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Calcitonin lowers blood levels of calcium by inhibiting the resorption of bone by osteoclasts, and its secretion is increased proportionally with the concentration of calcium. [7] Parafollicular cells are also known to secrete in smaller quantities several neuroendocrine peptides such as serotonin, somatostatin or CGRP.
Calcitonin is a hormone secreted by the thyroid in humans. Calcitonin decreases osteoclast activity, and decreases the formation of new osteoclasts, resulting in decreased resorption. [4] Calcitonin has a greater effect in young children than in adults, and plays a smaller role in bone remodeling than PTH. [4]
Calcitonin is a 32 amino acid peptide hormone secreted by parafollicular cells (also known as C cells) of the thyroid (or endostyle) in humans and other chordates [5] in the ultimopharyngeal body. [6] It acts to reduce blood calcium (Ca 2+), opposing the effects of parathyroid hormone (PTH). [7]
The calcitonin receptor (CT) is a G protein-coupled receptor that binds the peptide hormone calcitonin and is involved in maintenance of calcium homeostasis, [5] particularly with respect to bone formation and metabolism. [6] [7] [8]
Parafollicular cells produce calcitonin in response to high blood calcium. Calcitonin decreases the release of calcium from bone, by decreasing the activity of osteoclasts, cells which break down bone. Bone is constantly reabsorbed by osteoclasts and created by osteoblasts, so calcitonin effectively stimulates movement of calcium into bone.
The osteoclast disassembles and digests the composite of hydrated protein and mineral at a molecular level by secreting acid and a collagenase, a process known as bone resorption. This process also helps regulate the level of blood calcium. Osteoclasts are found on those surfaces of bone that are undergoing resorption.
Bone tissue is removed by osteoclasts, and then new bone tissue is formed by osteoblasts. Both processes utilize cytokine (TGF-β, IGF) signalling.In osteology, bone remodeling or bone metabolism is a lifelong process where mature bone tissue is removed from the skeleton (a process called bone resorption) and new bone tissue is formed (a process called ossification or new bone formation).
The matrix is hardened by the binding of inorganic mineral salt, calcium phosphate, in a chemical arrangement known as bone mineral, a form of calcium apatite. [8] [9] It is the mineralization that gives bones rigidity. Bone is actively constructed and remodeled throughout life by specialized bone cells known as osteoblasts and osteoclasts.