Development of the Adrenal Medullary Cells in Rats with Reference to Synaptogenesis

The adrenal medullae of rats were studied electron microscopically from day 13.5 of gestation. Synapses with thickening of pre-and post-synaptic membranes were first evidenced on the medullary cells of 15.5-day-old fetuses. They increased gradually in number with advancing age. In the medullary cells, a few membrane-limited granules were recognized on day 13.5 of gestation, and thereafter they increased in number. The appearance of the granules was not uniform; some of the granular contents consisted of fine or coarse particles and others contained homogeneous material of varying electron-densities. The population of these granules in the cytoplasm was different from cell to cell. Thus, the discrimination of cell types (NA-and A-cells) was not possible in the prenatal period. Granular discharge of the cells was totally absent in the normal untreated fetuses. However upon the intraperitoneal administration of insulin to the fetus on day 16.5, 18.5, or 21.5 of gestation, discharge by reverse pinocytosis was first evidenced in the medullary cells of the 21.5-day-old fetus.