[Formation of the external zone of the median eminence of rats in the perinatal period].

The formation of interrelations of the axons of neurosecretory cells and of ependyme cells with the capillaries of primary portal plexus in rats from the 14th day of embryogenesis till the 9th day of postnatal life was studied using the light and electron microscope methods. During the whole period under study, the basal processes of the ependyme cells reach the primary portal plexus of the capillaries. The terminals of the basal processes are usually separated from the endothelium of the capillaries by two basal membranes and enclosed pericapillary space. After the birth, some basal process penetrate in the pericapillary space and terminate on the endothelium. The surface of contact of the ependyme cell processes with the external basal membrane increases with the age, this being accompanied by the increase of pinocytotic activity. The neurosecretory axons are found in the median eminence already on the 14th day of embryogenesis, but by the 20th day only they reach the external basal membrane and penetrate sometimes in the pericapillary space. After the birth, the number of axons reaching the external basal membrane and the surface of contact between them increase gradually with, apparently, a concomitant intensification of the transport of neurohormones in the portal circulatory system of the hypophysial-hypothalamic complex.