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The adrenal gland has a very distinctive tissue architecture that is often readily recognizable on microscope slides even without using a microscope.  The adrenal cortex forms a layer of uniform thickness around the irregular adrenal medulla -- rather like a pita bread (the cortex) stuffed with filling (the medulla).

Adrenal blood supply flows from capsular arteries to medullary veins by percolating through cortical sinusoids.  Some arterial branches pass directly through the cortex to provide fresh blood to the medulla.

The adrenal cortex consists of epithelial cords arranged more-or-less perpendicular to the organ's thin connective tissue capsule.  The cords are separated by a network of vascular sinusoids which connect the capsular arteries with the medullary veins.

All of the cortical parenchymal cells are specialized for steroid secretion, with the typical steroid-cell appearance of vacuolated eosinophilic cytoplasm reflecting the presence of abundant lipid droplets (the raw material for steroid production), smooth endoplasmic reticulum, and mitochondria with tubular cristae.

The cords of cortical cells are differentiated by cell arrangement and by cell function into three poorly-defined zones.  The names reflect the predominant arrangement.

In the thin outer zona glomerulosa, the cords tend to form rounded knots (glomeruli).  Cells of the zona glomerulosa secrete mineralocorticoids such as aldosterone which regulate salt balance.

In the broader zona fasciculata, the cords tend to form straight parallel bundles (fascicles).  Cells of the zona fasciculata secrete glucocorticoids such as cortisol which regulate sugar balance.  Cells in this zone are often paler than those in either zona glomerulosa or zona reticularis, as a consequence of larger and more numerous lipid droplets.

In the inner zona reticularis, the cords tend to form an interconnected network (a reticulum).  Cells of the zona reticularis secrete androgenic steroids.

The classic mneumonic for zonal function is:  "Salt, sugar, sex  -->  deeper is sweeter."

The parenchyma of the adrenal medulla consists of cells loosely organized into clusters and cords.  These chromaffin cells are described as "epithelioid" (resembling epithelial cells), although they are derived from neural crest and functionally resemble sympathetic neurons.  Proper sympathetic-ganglion nerve cells can occasionally be found in the medulla.

The medullary chromaffin cells secrete catecholamines, either epinephrine or norepinephrine.

The medulla receives a dual blood blood supply -- from cortical sinusoids and also from arteries which pass directly through the cortex from capsular arteries. 

The medullary veins which drain the adrenal gland have walls containing peculiar longitudinal bundles of smooth muscle.

Comments and questions:

SIUC / School of Medicine / Anatomy / David King
Last updated:  18 May 2022 / dgk