Parafollicles

Parafollicles are microscopic structures found within the thyroid gland. They are also known as the C cells or parafollicular cells of the thyroid. These cells are distinct from the follicular cells that produce thyroid hormones like thyroxine and triiodothyronine. Parafollicles are typically located in the interstitial spaces between the thyroid follicles, or sometimes within the follicular epithelium itself. Their primary function is the production and secretion of calcitonin. Calcitonin is a hormone that plays a role in calcium and phosphate metabolism. It acts to lower blood calcium levels, primarily by inhibiting osteoclast activity, which are cells responsible for bone resorption, and by increasing calcium excretion by the kidneys. The secretion of calcitonin is stimulated by elevated blood calcium levels. While present in humans, the clinical significance of calcitonin in normal human physiology is debated compared to its more prominent role in some other vertebrates. However, parafollicles are the origin of medullary thyroid carcinoma, a rare type of thyroid cancer. Histologically, parafollicles can be identified by their characteristic granular cytoplasm and often eccentric nuclei. Their presence and distribution can vary slightly between individuals and species.