parafollicular

Parafollicular refers to a group of cells in the thyroid gland known as parafollicular cells or C cells. These neuroendocrine cells lie in the interfollicular stroma, between thyroid follicles, and are derived from neural crest cells during embryogenesis.

The primary secretory product of parafollicular C cells is calcitonin, a peptide hormone. Calcitonin lowers serum calcium levels mainly by inhibiting osteoclast-mediated bone resorption. Its renal effects are considered minor in humans, and overall calcium homeostasis is predominantly controlled by parathyroid hormone and vitamin D. Calcitonin release is stimulated by elevated calcium levels and is modulated by neural inputs.

Histologically, parafollicular cells are pale-staining, polygonal cells located in the connective tissue between follicles. They can be identified with immunohistochemical staining for calcitonin and related markers. In development, their abundance is higher in fetal tissue and decreases with age.

Clinical relevance centers on neoplasms and biomarkers. Medullary thyroid carcinoma (MTC) arises from parafollicular C cells and often produces calcitonin, which can serve as a tumor marker for diagnosis and postoperative surveillance. MTC can be associated with amyloid deposition derived from calcitonin-related peptides. Genetic mutations in the RET proto-oncogene are linked to familial syndromes that include MTC, underscoring the diagnostic and prognostic importance of C-cell pathology.