A key biological element of bone formation is an epithelial-like cellular organization which allows control of phosphate, calcium and pH during mineralization. All of these elements are present in vertebrate bone metabolism. Conversely mineral deposition at non-physiologic sites where calcium and phosphate are adequate has been shown to be regulated in large part by pyrophosphate.
Mineral accumulation in membrane vesicles is widely suggested, but does not correlate with a definitive stage of mineralization. Mineral accumulation in dense collagen is, at least in major part, mediated by amorphous aggregates - often called Posner clusters - of calcium and phosphate that are small enough to diffuse into collagen fibrils. Mineralization in true bone results from stochastic nucleation of hydroxyapatite crystals within the cross-linked collagen fibrils. Cartilage mineralization is coordinated with high expression of tissue nonspecific alkaline phosphatase and PHOSPHO1 to harvest available phosphate esters and support mineralization of collagen secreted locally. Cartilage and mineralized cartilage are skeletal elements separate from bone, but with metabolic features common to bone. In jawless fishes without skeletons, tooth formation included epithelial transport of phosphates, a process echoed later in bone physiology. Phylogenetically the most ancient densely mineralized tissue is teeth. Three physiologically mineralizing tissues - teeth, cartilage and bone - have critical common elements and important evolutionary relationships. It was concluded that osteoblasts expressed several tight junction-associated proteins, which possibly regulated ion transport across the bone membrane. Primary osteoblasts cultured in the Snapwell for 19-26 days were found to form a monolayer with measurable transepithelial resistance of approximately 110-180 Omegacm(2), confirming the presence of barrier functions of the tight junction. In addition, immunohistochemical studies in decalcified tibial sections demonstrated the expression of claudin-5, -11, -14, -15 and -16 in bone-lining cells (inactive osteoblasts). A confocal immunofluorescent study in undecalcified tibial sections confirmed that claudin-16 was localized on the trabecular surface, normally covered by osteoblasts and bone-lining cells. By using western blot analyses of selected claudins, expression of claudin-5, -11, -14 and -15, but not claudin-3, were identified in osteoblasts.
Quantitative real-time PCR showed that osteoblasts expressed ZO-1, -2, -3, cingulin, occludin, claudin-1 to -12, -14 to -20, -22 and -23. We therefore studied the expression of tight junction-associated genes in primary rat osteoblasts and bone tissues. However, the evidence for the presence of tight junction-associated proteins in osteoblasts is lacking. Osteoblasts were previously reported to form tight junctions, which may play an important role in the regulation of ion transport across the epithelial-like bone membrane.
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