Functional Anatomy of the Adrenal Gland
The two adrenal glands are located immediately anterior to the kidneys, encased in a connective tissue capsule and usually partially buried in an island of fat. Like the kidneys, the adrenal glands lie beneath the peritoneum (i.e. they are retroperitoneal). The exact location relative to the kidney and the shape of the adrenal gland vary among species.
Inspection of a mammalian adrenal gland that has been sectioned reveals two distinct regions, as demonstrated to the right with a sheep adrenal (click the image to flip between surface and cut views):
- An inner medulla, which is a source of the catecholamines epinephrine and norepinephrine. The chromaffin cell is the principle cell type. The medulla is richly innervated by preganglionic sympathetic fibers and is, in essence, an extension of the sympathetic nervous system.
- An outer cortex, which secretes several classes of steroid hormones (glucocorticoids and mineralocorticoids, plus a few others). Histologic examination of the cortex reveals three concentric zones of cells that differ in the major steroid hormones they secrete.
Despite their organization into a single gland, the medulla and cortex are functionally different endocrine organs, and have different embryological origins. The medulla derives from ectoderm (neural crest), while the cortex develops from mesoderm. The utility, if any, of having them together in one discrete organ is not obvious. In some species, amphibians and certain fish, for example, two separate organs are found.