The adrenal medulla is the most prominent part of the chromaffin system, which consists of aggregates of chromaffin cells, which stain brown with chrome salts. It is derived from ectodermal chromaffin tissue, and can be considered to be a modified sympathetic ganglion with post-ganglionic cells but no axons.
Ovoid and columnar chromaffin cells are arranged in clumps and cords, around blood vessels. The chromaffin cells contain dense-cored chromaffin vesicles storing catecholamines, which are synthesised by the hydroxylation and decarboxylation of phenylalanine and tyrosine. The process of amine precursor uptake and decarboxylation is a feature common to a variety of embry-ologically related neuroendocrine tissues derived from the neural crest. Tyrosine hydroxylase activity is the rate limiting step in catecholamine synthesis. It catalyses the para-hydroxylation of tyrosine to form hydroxyphenylalanine (DOPA).
The adrenal medulla is unique in containing phenylethanolamine N-methyl transferase (PNMT), which catalyses N-methylation of noradrenaline to adrenaline. The adrenal medulla secretes mostly adrenaline, containing 80% adrenaline-secreting cells and 20% noradrenaline-secreting cells.
Catecholamine release results from stimulus-secretion coupling. Acetylcholine is released by preganglionic nerve impulses. This increases membrane permeability, producing depolarisation and leading to calcium influx. Exocytosis of the secretory granules ensues.
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