Phenotype of smooth muscle cells in internal mammary arteries.

The volume fraction of synthetic organelles in smooth muscle cells (SMCs) from the intima and media of human internal mammary artery (IMA) was examined by ultrastructural morphometry. The distal ends of 8 IMAs were collected peroperatively from coronary bypass patients aged 43 to 67 years. The thickness of the intimas ranged from 3 to 49 microns. Even the thickest parts had only a few cell layers, the majority of which were SMCs. In most patients, the cytoplasmic volume fraction of synthetic organelles was significantly lower in the media than in the intima (0.11 and 0.21 respectively). The high degree of differentiation of the SMCs agrees with the absence of atherosclerosis in the IMAs.