The Adrenal Medulla

Structure

The adrenal glands, also known as supra-renal glands, are found immediately superior to the kidneys. They are retroperitoneal structures and composed of two major regions: the outer adrenal cortex and the inner adrenal medulla.

The main secreting cells of the adrenal medulla are called chromaffin cells, which are neuroendocrine cells that are modified sympathetic ganglia. The chromaffin cells are neural crest cell derivatives. Adrenaline is released in response to activation of the sympathetic nervous system, fibres of which are carried to the adrenal medulla by the thoracic splanchnic nerves.

Jpogi, CC0, via Wikimedia Commons

Fig 1
Histology of the adrenal gland.

Function

The adrenal medulla is mainly responsible for the synthesis of the catecholamines, adrenaline and noradrenaline, but also has other secretory functions such as the production of dopamine. Both adrenaline and noradrenaline are produced from the amino acid tyrosine, through multiple reactions.

The synthesised adrenaline is stored in vesicles before being released into the blood stream. Adrenaline is mainly associated with the “fight or flight response“, and noradrenaline also plays a role in the activation of the sympathetic nervous system as a neurotransmitter in post-ganglionic synapses.

It exhibits its actions through α and β adrenoreceptors (G protein coupled receptors), both in the central nervous system and in the periphery. The “fight or flight response” is a key survival mechanism, and causes a number of physiological changes, such as increased cardiac output and increased glycogenolysis in liver and muscle tissue.

Clinical Relevance

Phaeochromocytoma

A phaeochromocytoma is a neuroendocrine tumour of the adrenal medulla, specifically the chromaffin cells which secrete adrenaline. This leads to an excess of adrenaline, and constant activation of the “flight or fight response”. Hence, this can lead to symptoms such as:

• Tachycardia
• Hypertension
• Anxiety
• Palpitations
• Weight loss
• Hyperglycaemia

In some situations it can lead to a hypertensive crisis. Whilst the above symptoms are all possible, the most typical presentation is intermittent attacks of headaches, excessive sweating and tachycardia.

These patients present with extremely high blood pressures, typically greater than 180/120 mmHg. Hypertension can lead to increased pressure in vital circulations, such as in the brain and in the kidney, which can be lethal.

Typically, treatment involves surgical resection of the tumour, however alpha adrenoceptor blockers often need to be given prior to surgery to minimize complications.

By Michael Feldman, MD, PhD University of Pennsylvania School of Medicine [CC BY 2.0 (http://creativecommons.org/licenses/by/2.0)], via Wikimedia Commons

Fig 2
Adrenal gland with a phaeochromocytoma present in the medulla.