Parafollicular cells are small biological structures that reside in the thyroid and are responsible for releasing a hormone called calcitonin. They are classified as endocrine cells since the thyroid is an endocrine gland. Additionally, parafollicular cells are sometimes referred to as C cells.
The term "parafollicular" is derived from the precise location and status of the cells. These types of cells are found outside the thyroid's pouch-like cavities, or follicles, in the connective tissue. The thyroid, or thyroid gland, is located at the neck and is regarded as one of the biggest endocrine glands. This is a group of glands that includes the pancreas, pituitary gland, ovaries and testes, and is responsible for releasing various types of hormones into the blood. Parafollicular cells are similar to other endocrine cells in that they are stationed close to the capillaries, on which the cells rely to release hormones to the blood.
Parafollicular cells release a hormone called calcitonin into the blood. This is a polypeptide hormone composed of 32 amino acids that is perhaps best known for reducing the level of calcium in the blood plasma. This function serves to balance the function of the parathyroid hormone, which is to increase blood calcium levels. Also known as parathormone or parathyrin, the secretion of the parathyroid hormone is produced at the parathyroid glands, which are four small endocrine glands residing at the thyroid’s rear surface.
Calcitonin is produced by the parafollicular cells when they detect high levels of calcium in the blood. Left unchecked, one could develop hypercalcemia. This is triggered by primary hyperparathyroidism, which occurs when there is an excessive secretion of parathyroid hormone. Calcitonin counter acts high levels of calcium by suppressing osteoclastic activity. This refers to the action of osteoclasts, which are bone cells responsible for the resorption, or breaking down, of bone. The calcium produced during this process is then released into the blood, therefore contributing to the increase in blood calcium.
Parafollicular cells can become cancerous, resulting in a condition called medullary carcinoma of the thyroid, or medullary thyroid carcinoma. Genetics is responsible for about a quarter of the cases involving this type of cancer. When the cause is genetic, the cancer is triggered by a mutation occurring in the RET proto-oncogene. Physicians believe that the excessive production of calcitonin causes diarrhea, which is the primary symptom of medullary thyroid carcinoma. Treatment typically involves surgery and success generally depends on how early the cancer is diagnosed.