Anatomy, Structure, and Synthesis of Calcitonin (CT)
Calcitonin is produced by parafollicular cells that surround the thyroid follicular cells. These parafollicular cells, or C cells, are derived from the neural crest and migrate into the thyroid gland. In submammalian species, CT is produced by a discrete ultimobranchial body. CT is a small peptide of 32 amino acids, has a molecular weight of 3500, and is unique in that it has a disulfide bridge between amino acids 1 and 7 (figure). Species differences in structure are small, but salmon CT is different from human in 16 amino acids. CT from salmon ultimobranchial bodies has greater biological activity and a longer half-life than human CT. Analogs of salmon CT are used in clinical medicine for treating of metabolic bone disease.
CT is cleaved from a prohormone that also contains two other peptides, katacalcin and calcitonin gene-related peptide (CGRP). These peptides are secreted 1:1 with CT and are therefore found in equimolar ratio with CT in the circulation. Some evidence suggests that CGRP is a vasodilator. CT and CGRP have been isolated from other organs, including the pituitary, suggesting that they may have other functions besides lowering plasma calcium.