Parafollicular cells (also called C cells) are cells in the thyroid that produce and secrete calcitonin. They are located adjacent to the thyroid follicles and reside in the connective tissue. These cells are large and have a pale stain compared with the follicular cells or colloid. In teleost and avian species these cells occupy a structure outside of the thyroid gland named the ultimobranchial body.
In a series of elegant experiments, Nicole LeDouarin (ref), transplanted neural crest cells from quail, with unique and easily identified nuclei, into non-quail neural crest. She subsequently demonstrated the presence of cells with quail nuclei populating the ultimobranchial body and concluded that C cells migrate during embryologic development from the neural crest.
Embryologically, they associate with the ultimobranchial body, which itself is a ventral derivative of the fourth (or fifth) pharyngeal pouch. Parafollicular cells themselves are derived from Neural Crest cells. They are not numerous in the thyroid and are typically situated basally in the epithelium, without direct contact with the follicular lumen. They are always situated within the basement membrane, which surrounds the entire follicle.
When parafollicular cells become cancerous, they lead to medullary carcinoma of the thyroid.
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