Adrenal cortex of fetal lamb: changes after hypophysectomy and effects of Synacthen on cytoarchitecture and secretory activity.

The effects of hypophysectomy on the cell population of the adrenal cortex has been examined with light and electron microscopy. The sensitivity of the adrenal cortical cells to exogenous ACTH in both normal and hypophysectomized fetuses has been investigated by comparing plasma cortisol concentrations with structural changes in the adrenal cortex. Hypophysectomy was carried out at about 100 d and the subsequent analysis was made at about 135 d on fetuses catheterized at least 6 d earlier. The zona fasciculata of intact fetuses contained about 25% mature and 75% immature cells at 130-136 d. After hypophysectomy this zone contained a uniform population of immature cells and no mature cells; there was a significant reduction in both adrenal weight and cortical thickness. The zona glomerulosa was unaffected by hypophysectomy. The infusion of Synacthen 0 . 25 mg/d I.V. for 48 h did not affect the zona glomerulosa but resulted in a 2-fold increase in the thickness of the zona fasciculata in hypophysectomized and intact fetuses. This cortical growth was due to both hypertrophy and hyperplasia. All the cells in the zona fasciculata were mature following Synacthen treatment in both groups. The most striking difference between the secretory response of the adrenal cortex to Synacthen in the two groups was found within 3-6 h of the onset of infusion. Within this period there was a significant increment in the plasma cortisol concentration in the intact fetuses while no response was observed in the hypophysectomized animals. After 12 h infusion by contrast, a steadily rising response was observed in both groups. There was a significant correlation between the final plasma cortisol concentration and both the adrenal weight and cortical thickness after 48 h treatment. The increased sensitivity of the adrenal cortex to exogenous ACTH as term approaches in the sheep is considered to be dependent upon the increasing proportion of mature zona fasciculata cells within the adrenal cortex.