Histologically, these cells and granules have a high affinity for chromium salts ( chromaffin reaction) and thus are called chromaffin cells and contain chromaffin granules. Medullary parenchymal cells accumulate and store their hormone products in prominent, dense secretory granules, 150–350 nm in diameter. This relationship is analogous to the other sympathetic paraganglions, which connect preganglionic cholinergic sympathetic nerve fibers with postganglionic fibers using catecholamines (mainly norepinephrine) as neurotransmitters. 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 (mainly epinephrine) directly into the circulation. Medullary cells are innervated by cholinergic preganglionic nerve fibers that reach the gland via the splanchnic nerves. Embryologically, the adrenal medullary cells derive from neural crest cells. The adrenal medulla is made up of polyhedral cells arranged in cords or clumps.
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