Ultrastructure of the capillary pericytes and the expression of smooth muscle alpha-actin and desmin in the snake infrared sensory organs.

The infrared sensory membranes of pit organs of pit vipers have an extremely rich capillary vasculature that forms many vascular loops, each serving a small number of infrared nerve terminals. We clarified the ultrastructure of capillary pericytes in the pit membranes by scanning and transmission electron microscopy, and examined the immunoreactivity in their cytoplasm to two contractile proteins: smooth muscle alpha-actin (SM alpha-actin) and desmin. The capillary pericytes had two major cytoplasmic processes: thickened primary processes that radiate to embrace the endothelial tube and flattened secondary processes that are distributed widely on the endothelium. Coexpression of SM alpha-actin and desmin was observed in the pericytes of entire capillary segments, and SM alpha-actin was characterized by prominent filament bundles directed mainly at right angles to the capillary long axis. This expression pattern was different from that of capillary pericytes of the scales, where SM alpha-actin was expressed diffusely in the cytoplasm. In a series of electron microscopic sections, we often observed the pericyte processes depressing the endothelial wall. We also observed a close relationship of the pericytes with inter-endothelial cell junctions, and pericyte processes connected with the endothelial cells via gap junctions. From these findings, we surmised that capillary pericytes in the pit membrane have a close functional relationship with the endothelium, and through their contractile and relaxing activity regulate capillary bloodflow to stabilize production of infrared nerve impulses.