Adrenal
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.
Cortex
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. These cells have
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. (For electron micrographs of these cells, see
Electron Microscopic Atlas of cells, tissues and organs in the internet.)
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."
Medulla
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. (For electron micrographs of these cells, see
Electron Microscopic Atlas of cells, tissues and organs in the internet.)
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.
