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An osteoclast is a large multinucleated cell and human osteoclasts on bone typically have four nuclei [5] and are 150–200 μm in diameter. When osteoclast-inducing cytokines are used to convert macrophages to osteoclasts, very large cells that may reach 100 μm in diameter occur. These may have dozens of nuclei, and typically express major ...
The osteoclast then induces an infolding of its cell membrane and secretes collagenase and other enzymes important in the resorption process. High levels of calcium , magnesium , phosphate and products of collagen will be released into the extracellular fluid as the osteoclasts tunnel into the mineralized bone.
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).
Osteoclasts mature and/or migrate to discrete bone surfaces. Upon arrival, active enzymes, such as tartrate-resistant acid phosphatase , are secreted against the mineral substrate. [ citation needed ] The reabsorption of bone by osteoclasts also plays a role in calcium homeostasis .
Many believed osteoclasts and osteoblasts came from the same progenitor cell. Because of this, osteoclasts were thought to be derived from cells in connective tissue. Studies that observed that bone resorption could be restored by bone marrow and spleen transplants helped prove osteoclasts' hematopoietic origin. [3]
Osteoclasts break down bone tissue, and along with osteoblasts and osteocytes form the structural components of bone. In the hollow within bones are many other cell types of the bone marrow . Components that are essential for osteoblast bone formation include mesenchymal stem cells (osteoblast precursor) and blood vessels that supply oxygen and ...
It is postulated that osteoclasts are the cells responsible for the resorption of the root surface. [7] Osteoclasts can break down bone, cartilage and dentin. [8] Receptive activator of nuclear factor kappa-B ligand , also called osteoclast differentiation factor (ODF) and osteoprotegerin ligand (OPGL), is a regulator of osteoclast function.
Under normal conditions, osteocytes express high amounts of TGF-β and thus repress bone resorption, but when bone grows old, the expression levels of TGF-β decrease, and the expression of osteoclast-stimulatory factors, such as RANKL and M-CSF increases, bone resorption is then enhanced, leading to net bone loss. [24]