ABSTRACT
Bone, a highly specialized connective tissue, consists of cells housed in a mineralized extracellular matrix. The cells belong to two distinct populations: the bone-forming cells (osteoblasts, osteocytes and bone-lining cells) and the bone resorbing cells (the osteoclasts). Osteoblasts differentiate from pluripotent stem cells. They produce an unmineralized extracellular matrix, the osteoid, that mineralizes under their influence. Osteoblasts differentiate into quiescent bone-lining cells, or osteocytes, while some undergo apoptosis. Osteocytes, surrounded by mineralized matrix, show a characteristic stellate morphology. Resorbing cells, the osteoclasts, originate from the fusion of mononuclear cells of the hematopoietic stem cell lineage. These large multinucleate cells develop deep infoldings of the plasma membrane, such as the ruffled border and the sealing zone surrounding the ruffled border. The bone extracellular matrix consists of an organic matter containing collagenous proteins (90%), predominantly type I collagen, non-collagenous proteins, and lipids. The mineral phase consists of crystals of hydroxyapatite, a calcium phosphate containing impurities such as carbonates, bicarbonates, fluorite, sodium, potassium and magnesium. Unravelling the structure and function of bone cells and their relationships to the matrix should help us understand the dynamic nature of bone tissue.
