Osteocytes/osteoclasts

Primary bone cells osteoblasts, osteocytes, osteoclasts /... 

The result of HE staining shows that cancellous blocks don t have any cells. All blood and bone cells are removed. The control specimens are red because they contain some kinds of cells including blood cells such as erythrocytes, leukocytes and bone cells such as osteoblasts, osteocytes, osteoclasts. HE staining show clearly that cancellous bone contain bony trabeculae and cells including blood cells around bony trabeculae and bone cells on bony trabeculae. 

Bone, or osseous tissue, is composed of osteocytes and osteoclasts embedded in a calcified matrix. Hard tissue consists of about 50% water and 50% solids. The solids are composed of cartilaginous material hardened with inorganic salts, such as calcium carbonate and phosphate of lime. 

Systemic regulators of osteoblast, osteocyte and osteoclast functions, and therefore of bone metabolism. The major bone-seeking hormones are parathyroid hormone (PIH), 1,25-dihydroxy vitamin D3 (calcitriol) and the various ex hormones. 

The major cell types involved in bone resorption and deposition are osteoclasts and osteoblasts (Figure 48-11). The former are associated with resorption and the latter with deposition of bone. Osteocytes are descended from osteoblasts they also appear to be involved in maintenance of bone matrix but will not be discussed further here. 

Figure 7.2. Schematics of bone anatomy (a) the structure of a long bone demonstrating the distribution of the two different tissue structures, cortical and cancellous bone, and (b) the cells present in bone osteoblasts, bone-forming cells found on surfaces osteocytes, bone cells embedded in the mineralised matrix and osteoclasts, bone-removing cells.

Nakamura, H. (1995) Localization of CD44, the hyaluronate receptor, on the plasma membrane of osteocytes and osteoclasts in rat tibiae. Cell Tissue Research 280 225-233... 

Figure 1. The cellular activity during bone remodelling. At the tip (cutting cone) multi-nucleated osteoclasts (OCLs) excavate the mineralised bone tissue. At some distance, after the resting zone, osteoblasts (OBLs) refill the tunnel with (osteoid) that is subsequently mineralised. Osteocytes (OCYs) are former osteoblasts that were entombed within the bone matrix, but remained connected to the bone surface by numerous long slender protrusions (not visible). Courtesy R. Schenk.

Bone sialoprotein, osteopontin, and osteocalcin are synthesized and deposited as the mineralization process begins and mineral nodules form (Stein and Lian, 1993). Bone sialoprotein contains the cell-adhesive arginine-glycine-aspartic acid peptide sequence and may thus mediate osteoblast adhesion on the extracellular matrix (Gehron-Robey, 1989). Osteocalcin, a calcium-binding protein, interacts with hydroxyapatite and is thought to mediate coupling of bone resorption (by osteoclasts) and bone formation (by osteoblasts and/or osteocytes) (Stein and Lian, 1993). 

Within cell lineage differentiation pathways, the inherent potency of the starting material may be restricted based on original source. Bone marrow derived cells have the ability to differentiate into eosinophils, erythrocytes, megakaryocytes, osteoclasts, and lymphoid cells [20]. However, these hematopoietic lineages have limited potential to become a distinct tissue such as liver. Mesenchymal stem cells, on the other hand, can differentiate into a broader array of distinct tissues such osteocytes, chondrocytes, adipocytes, myocytes, and bone marrow stromal cells [20]. This plurality of outcomes requires identification of circumstances under which an aberrant outcome is manifested. 

Figure 25 TEM microradiographs of individual bone cells (a) osteoclast, a multinucleate cell (b) osteoblast, mononucleate with surrounding matrix only partially mineralized (dark spots in fibrous matrix) (c) osteocyte, mononucleate, completely embedded in darkly colored mineralized tissue. Scale the nuclei of these cells are 5 p,m in diameter (courtesy of Lynn Ann Neff, Department of Orthopaedics, Yale University).

Bone cells and products collagen. With the advent of high-resolution transmission and scanning electron microscopy, the intimate relationships between the cells (Figures 25 (a)-(c)) that form (osteoblasts), maintain (osteocytes), and remodel (osteoclasts) mineralized bone tissues have been depicted and their products and reactions studied. The first extracellular products of the osteoblast are bioorganic molecules dominated by the fibrous asymmetric protein... 

Parathyroid hormone acts chiefly on kidney increasing renal tubular resorption and bone resorption of calcium it increases calcium absorption from the gut, indirectly, by stimulating the renal synthesis of la-25-dihydroxycholecalciferol (see Vitamin D). It increases the rate of bone remodelling (mineral and collagen) and osteocyte activity with, at high doses, an overall balance in favour of resorption (osteoclast activity) with a rise in plasma calcium con-... 

The skeleton is the body s principal reservoir of calcium and phosphorus. Contrary to its appearance, bone is a dynamic tissue, and calcium and phosphate are continuously deposited and released. Bone is a modified connective tissue consisting of a cellular component, an organic matrix and an inorganic (mineral) phase. Its cells are osteoblasts, osteoclasts, osteocytes, and osteoprogenitor cells. The last are a type of mesenchymal cell that can differentiate into any of the other three types and to which the other types can revert. 

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