Clinical physiology

The fetal adrenal cortex becomes active during the second trimester and, at birth, it occupies 80% of the enlarged adrenal gland. The adult or definitive zones are scattered in groups around the periphery of the gland, and the adult configuration of three zones evolves with time, the zona reticularis not differentiating until middle childhood. It seems likely that cells gradually progress towards the centre of the adult adrenal cortex changing their function as they go. The enzymes of the adrenocortical cells are contained variously in microsomes and mitochondria so steroidogenesis requires coordination of mitochondrial and microsomal enzyme systems.

The outer zone of the adult cortex, the zona glomerulosa, comprises 10% of the cortical thickness. Its cells produce aldosterone and are under the control of the renin–angiotensin system which regulates both cholesterol uptake by the adrenal gland and also 18 hydroxylation of corticosterone (Fig. 5.1). This enzyme is not present elsewhere in the adrenal gland. The zona glomerulosa cells do not catalyse 17α-hydroxylation of progesterone, which is the principal substrate for the synthesis of the glucocorticoid and androgenic adrenal steroids.

Renin is synthesized by the juxtaglomerular apparatus and stored as a proenzyme in cells of the macula densa. When pulse pressure falls, plasma sodium concentration falls or plasma potassium concentration rises, this proteolytic enzyme hydrolyses angiotensin I from renin substrate (angiotensinogen), which is synthesized in the liver.