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Bone Remodelling in Menopause Current Topics in Menopause 85 coupled. The half-life of the BMU is between six and nine months. It is estimated that at any given time are active between 1.5 and 2 million of these units in different places and functional times. After a year, 3-4% of cortical bone and 25- 30% of trabecular is renewed [1, 2]. Bone Structure The strength of bone is determined by its material composition and structure. Bone must be stiff and able to resist deformation, thereby making loading possible. Bone must also be flexible: it must be able to absorb energy by deforming, to shorten and widen when compressed, and to lengthen and narrow in tension without cracking. Bone must also be light to facilitate movement. A unique feature of bone is that it can serve these contradictory needs of stiffness yet flexibility and lightness yet strength [3]. Bone Matrix and Mineral Bone is composed of an orderly collagen matrix on which calcium and phosphate are deposited in the form of hydroxyapatite [3]. Collagen is deposited in a lamellar fashion and strengthened by multiple crosslinks, both within and between the triple-helical collagen molecules. The matrix also contains noncollagen proteins that are critical for regulating mineralization and strengthening the collagen backbone. The calcium-binding proteins include osteocalcin (bone Gla protein) and matrix Gla protein, which contain gamma carboxyglutamic acid and are vitamin K-dependent. The bone mineral is composed of complex, often incomplete, crystals of hydroxyapatite. They are relatively small, which is appropriate for a structure that can undergo strain with minimal microdamage. Bone Cells Osteoblasts (Fig. 1A) are derived from mesenchymal stem cells which are initially pluripotential and can also differentiate into muscle, adipocytes, cartilage, or fibrous tissue. There appears to be a population of precursor cells that have partially differentiated and ordinarily can further differentiate only into osteoblasts. These "determined osteoblast progenitor cells" are present in the periosteum and in the marrow stroma. In these initial stages play an important role Wnt proteins and bone morphogenetic protein (BMP) [4]. Cultured preosteoblasts and osteoblasts have a reproducible temporal pattern of collagen and alkaline
86 Current Topics in Menopause Muñoz-Torres and Martín phosphatase synthesis followed by synthesis of osteocalcin. Active osteoblast are directed to the bone surface, down regions of new bone formation with deposition of bone matrix (osteoid) in orderly lamellae and induce mineralization. Osteocytes (Fig. 1B) are osteoblasts converted in the process of remodelling [5]. The conversion of osteoblasts to osteocytes involves a reduction, but not a complete loss, of metabolic activity [6]. A critical feature of osteocyte differentiation is the development of an extensive network of cytoplasmic connections. This syncytium of interconnected cells is probably critical for sensing mechanical changes. Sclerostin is a humoral factor inhibitor of Wnt signalling produced by the osteocytes and inhibits osteoblastogenesis. Osteoclasts (Fig. 1C) are large, multinucleated cells that resorb bone by dissolving mineral and degrading matrix. These cells are related to monocyte/macrophage cells and are derived from granulocyte/macrophageforming colony units (CFU-GM) [7, 8]. Macrophage colony-stimulating factor (M-CSF) is required to initiate osteoclast differentiation. This stimulation activates the signalling pathways for survival and proliferation of osteoclast precursors and determined the expression of receptor activator of nuclear factor B (RANK). The emergence of RANK means the irreversibility of the differentiation into osteoclasts. Osteoclast progenitors are present in the marrow, spleen, and in small numbers in the circulation and during development, probably migrate to the bone from extramedullary haematopoietic sites [9]. A B C Figure 1: Bone cells: osteoblasts (A), osteocytes (B) and osteoclasts (C) (downloaded from http://citologiaehistologiatm1.blogspot.com/2009/04/la-celula-una-celula-del-latin-cellula.html). Bone Remodelling Cycle Remodelling cycle can be divided into four steps: activation, resorption, reversal and formation. Bone is not originally mineralized (osteoid) and subsequently mineralizes
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