desmin. Nor was desmin detectable with SDS gel electrophoresis followed by immunoblotting, while clear bands corresponding to vimentin, myosin, and actin were present. In isoelectric focusing and two-dimensional gel electrophoresis beta-actin was the most prominent of the 3 actin isoforms in all cases. The present results show that SMCs in the media of fetal human aorta have a poorly differentiated phenotype, which morphologically and biochemically resembles that previously described in the aorta of fetal and newborn rat, in the arterial intima after endothelial injury, in atherosclerotic lesions, and after spontaneous modulation of medial SMCs in culture.