Light- and electron-microscopic characteristics of artrial smooth muscle cell cultures subjected to hypoxia or carbon monoxide.

Smooth muscle cells from the tunica media of piglet aortae grown under hypoxic conditions undergo the following changes: First, they become modified by partial loss of myofilaments and proliferation of organelles, which are characteristics of young primitive cells. Second, an increase in number of pinocytotic vesicles at and below the cell surface, indicating increased extracellular uptake of material, can be detected. This is followed by accumulation of Oil Red O positive intracytoplasmic granules and vacuoles as well as the subsequent formation of mount-like protrusions. The latter consist of a core of extracellular material and necrotic debris covered with a cap of viable cells. A third feature of the cells subjected to hypoxia is a conspicuous rise in the number of lysosomes. This is considered to be a manifestation of a defense mechanism of the cells to remove undesirable material from cytoplasm. Cells exposed to an atmosphere rich in carbon monoxide exhibit basically the same alterations as those grown under hypoxic conditions; however, formation of mound-like aggregates is less prominent, while the rise in the number of lysosomes is more evident than in the hypoxic cells. The above alterations are similar to changes observed in smooth muscle cells of rabbit with experimental atherosclerosis. It is suggested that whereever the arterial smooth muscle cell is subjected to adverse conditions basically the same mechanism, consisting of dedifferentiation, increased permeability and lysosomal defense reaction, takes place.