parafolikulära

Parafolikulära, also known as parafollicular cells or C cells, are a distinct population of neuroendocrine cells embedded in the thyroid gland. They are situated in the connective tissue between thyroid follicles and are derived from neural crest cells during development. Their primary product is calcitonin, a peptide hormone that helps regulate calcium homeostasis.

Calcitonin is released in response to elevated blood calcium and acts to lower serum calcium by inhibiting osteoclast-mediated bone resorption and by enhancing calcium excretion in the kidneys. In humans, the physiological role of calcitonin is relatively minor compared with that of parathyroid hormone and active vitamin D, and disruptions in calcitonin signaling generally have limited clinical consequences under normal conditions.

Clinical relevance and pathology: Parafollicular cells are the cell of origin for medullary thyroid carcinoma (MTC), a neuroendocrine tumor that frequently secretes calcitonin. Serum calcitonin levels, often alongside CEA, are used as tumor markers for diagnosis and monitoring of MTC. Histologically, MTC typically presents as nests or sheets of polygonal to spindle-shaped cells that express neuroendocrine markers, including calcitonin, chromogranin A, and synaptophysin. The tumor stroma may contain amyloid deposits derived from calcitonin-derived peptides, a feature that can aid in diagnosis.