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As we know that the human skeleton is mainly composed of bone tissues which provides support for the body, and gives protection to the vital organs such as those in the cranial and thoracic cavities, and encloses medullary cavity containing bone marrow ,as well as bone tissues, also serve as reservoir of phosphate, calcium, and other ions. Bone tissues also provide mechanical and metabolic function to skeleton. Bone is a specialized connective tissue which comprises bone matrix, calcified extracellular material, and three major cell types (osteocytes, osteoblasts and osteoclasts). All bones on their external and internal surface are lined by the layer of connective tissues, (internal surface is surrounded by the endosteum and external surface is surrounded by the periosteum). Now I will write about the types of bone cell.
Osteoblast
It is a types of bone cell which helps to secrete the substance of bone like bone matrix, including type 1 collagen fibers, proteoglycans, and matricellular glycoproteins. Active osteoblast is located entirely at the surface of bone matrix, and wrapped by integrins by forming a single layer of cuboidal cells which is joined by the adherent and gap junction. After the completion of their synthetic activity some osteoblasts differentiate as osteocytes and captured in matrix-bound lacunae while some flatten and cover the matrix surface as bone lining cells. Osteoblast are large cell which is responsible for the synthesis and mineralization of bone during the initial bone formation and later bone remodelling, it also forms a closely packed sheet on the surface of the bone, from which cellular processes expand through the developing bone.
Osteocytes
As I have mentioned that some osteoblasts differentiate into osteocytes and enclosed in the lacunae. During the change from osteoblast to osteocytes, the cell continues many long dendritic processes which is surrounded by calcifying matrix, and come to occupy the many canaliculi. The almond shaped osteocytes are most abundant cells in bone exhibit crucially less Rough endoplasmic reticulum, smaller Golgi complexes, and more condensed nuclear chromatin than osteoblasts. Diffusion of metabolites between osteocytes and blood vessels which takes place through the small amount of interstitial fluid in the canaliculi. The communication of osteocytes takes place via osteoblast and bone lining. Osteocyte maintain the calcified matrix and their death is followed by matrix reabsorption. Osteocyte express many proteins with paracrine and endocrine effect in order to regulate bone remodeling.
Osteoclast
hey are very large, motile cells with multiple nuclei, and they are responsible for matrix reabsorption during bone growth and remodeling. The development of osteoclast requires two polypeptide which is produced by osteoblast: macrophage-colonystimulating factor (M-CSF), and the receptor activator of nuclear factor-ºB ligand (RANKL). In the active osteoclast the membrane domain contacts the bone to forms a circular sealing zone in order to binds the cell tightly to the bone matrix and surrounds an area with many surface projections, called the ruffled border at this process a specialized microenvironment is created between osteoclast and matrix where bone reabsorption takes place. Osteoclast pumps protons to acidify and promote dissolution of the adjoining hydroxyapatite, and releases matrix metalloproteinases and other hydrolytic enzymes from lysosome-associated secretory vesicles for the localized digestion of matrix proteins.
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