EPIGENETIC MECHANICAL FACTORS IN THE EVOLUTION OF LONG-BONE EPIPHYSES

In developing vertebrate long bones in which endochondral ossification occurs, it is preceded or accompanied by perichondral ossification. T he speed and extent of perichondral apposition relative to endochondra l ossification varies in different taxa. Perichondral ossification dom inates early long bone development in extinct basal tetrapods and dino saurs, extant bony fish, amphibians, and birds. In mammals and lizards , perichondral and endochondral ossification proceed more synchronousl y. One of the most important epigenetic factors in skeletogenesis is m echanical loading caused by muscle contractions which begin in utero o r in ovo. It has been previously shown that the stress distributions c reated perinatally in the chondroepiphysis during human skeletal devel opment can influence the appearance of secondary ossification centres. Using finite element computer models representing bones near birth or hatching, we demonstrate that in vertebrates in which perichondral os sification significantly precedes endochondral ossification, the distr ibution of mechanical stresses in the ossifying cartilage anlagen tend s to inhibit the appearance of secondary ossification centres in the e nds of long bones. In models representing vertebrates in which endocho ndral ossification keeps pace with perichondral apposition, the appear ance of secondary centres is promoted. The appearance of secondary cen tres leads to the formation of bony epiphyses and growth plates, which are most common in mammals and extant lizards. We postulate that geno typic factors influencing the relative speed and extent of perichondra l and endochondral ossification interact with mechanical epigenetic fa ctors early in development to account for many of the morphological di fferences observed in vertebrate skeletons. (C) 1998 The Linnean Socie ty of London.