The role of smooth muscle cells and hematogenous macrophages in atheroma

Two types of lipid-containing cells were found in diet-induced atheromata of rabbits. The cells of the first type contained little acid esterase activity, were situated mainly in the deep and middle layers of the intima and constituted the overwhelming majority of foam cells. The lipid contained in them was bulky, indicating inefficient lipid emulsification and metabolism. The cells of the second type were rich in acid esterase activity and were situated in the superficial layer of the intima, often in subendothelial rows. These cells contained lipid which was finely emulsified and present in much smaller amounts than the lipid present in the first type of cell. The localisation of the two types of cells, the observation that only the second type of cell exhibited marked AS-D acetate esterase activity and the study of Adams and Bayliss (1976) indicate that, while the cells of the first type are myogenous, the cells of the second type are hematogenous macrophages. It is proposed that relative insufficiency of acid esterase activity in the myocytes is an important factor in the formation of atheroma. The intense acid esterase activity in the macrophages may play a major role in the healing of atheroma and its transformation into a fibrous plaque.