Intracellular and extracellular control of the differentiation of cartilage and bone.

This paper provides an overview of one aspect of the differentiation of cartilage and bone, namely, the degree of control provided by the extracellular matrix and microenvironment. A brief review of the diagnostic features of cartilage and bone is followed by a discussion of stem cells, emphasizing how to identify them using cytochemical, ultrastructural or experimental procedures. The role of extracellular matrices in the initiation of differentiation is discussed with reference to the initiation of chondrogenesis in the vertebral skeleton of the embryonic chick and of osteogenesis in the mandibular skeletons of embryonic chick and mice. The role of extracellular matrices in the maintenance of the differentiated state is discussed with reference to the ability of chondrocytes to compensate for depletion of their extracellular matrices and to the maintenance of altered differentiated states in achondroplasia. Some emphasis is placed on the notion that skeletal cells can neither be considered nor studied in isolation. The epigenetic approach used in studies of growth and morphogenesis needs to be applied to studies on both the initiation and the maintenance of cytodifferentiation.