Fine structure of the vessels of the hypophysial portal system of the White-crowned Sparrow,Zonotrichia leucophrys gambelii

Summary The angioarchitecture of the hypophysial portal system of the White-crowned Sparrow, Zonotrichia leucophrys gambelii, was investigated by electron microscopy in conjunction with light microscopy of serial thick sections.The small arteries or arterioles supplying the primary capillary plexus of the median eminence have the typical form of arterioles.The vessels of the primary capillary plexus, on the surface of the median eminence, with their many fenestrations and pinocytotic vesicles, are typical of the form of capillary usually found in other endocrine organs.The portal vessels in the pars tuberalis have wide perivascular spaces between the basement membrane of the endothelium and that of parenchymal lobules of the pars tuberalis. These perivascular spaces are occupied usually by the perivascular cells, but sometimes contain erythrocytes.The endothelial cells of the portal vessels often protrude into vascular lumen giving the appearance of valve-like structures. These may have a role in the regulation of blood flow.The endothelial cells of the portal vessels are invested by a definitive basement membrane and by the cytoplasm of pericytes which are oriented spirally to the longitudinal axes of the vessels. The pericytes may have a function in the mechanical support of the vascular wall and a contractile function that might regulate the flow rate of blood.The investigation reported herein was supported by a scientific research grant (No. 291049) from the Ministry of Education of Japan to Prof. Mikami; by a grant from the Deutsche Forschungsgemeinschaft to Prof. Oksche; by a grant (5 ROI-NB 06817) from the National Institutes of Health to Prof. Farner, and by a research grant (5 ROI-HE 07240 NEUA) from the National Institutes of Health to Prof. Vitums.