Wednesday, March 17, 2010

Parafollicular Cells

Parafollicular cells, also known as C cells, are cells situated around the follicles of the thyroid gland. They are rich in mitochondria and synthesize and secrete calcitonin. Parafollicular cells are large and have a pale stain compared with the follicular cells. Embryologically, they associate with the ultimobranchial body, which itself is a ventral derivative of the fourth pharyngeal pouch. Parafollicular cells themselves are derived from Neural Crest cells. C cells 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.

Thyroid parafollicular cells are neural crest-derived endocrine cells which produce calcitonin and serotonin. The secretory vesicles of parafollicular cells acidify when secretion is induced by increased extracellular Ca2+ or TSH. Scientists have tested the hypothesis that acidification is regulated by secretogogue-gated Cl- channels in vesicular membranes. Cl- channel (p64) immunoreactivity was enriched in purified PF vesicles. X-Ray microanalysis showed a change in chlorine level in C cells vesicles in response to secretogogue-stimulation of isolated cells. Secretogogue stimulation also altered the degree of p64 channel phosphorylation. Protein kinase and phosphatase inhibitors antagonized secretogogue- induced vesicle acidification and secretion; however, secretion could occur even when acidification was blocked. So, it has been concluded that acidification of C cells vesicles is regulated by a gatable Cl- channel in vesicle membranes and that protein phosphorylation and dephosphorylation are involved in channel activation. Acidification of vesicles is not required for exocytosis.