zona fasciculata

The zona fasciculata is the thick middle layer of the adrenal cortex and the largest of its three zones. Its name comes from its cells being arranged in long straight columns, or bundles (fasciculi), running inward like the strings of a harp. This is the cortex's stress-and-metabolism department, and it produces the body's most important steroid hormone.

Its main product is cortisol, the principal glucocorticoid. The fasciculata cells are packed with lipid droplets of stored cholesterol — the raw material for steroid synthesis — which gives them a pale, foamy appearance under the microscope and earns them the nickname clear cells. They convert that cholesterol stepwise into cortisol and, to a lesser degree, some androgens.

Production here is driven by ACTH from the anterior pituitary, placing the fasciculata at the working end of the HPA axis. When ACTH is chronically high — for instance from a pituitary tumor — this zone enlarges and overproduces cortisol, a major cause of Cushing's syndrome. When ACTH is lacking, the zone shrinks and cortisol falls.