Secretion of Osteoid Matrix

Skeletal tissue mineralization (bone formation) is initiated by osteoblasts, which secrete the osteoid matrix (Fig. 9.4). They express type I procollagen in secretory vesicles together with matrix vesicles that pinch off from the membrane. The matrix vesicles are pushed away from the cell surface, possibly by the flow of fluid containing calcium and phosphate ions that are also transported through the cell from the extracellular fluid on the outer surface. Collagen fibers develop further away from the cell surface than from fibroblasts. 

Type I collagen secretion Osteoid (uncalcified bone) matrix Osteoblast process Calcified bone matrix 

Cell process in canaliculus Osteocyte (end-stage osteoblast) 

Osteoblast Transport of Calcium and Phosphate Ions to Matrix Vesicles 

Osteoblasts take up Ca2+ ions from the periosteal extracellular fluid using Na+/Ca2+-exchangers NCX1 and NCX3. Once in the cytosol, the Ca2+ ions must be transported to the osteoid matrix side (basal side) by calbindins, which require the active form of vitamin D (calcitriol) for synthesis and expression. The Ca2+ ions are passed out to the osteoid matrix through an ATP-dependent plasma membrane Ca2+-ATPase lb (PMCAlb). The orientations of the cells, the transporters, and the calbindins are described in detail in Sect. 10.4.1. Once in the osteoid matrix, the matrix vesicles take up the Ca2+ ions via an annexin transporter. 

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