paraganglia

Paraganglia are small clusters of neuroendocrine cells that originate from neural crest cells and accompany components of the autonomic nervous system. They form a diffuse system outside the adrenal glands and include both sympathetic and parasympathetic groups. The adrenal medulla is a specialized paraganglion that functions as part of the sympathetic system and is composed of chromaffin cells that secrete catecholamines.

Paraganglia are traditionally divided into two major categories. Sympathetic paraganglia are typically located along the paravertebral sympathetic chain and near major vessels in the thorax and abdomen, including the organ of Zuckerkandl near the aorta and paraaortic regions. These paraganglia are usually chromaffin and capable of producing catecholamines such as norepinephrine and epinephrine. Parasympathetic paraganglia occur mainly in the head and neck, for example the carotid bodies and other carotid and jugulotympanic region clusters; these are typically nonchromaffin and do not regularly secrete catecholamines.

Functionally, paraganglia act as neuroendocrine and chemoreceptor tissues. Sympathetic paraganglia can respond to stress by releasing catecholamines, contributing to systemic cardiovascular and metabolic responses. Parasympathetic paraganglia, particularly in the head and neck, primarily serve sensory and regulatory roles such as chemoreception, rather than catecholamine secretion.

Clinical relevance centers on paragangliomas, tumors arising from paraganglia. Head and neck paragangliomas are common extra-adrenal tumors, while sympathetic paraganglia may give rise to catecholamine-secreting paragangliomas. Genetic predispositions, including mutations in SDHx, VHL, RET, and NF1 genes, are recognized. Diagnosis often involves biochemical testing for metanephrines and imaging, with treatment typically involving surgical resection and, when appropriate, radiotherapy. The term pheochromocytoma is reserved for adrenal tumors of chromaffin tissue, while paraganglioma denotes extra-adrenal counterparts.