Earliest evidence of cartilage and bone development in embryonic life.

Some aspects of the development of cartilage and bone during embryonic life are discussed in this review and an attempt is made to show that studies of development, even when performed on species far removed from humans, are relevant to clinical orthopedic surgery. Initially, some definitions of skeletal tissues and cells are presented to illustrate the nontrivial problem of how to tell whether cells are capable of becoming osteoblasts or chondroblasts and of depositing bone or cartilage. This leads to a discussion of the best criteria to use to identify differentiating osteogenic and chondrogenic cells. Cytodifferentiation is immediately preceded by the appearance of the membranous skeleton, consisting of the mesenchymal condensations in which bone and cartilage will develop. Condensation formation in normal development and defective condensations leading to abnormal skeletogenesis are related to cellular properties of mesenchymal cells. The remainder of the review is on the development of membrane bone in the craniofacial skeleton, subperiosteal bone in embryonic avian long bones, and subperiosteal and endochondral bone in developing mammalian long bones. In each case the emphasis is on the embryologic origins of the skeletogenic cells, cell migration, and the factors and influences involved in the initiation of cell differentiation. The relevance of developing systems to clinical practice is stressed throughout.