Pericyte-endothelial relationships in cardiac and skeletal muscle capillaries

In order to elucidate the structure and function of pericytes, we have utilized electron microscopy and morphometric techniques to examine pericyte ultrastructure and to characterize their relationship to endothelial cells in rat hearts and rat and human skeletal muscle. Ultrastructural features of pericytes (in all three tissues) and their distribution along capillaries (84 and 95% of sectioned vessels from rat cardiac and skeletal muscle, respectively) were similar. On the other hand, pericyte secondary processes in heart and skeletal muscle differed markedly; those in skeletal muscle were larger and encircled capillaries to a greater extent than in rat hearts (20 vs 10% of vessel circumferences, respectively) and their interactions with endothelium were more intimate and complex. In both rat and human skeletal muscle, pericyte secondary processes penetrate deeply into endothelial cells and interdigitate with them extensively; likewise, endothelial evaginations protrude deeply into pericytes. These phenomena are rarely observed in rat cardiac muscle. Both myelinated and unmyelinated axons are closely associated with cardiac capillaries and pericytes, but are virtually absent from skeletal muscle microvasculatures. These differences documented in pericyte-endothelial topological relationships in heart and skeletal muscle support the concept that pericyte function may vary in different tissues.