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Calcitriol is a hormone and the active form of vitamin D, normally made in the kidney. [8] [9] [10] It is also known as 1,25-dihydroxycholecalciferol.It binds to and activates the vitamin D receptor in the nucleus of the cell, which then increases the expression of many genes. [11]
Bone tissue is a dynamic system with active metabolism. [24] Bone tissue remodelling or bone remodeling is a successive chain of old bone matrix removal and its replacement with a new one. [25] These processes make a child’s skeleton grow and extend, while childhood is characterized by bone tissue growth rather than its resorption.
At a site of active bone resorption, the osteoclast forms a specialized cell membrane, the "ruffled border", that opposes the surface of the bone tissue. This extensively folded or ruffled border facilitates bone removal by dramatically increasing the cell surface for secretion and uptake of the resorption compartment contents and is a ...
The vitamin D receptor (VDR also known as the calcitriol receptor) is a member of the nuclear receptor family of transcription factors. [5] Calcitriol (the active form of vitamin D , 1,25-(OH) 2 vitamin D 3 ) binds to VDR, which then forms a heterodimer with the retinoid-X receptor .
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 cells of our bone that is involved in bone formation and bone breakdown is osteoblast and osteoclast respectively. Osteoclasts are cells of bones that promote bone demineralization or bone resorption. [3] In contrast, Osteoblast promotes calcium absorption by the bone therefore, promoting bone mineralization and formation of new bones. [2]
They aim to reduce the classical renal and intestinal effects of calcitriol on calcium and phosphate homeostasis, from its effects on other biologic processes. [5] They target cell proliferation and differentiation, especially in skin, or other effects in the parathyroid gland (secondary hyperparathyroidism) or bone cells. [5]
The reduction in the blood calcitriol concentration acts (comparatively slowly) on the epithelial cells (enterocytes) of the duodenum, inhibiting their ability to absorb calcium from the intestinal contents. [2] [5] [28] [29] The low calcitriol levels also act on bone causing the osteoclasts to release fewer calcium ions into the blood plasma. [25]