Progressive involution and physiological death of smooth muscle cells of rat incisal arterioles

The sequential morphology of vascular smooth muscle cell involution and physiological cell death was studied by electron microscopy on rat incisors. Concurrently with changes in the pulp of the continuously growing incisor, its arterioles pass through a cycle of growth, remodeling, regression and decay. During the cycle, smooth muscle cells of the arterioles were observed to involute by segregation of cytoplasmic components, formation of autophagic and digestive vacuoles and shedding of cell fragments. Phagocytic vacuoles were found in neighbouring muscle cells and in adventitious macrophages. Concomitant with the involution in some cells, physiological death was observed among other smooth muscle cells. This appears as an apoptosis with sequential transformations of polyribosomes into monoribosomes, nuclear and cytoplasmic condensation, and finally fragmentation followed by phagocytosis mainly by adventitious macrophages. Knowledge of the features of physiological involution and cell death of the vascular smooth muscle cells may be of importance not only in studies of functional remodeling and adaption, but also in the interpretation of vascular lesions.