HISTOGENESIS OF THE ADRENAL-CORTEX IN THE FETAL SHEEP

The cortex of sheep foetal adrenal glands from 25 days gestation until newborn (term equals 147±3 days) were examined by light and electron microscopy. Three stages of development are of particular importance in relating structure to function: 1) from 35 to 60 days, 2) from 60 to 120 days and 3) from 120 days to term. Between 35 and 60 days one cortical cell type predominated. It contained mitochondria with lamellar and vesicular cristae, scattered long strands of granular endoplasmic reticulum and only small amounts of smooth endoplasmic reticulum. After about 60 days two zones were apparent in the cortex and chromaffin cells became concentrated in the medulla. After 80 days the outer zone contained cells which resembled mature zona glomerculosa cells and the cells in the inner zone remained like those seen between 35 and 60 days, except they contained even less smooth endoplasmic reticulum. However, after about 90 days a small number of deep inner zone cells contained mitochondria with vesicular cristae which thus resemble mitochondria in the mature zone fasciculata. From about 120 days there was an increase in the number of cells in the inner zone that contained mitochondria with vesicular cristae. These cells also contained substantial quantities of smooth endoplasmic reticulum. At term most inner zone cells have this mature appearance. Thus there is no 'foetal cortex' in the sheep analogous to that found in human adrenal development, i.e. there is no prominent zone of cells containing large amounts of smooth endoplasmic reticulum which is present throughout most of the foetal period of development, and which regresses at birth. The structure of the cells present between 35 and 60 days was unexpected because it has been shown previously that sheep foetal adrenals of this age are capable of producing relatively large quantities of steroid hormones. However, the appearance of cells resembling mature zona glomerulosa cells at about 80 days correlates with the previously demonstrated ability of sheep adrenal glands of this age, to produce relatively large quantities of aldosterone. The rapid development of numbers of mature cells in the last 3 weeks of gestation correlates with the previously described ability of near term sheep foetal adrenals to produce very large quantities of steroid hormones.