Mammalian adrenal medullary tissue is composed of endocrine secretory units called chromaffin cells. These cells synthesize and store catecholamines and release them in response to neural impulses carried in the splanchnic nerves. The designation “chromaffin,” which implies an affinity for chrome, is derived from histopathology and refers to a characteristic darkening that occurs when tissues rich in chromaffin cells are exposed to aqueous solutions of potassium dichromate. It is now recognized that this peculiar darkening is caused by oxidation of the large quantities of catecholamines contained within the chromaffin cells and by the subsequent polymerization of the oxidized catecholamine derivatives.
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