Morphometric analyses of adrenal gland growth in fetal and neonatal sheep. II. The adrenal medulla, with some observations on its ultrastructure.

This account of fetal and neonatal sheep adrenomedullary development is the first such study in mammals using both morphometric and microscopic techniques. At 53 days gestation some cells in the migratory whorls and columns contained noradrenaline (NA) granules whereas by 100 days the medulla, now enclosed by the cortex, was composed of elongated juxtacortical cells and rounded central medullary cells, both populations of cells containing NA granules. In the 130 days glands, many of the juxtacortical cells contained adrenaline granules and had synaptic connection with axons of the preganglionic sympathetic nerve fibres. Later development was essentially growth-related. While the juxtacortical cells decreased from 33% of the medulla at 100 days to 22% at 144 days, the central medullary cells increased from 19% to 30% over the same period. Both cell populations exhibited hypertrophic growth over the study period; but the central cells multiplied at a faster rate. We conclude that the development of the cortical and medullary compartments of the adrenal gland are closely linked, for both showed rapid mid-gestational growth which slowed with the attainment of definitive tissue organisation. Then a second phase of growth, associated with increased and controlled catecholamine secretion in the medulla and cortisol secretion in the cortex, occurred in late gestation.