ABSTRACT
Bone is a complex living tissue that provides internal support for all higher vertebrates. It develops by osteogenesis, the process of ossification, starting out as a highly specialized form of connective tissue. Two major players in the formation of bone are the bone cells called osteoblasts (bone-forming) and the osteoclasts (bone-resorbing). During the process of ossification, osteoblasts secrete type I collagen, in addition to many noncollagenous proteins such as osteocalcin, bone sialoprotein, and osteopontin. Osteoblast-secreted extracellular matrix may initially be amorphous and noncrystalline, but it gradually transforms into more crystalline forms [1]. Mineralization is a process of bone formation promoted by osteoblasts and is thought to be initiated by the matrix vesicles that bud from the plasma membrane [2] of osteoblasts to create an environment for the concentration of calcium and phosphate, allowing crystallization. Collagen serves as a template and may also initiate and propagate mineralization independent of the matrix vesicles [3,4]. Eventually, some osteoblasts are surrounded by the bone matrix that they help to form and are called osteocytes. Despite their location, osteocytes are not metabolically inactive; they dissolve and resorb some bone mineral though osteolysis [5]. Bone resorption is in fact the primary function of another bone cell, the osteoclast, which can also digest calcified cartilage and is then called the chondroclast. Formation by the osteoblasts and resorption by the osteoclasts maintains bone in constant renewal as a dynamic tissue.
