Newer knowledge of skeletogenesis: macromolecular transitions in the extracellular matrix

Morphogenesis of skeletal tissues is accompanied by dramatic changes in the types and amounts of extracellular macromolecules synthesized. These changes correlate with the morphological and structural characteristics of each tissue type involved in skeletogenesis, viz. mesenchyme, cartilage and bone. At the beginning of skeletogenesis, a hyaluronate-rich extracellular matrix may provide a beneficial milieu for mesenchymal cell migration and proliferation, and prevent precocious differentiation. This matrix also contains type I collagen and possibly a unique sulfated proteoglycan. Cartilage differentiation, during endochondral osteogenesis, involves the removal of hyaluronate and the production of large amounts of type II collagen and a characteristic chondroitin sulfate-proteoglycan. Further complex transitions in arrangement and concentration of proteoglycan occur in the epiphyseal growth plate followed by sudden depletion at the site of initiation of bone formation. In addition, at this site, the type of collagen synthesized reverts from type II back to type I. Bone formation at periosteal sites also involves removal of proteoglycan. The collagen component of both osteoid and bone matrix is type I but the level of hydroxylation of lysine moieties may be significantly higher in osteoid. It is proposed that changes in extracellular matrix composition are important factors in the control of morphogenesis as well as in providing suitable structural properties to the developing skeletal tissues.