Typification and differentiation of medullary cells in the developing rat adrenal. A histochemical and electron microscopic study.

Various light and electron histochemical techniques were applied to the study of developing rat adrenal medulla. Adrenal glands or rudiments were examined at 14, 16, 18, 20 days of intrauterine life, at birth and at 1 week after birth. Material was processes for light microscopy as follows: (1) for the demonstration of cortical lipids by Sudan black B; (2) by immersion in Muller's formol-dichromate for the direct chromaffin reaction (CHR); (3) by sequential glutaraldehyde fixation and subsequent dichromate treatment for the indirect CHR. For electron microscopy, material from all developmental stages was fixed in glutaraldehyde and further processed in three ways: subsequent dichromate treatment (GD); sequential dichromate and osmium tetroxide (GDO); subsequent osmium tetroxide (GO). A positive direct CHR in the developing medullary; cells and discrete sudanophilic lipid droplets in the cortical cells were observed in 18-20 day embryos. Although the intensity of the direct CHR increased through the developmental stages, clear distinction between adrenaline (A) and noradrenaline (NA) cells was not observed until after birth by the indirect CHR. At an ultrastuctural level 'light' and 'dark' parenchymal cells were found in the sympathomedullary rudiment medial to the cortical anlage in the 14 day embryo: these 'light' cells, the phaeochromoblasts, together with nerve fibres, invade the cortex as from this stage. Phaeochromoblasts, the adrenomedullary precursor cells, were actively dividing and possessed a cytoplasmic content of numerous polyribosomes and a few (140 nm diameter) membrane-bounded inclusion granules. In embryos of 16-18 days, medullary cells were variously grouped as phaeochromoblasts, as the more differentiated phaeochromocytes and as intermediate forms. Phaeochromocytes contained a mixture of high and low density secretory granules (200 nm diameter) which in GD and GDO preparations were interpreted as NA and A granules. Cholinergic nerve terminals on medullary cells, the establishment of an endocrine-type relationship between regional capillaries and medullary cells and the presence of exocytotic profiles at the surface of these cells were all features of the 18 day and subsequent developmental stages. It was not possible to differentiate in GD and GDO preparations between A and NA cells on the basis of their granule typification until after birth. 'Light' and 'dark' variants of both A and NA cells observed at birth and thereafter were regarded as expressions of phase differences in the secretory cycles of these cells. The secretory granule population of medullary cells showed an overall increase during development and was especially high in the 'dark' variants of A and NA cells. Another developmental trend in the majority of medullary cells was an increase in the size of these granules (diameter range at 1 week, 160-330 nm); but in a few NA cells the mean granule diameter remained small (180 nm).