Overview of Adrenal Histology

The adrenal gland is encased in a connective tissue capsule that extends septae into the substance of the gland. The organ is richly vascularized and capsular blood vessels, nerves and lymphatics penetrate along with the connective tissue septae.

The most distinctive feature of the adrenal is its partitioning into cortex and medulla. The medulla is fairly homogeneous, but even when viewed a low power, three concentric zones can be distinguished in the cortex:

• zona glomerulosa - thin, outermost zone
• zona fasiculata - thick, middle zone
• zona reticularis - thin, inner zone

The three cortical zones and part of the medulla are evident in the section of rabbit adrenal gland seen below.

A higher magnification montage of a feline adrenal gland is available.

Based on embryologic origin and type of hormones produced, the cortex and medulla are best though of as separate endocrine organs. The medulla produces catecholamines and the cortex produces several steroid hormones, as depicted in the table below. There is some overlap in hormones synthesized by the zonae fasiculata and reticularis (i.e. cells in the fasiculata produce a small amount of androgens and cells in the reticularis secrete some cortisol).

Cortex	zona glomerulosa	mineralocorticoids (aldosterone)
	zona fasiculata	glucocorticoids (cortisol)
	zona reticularis	sex steroids (androgens)
Medulla		catecholamines (epinephrine and norepinephrine)

Histologic examination of the adrenal reveals a rich vasculature. Numerous small arteries from several sources ramify over the surface of the gland and penetrate into the gland into two ways: Cortical arteries and arterioles branch into capillary beds within the cortex to supply that area, then coalesce into veins at the corticomedullary junction. Medullary arteries and arterioles penetrate the cortex without branching, then form capillary beds in the medulla. Blood from both cortical and medullary veins empties through a single large central vein, which leaves the adrenal and anastomoses with either the vena cava or renal vein.

Send comments to Richard.Bowen@colostate.edu