[Development of the ultrastructural organization of the rat neurohypophysis during postnatal ontogenesis].

The development of structural components of the posterior hypopysis lobe of rats, the development of the neurosecretory activity of the hypothalamo-hypophyseal nervous system and the establishment of its function as a single system within the first three weeks after birth (1, 7, 14 and 21 days) as well as embryos of the 20th and 21st days of development were studied electron-microscopically. It has been established that the pituicytes of the posterior lobe of the hypophysis of newborn rats are represented by a single type of cells and are completely differentiated. On the 21st day they are in different morpho-functional states and consist of dark and light cells. By the 14th day of development a considerably increased amount of the neurosecretion is observed. By the 21st day the density of neurosecretion granules per a preparation unit does not change. The basal layer of the capillaries is formed not definitely by the birth time; on the 14th day the organization of the vessel wall is almost similar to that of mature animals. The Herring bodies make their appearance on the 21st day after birth. The hypothalamus and neurohypophysis of rats are functionally active before birth. After birth their neurosecretion activity is well pronounced while the hypothalamo-hypophyseal nervous system is not mature yet. By the time of getting sight this system is almost formed and sufficiently active.