Adrenal medulla

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Main organs of the endocrine system.

The adrenal (suprarenal) medulla, or the medulla of the suprarenal gland, refers to the inner central part of each adrenal gland and is derived from neural crest cells. It is a thin region and forms approximately one-tenth of the gland. The adrenal medulla is nearly completely surrounded by the yellowish cortex, with the exception of the hilum which transmits the neurovasculature of the gland. The adrenal medulla is extensively vascularized, and depending on the amount of blood within it, the adrenal medulla may appear as either dark red, brownish or greyish.

Histologically, the adrenal medulla consists of clusters and columns of large spherical chromaffin cells, also called pheochromocytes, between which run capillaries and sinusoids, within a supporting network of reticular fibres. The chromaffin cells are regarded as modified ganglionic cells of the sympathetic nervous system since they are derivatives of neural crest cells.

Functionally, the chromaffin cells are similar to postganglionic sympathetic neurons. They synthesize and store the catecholamine hormones noradrenaline (norepinephrine) and adrenaline (epinephrine) in granules. Preganglionic sympathetic nerve fibers within the adrenal medlla form synapses with the chromaffin cells. In response to signals from these preganglionic neurons, the chromaffin cells release the hormones, mainly adrenalin, from their granules into the venous sinusoids and ultimately into circulation via the central medullary vein and hilar suprarenal vein to affect target cells.

The hormones noradrenaline and adrenaline stimulate a flight-or-fight response that prepares the body for an emergency defensive reaction. Specifically, these hormones result in various target organ effects including an elevated heart rate and blood pressure, bronchodilation, increased blood glucose and redistribution of blood flow to vital organs.