Adrenal medulla
Medulla glandulae suprarenalis
Also known as: chromaffin cells
The core of the adrenal gland — a modified sympathetic ganglion of chromaffin cells that secrete epinephrine (≈80 %) and norepinephrine (≈20 %) into the blood during stress.
Overview
The medulla is composed of clusters of chromaffin cells (named for the brown colour their granules take with chromium salts) surrounding wide sinusoids and venules. They are postganglionic sympathetic neurons without axons: preganglionic sympathetic fibres from the thoracic splanchnic nerves synapse directly on them, releasing acetylcholine onto nicotinic (N_N) receptors. Depolarisation opens Ca²⁺ channels and the cells exocytose granules containing catecholamines, ATP, chromogranin A and enkephalins.
Catecholamine synthesis: tyrosine → L-DOPA (tyrosine hydroxylase, rate-limiting) → dopamine (aromatic L-amino acid/DOPA decarboxylase) → norepinephrine (dopamine β-hydroxylase, inside granules) → epinephrine (PNMT, in the cytosol). PNMT is induced by the high cortisol concentration in blood flowing from the cortex. Catecholamines are inactivated by COMT and MAO to metanephrines, normetanephrines and vanillylmandelic acid (VMA), which are measured in diagnosis.
- Location
Centre of each adrenal gland, surrounded by the zona reticularis.
Parts & subdivisions
- Majority of chromaffin cells; smaller, less dense granules.
- Minority; granules with an eccentric dense core.
- Sustentacular cells and ganglion cellsSupporting glia-like cells and scattered sympathetic neurons.
Anatomy
- Innervation
Preganglionic sympathetic (cholinergic) fibres from lower thoracic spinal segments via the greater, lesser and least splanchnic nerves and coeliac plexus — no synapse in a ganglion.
- Histology
Basophilic polygonal cells in clusters and cords around sinusoids; chromaffin reaction positive; chromogranin and synaptophysin immunoreactive.
- Development
Neural crest cells that migrate into the fetal cortex (same origin as sympathetic ganglion neurons).
Function
- Releases epinephrine and norepinephrine within seconds of sympathetic activation (fight or flight).
- Amplifies and prolongs sympathetic effects throughout the body, including tissues with no direct sympathetic innervation.
Clinical significance
Phaeochromocytoma
Chromaffin-cell tumour secreting catecholamines: paroxysmal hypertension, headache, sweating, palpitations. Diagnose with plasma/urine metanephrines; α-block before β-block before surgery. Associated with MEN2, VHL, NF1 and SDH mutations.
Adrenalectomy and epinephrine
After bilateral adrenalectomy, circulating epinephrine nearly disappears, but norepinephrine (from sympathetic nerves) remains — showing that nerves, not the medulla, are the main source of plasma norepinephrine.
Test yourself
Why does the adrenal medulla secrete mainly epinephrine rather than norepinephrine?
- Chromaffin cells lack dopamine β-hydroxylase
- Cortisol reaching the medulla via the intra-adrenal portal circulation induces PNMT
- Aldosterone activates tyrosine hydroxylase
- Preganglionic fibres release epinephrine directly
Answer: Cortisol reaching the medulla via the intra-adrenal portal circulation induces PNMT
Cortisol-rich blood draining from the cortex into medullary sinusoids induces phenylethanolamine N-methyltransferase, which methylates norepinephrine to epinephrine. Extra-adrenal paragangliomas, lacking this cortisol exposure, secrete mainly norepinephrine.
Compiled from standard anatomy and physiology references — see the Endocrine System sources. For education only; not medical advice.