Special structural features of the rat portal vein

Unlike other rat veins of comparable diameter, the smooth muscle cells of the extrahepatic segment of the portal vein are arranged in two distinct mutually perpendicular layers. The relatively narrow inner layer is nearly circumferential and devoid of vasa vasorum; the cells resemble those seen in other veins. In the longitudinal outer layer, vasa are abundant and the cells contain unusual numbers of mitochondria and abundant pinocytotic vesicles, indicating relatively increased metabolic activity. Between the layers, the connective tissue contains nerve fibers, suggesting coordinated reflex activity.