Ultrastructure of the hypophyseal portal vessel in mature rats--SEM and TEM observations.

After chemical digestion of the isolated specimen of the hypophysis of the rat, external surface of the hypophyseal portal vessel in the mature rat was investigated with scanning electron microscope (SEM). The portal vessel lacked smooth muscle fibers, it was accompanied by many pericytes with highly ramifying processes along its whole course. Transmission electron microscopic (TEM) study revealed that the endothelium of this vessel has many fenestrations and channels. These findings suggest that the hypophyseal portal vessel in the mature rat is a capillary sinus with pericytes rather than a vein as suggested previously.