The adrenal cortex is subdivided into the following three ill-defined zones: the zona glomerulosa under the capsule, the zona fasciculata and the zona reticularis next to the medulla.

The glomerulosa is composed of irregular globular groups of deeply stained glandular cells. Such groups frequently take a vague, arched configuration.

The fasciculata is composed of anastomotic plates of vacuolated cells. The cytoplasmic vacuoles are formed by the extraction of lipid droplets.

The reticularis shows numerous small deeply stained cells, often containing lipofuscin granules.

These cells are grouped in clusters or plates. In the three zones, the groups of cells are separated by loose connective tissue containing networks of small vessels (capillaries, post-capillary venules or sinuses) which collect the steroids secreted by the cells. The medulla is composed of irregular clusters of large spheroidal glandular cells, separated by connective tissue containing numerous small vessels. These cells secrete catecholamines under the influence of the autonomic nervous system.