The adrenal medulla secretes catecholamines (epinephrine, norepinephrine,
and dopamine). The catecholamines help prepare the individual to
deal with emergency situations. The major disorder of the adrenal
medulla is pheochromocytoma, a neoplasm characterized
by excessive catecholamine secretion.
The adrenal medulla is the reddish-brown central portion of the
adrenal gland (see Figure 21–2).
Accessory medullary tissue is sometimes located in the retroperitoneum
near the sympathetic ganglia or along the abdominal aorta (paraganglia) (Figure 12–1).
Anatomic distribution of extra-adrenal chromaffin tissue
in the newborn.
(Redrawn, with permission, from Coupland R. The
Natural History of the Chromaffin Cell. Longman, Green,
The adrenal medulla is made up of polyhedral cells arranged in
cords or clumps. Embryologically, the adrenal medullary cells derive
from neural crest cells. Medullary cells are innervated by cholinergic
preganglionic nerve fibers that reach the gland via the splanchnic
nerves. The adrenal medulla can be regarded as a specialized sympathetic
ganglion, where preganglionic sympathetic nerve fibers (using acetylcholine
as a neurotransmitter) directly make contact with postganglionic
cells, which secrete catecholamines directly into the circulation. This
relationship is analogous to the other sympathetic paraganglions,
which connect preganglionic cholinergic sympathetic nerve fibers
with postganglionic fibers using catecholamines as neurotransmitters.
Medullary parenchymal cells accumulate and store their hormone products
in prominent, dense secretory granules, 150–350 nm in diameter.
Histologically, these cells and granules have a high affinity for chromium
salts (chromaffin reaction) and thus are called chromaffin
cells and contain chromaffin granules. The granules
contain the catecholamines epinephrine and norepinephrine. Morphologically,
two types of medullary cells can be distinguished: epinephrine-secreting
cells, which have larger, less dense granules, and norepinephrine-secreting
cells, which have smaller, very dense granules. Separate dopamine-secreting
cells have not been identified. Ninety percent of medullary cells
are the epinephrine-secreting type and 10% are the norepinephrine-secreting
The catecholamines help to regulate metabolism, contractility of
cardiac and smooth muscle, and neurotransmission.
& Metabolism of Catecholamines
The adrenal medulla secretes three catecholamines: epinephrine,
norepinephrine, and dopamine. Secretion occurs after release of
acetylcholine from the preganglionic neurons that innervate the
medullary cells. The major biosynthetic pathways and hormonal intermediates
for the catecholamines are shown in Figure 12–2.
In humans, most (80%) of the catecholamine output of the
adrenal medulla is epinephrine. Norepinephrine is principally found
in nerve endings of the sympathetic nervous system and in the CNS,
where it functions as a major neurotransmitter.
Biosynthesis of catecholamines.
(Redrawn, with permission, from Greenspan FS,
Gardner DG [editors]. Basic and Clinical
Endocrinology, 7th ed. McGraw-Hill, 2004.)
Approximately 70% of the epinephrine and norepinephrine and