Parafollicular cells, thyroid

Inhibition of calcitonin secretion from thyroid parafollicular cells... 

Calcitonin. This hormone, which is also secreted from the thyroid gland, is synthesized by the parafollicular cells (C cells) located between the follicles. The primary effect of calcitonin is to decrease the blood levels of calcium and phosphate. The mechanism of action involves the direct inhibition of osteoclast activity, which decreases bone resorption. This results in less demineralization of the bone and therefore a decrease in the release of calcium and phosphate from the bone into the blood. Calcitonin has no direct effect on bone formation by osteoblasts. 

In most mammals, calcitonin is synthesized by specialized parafollicular cells in the thyroid. In sub-mammalian species, it is synthesized by specialized anatomical structures known as ulti-mobranchial bodies. 

Calcitonin is a single chain polypeptide of 32 amino-acids. It is secreted by the parafollicular cells of the thyroid gland. However in the circulation various forms of calcitonin are present, probably including several precursors. Calcitonin inhibits osteoclastic resorption of bone and it increases calcium and... 

The hormone that opposes the action of PTH is CT. It is produced in the parafollicular cells of the thyroid gland, whereas thyroxine is produced by the follicular cells. Human CT is a 32 amino acid peptide with one disulfide bond. CT is secreted in response to increased serum [Ca2+], especially following a meal. Its role is to prevent hypercalcemia by decreasing the serum [Ca2+]. This is achieved by counteracting the action of PTH on osteoclasts while having no effect on osteoblasts. In addition, CT affects the movement of serum Ca2+ into a labile Ca pool on the bone surface, which then releases Ca back into the circulation when [Ca2+] in serum declines. 

Calcitonin (ihyrocalcilonin) is a 32-amino-acid polypeptide hormone secreted by parafollicular cells of the thyroid glands in response lo hypocalcemia. The entire 32-residue peplide appears to be required for activity, bccau.se smaller fragments are totally inactive. Common structural features of calcitonin isolated fntm different species are a COOH-lerminal prolinamide. a disulfide bond between residues I... 

The follicle cells, called thyrocytes, produce the thyroid hormones and are derived from the entodermal pharynx. Interspersed between follicles are specialized APUD cells derived from the neural crest, called C-cells or parafollicular cells. These cells produce calcitonin (CT), a polypeptide hormone discussed in Chapter 37. 

Calcitonin A polypeptide honnone of 32-amino acid residues that is synthesized in the parafollicular cells (C cells) of the thyroid gland. Calcitonin is seaeted in response to elevated blood Ca levels. 

Ljungberg O, Bondeson L, Bondeson AG. Differentiated thyroid carcinoma, intermediate type A new tumor entity with features of follicular and parafollicular cell carcinomas. Hum Pathol. 1984 15 218-228. 

The majority of the thyroid giand contains follicular cells responsible for the production of the thyroid hormones. A second population of endocrine cells within the thyroid known as C (clear) cells, or parafollicular cells, produce the hormone calcitonin (CT), which has an opposing action to that of PTH in that it decreases the Ca concentration in body fluids. It accomplishes this by inhibiting the activity of osteoclasts (i.e., decreasing Ca release from bone by inhibiting bone resorption). The actual biosynthesis and release of CT is regulated by the concentration of Ca in plasma (i.e., when it is high, CT secretion increases). 

C call (parafollicular cell) Anyone of a group of calcium-secreting cells in vertebrates that are derived from the terminal pair of gill pouches. In mammals these cells are incorporated into the thyroid gland and the parathyroid gland. 

Contrary to PTH, calcitonin has a lowering action on blood Ca activity. It is synthesized in the parafollicular cells (C cells) in the thyroid. An increased blood Ca activity leads to calcitonin secretion, and within a few minutes both calcium and phosphate concentrations in plasma are lowered. This effect is accomplished by an effect on the bone cells, where calcium is bound as hydroxyapatite. Besides this effect, calcitonin also decreases intestinal uptake and renal reabsorption. Calcitonin inhibits the osteoclasts ( bone eater cells ) and hence reduces the amount of calcium and phosphate released from bone to the extracellular fluid. The effect on the kidneys leads to an increased excretion of calcium, phosphate, sodium, chloride, and water. 

Autoradiographs taken many hours after injection of the labelled compound and containing areas of high localisation may indicate retention of a metabolite. Thus, 24 hours after injection of labelled 5-hydroxytryptophan (5-HTP), there is a pronounced retention of radioactivity in the thyroid which is confined to the parafollicular cells [131], As no free amino acid is demon-trable biochemically at this time, then the radioactivity points to the presence of serotonin (5-HT) which is known to be present in these cells [132, 133]. 

Inhalable co-spray dried powders of salmon calcitonin-loaded chitosan nanoparticles with mannitol have been prepared and analyzed. Salmon calcitonin or Calcitonin (also known as thyrocalcitonin) is a 32-amino acid linear polypeptide hormone that is produced in humans primarily by the parafollicular cells (also known as C-cells) of the thyroid, and in many other animals in the ultimobranchial body. It acts to reduce blood calcium (Ca ), opposing the effects of parathyroid hormone (PTH) [118]. The... 

A similar mechanism of secretion is thought to occur in the anterior and posterior pituitary gland and in the parafollicular cells of the thyroid (Trifaro, 1977). The process of exocytosis occurs in the neurohypophysis, with the release of oxytocin, vasopressin, and neurophysins I and II (Uttenthal et al., 1971 Nordmann et al., 1971) in the anterior pituitary, with the release of thyrotropin (Farquhar, 1969), adrenocor-ticotropin (Rennels and Shiino, 1968), and growth hormone (Farquhar, 1961) and in the alpha (Gomez-Acebo et al., 1968), beta (Lacy, 1961), and delta (Gomez-Acebo et al., 1968) cells of the pancreas. 

The regulation of calcitonin synthesis and release from the parafollicular C cells of the thyroid gland is calcium dependent. Rising serum calcium is the principal stimulus responsible for calcitonin synthesis and release. Other hormones, such as glucagon, gastrin, and serotonin, also stimulate calcitonin release. Calcitonin has been isolated in tissues other than the parafollicular C cells (parathyroid, pancreas, thymus, adrenal), but it is not known whether this material is biologically active. 

A calcitonin precursor has been identified within the thyroid parafollicular C cells. It is thought to function in a manner analogous to that of proPTH to facilitate intracellular transport and secretion of the hormone. The metabolic degradation of calcitoifin appears to occur in both the fiver and kidney. 

Several years ago, it was discovered that the thyroid gland was also the source of a hypocalcemic hormone having effects in general opposition to those of the parathyroid hormone. This hormone is produced in mammals by the parafollicular C-cells and in other vertebrates by the ultimobrachial bodies (45). Originally called thyrocalcitonin, it is now referred to as calcitonin (CT). 

Calcitonin is secreted by the parafollicular or C cells, which arise from the neural crest and are distributed throughout the thyroid gland. These cells are included in the APUD (cmine precursor uptake and decarboxylation) family, which explains the association of medullary thyroid carcinoma (a tumor of the C cells) and other tumors of the APUD family in multiple endocrine neoplasia type 2A and -2B (MEN-2A and MEN-2B). Release of calcitonin is stimulated by calcium, and it has been used pharmacologically as an inhibitor of bone resorption. 

The thyroid gland also contains another type of cell known as parafollicular or C cells. These cells have been shown to produce the polypeptide hormone calcitonin (see Chapter 49). These cells never border the follicular lumen but are confined within the follicular basement lamina or exist in clusters in the interfollicular spaces,... 

---