Three-dimensional cytoarchitecture of the media of arteriovenous anastomoses in the rabbit ear.

Arteriovenous anastomoses in the rabbit ear were examined with scanning electron microscopy to elucidate the structural differentiation of the media of the shunt. Arterial, intermediate, and venous segments in the shunt and two layers of the media in the intermediate segment were differentiated based on cell shape and cell organization. In the arterial segment, smooth muscle cells were spindle-shaped, either elongated or short, with a few branches, and were arranged circularly or diagonally with respect to the vessel's long axis. There were also stellate muscle cells with radiating processes. In the intermediate segment, the smooth muscle cells of the outer layer of the media were also arranged circularly and resembled the elongated cells in the arterial segments, but they were more irregular in shape and had more processes than those of the arterial segment. The epithelioid cells of the inner layer of the media were oval or polygonal and oriented irregularly with respect to the vessel's long axis, clustering to form longitudinal plicae. The smooth muscle cells of the venous segment were flat with many lateral processes and formed a thin, discontinuous layer. The smooth muscle cells in the arterial segment and those of the outer layer of the intermediate segment exhibited a highly rugged surface texture, indicating their strong contractility; the epithelioid cells and the smooth muscle cells in the venous segment exhibited a generally smooth surface, indicating less contractility. The intermediate segments were supplied with a dense nerve plexus. The intermediate segments, therefore, may be actively involved in the regulation of blood flow under neuronal influence.