Patterns of chondrogenesis and calcification in the developing limb of the lizard, Calotes versicolor

The first skeletal condensation appears deep at the base of the limb bud near the somites, when the apical ectodermal ridge (AER) is maximally developed. Thereafter the skeletal elements generally appear in a proximodistal sequence but most of the mesopodial cartilages appear well after the metapodial ones and one of them, tarsalia-1, even after the phalangeal ones. The skeletal elements that fuse or "disappear" during the development are the cartilaginous condensation of fibulare, and the precartilaginous condensation of the distal centrale in the tarsus, and possibly the mesenchymatous condensation of the intermedium in the carpus. The calcification of all the long cartilages is perichondral and osseous while that of all the mesopodial and other cartilages, like epiphyses and sesamoids, is endochondral and nonosseous except the partly osseous astragalus and fibulare. The limbs of the mature adult have many sesamoids and metaplastic calcifications. The AER starts regressing after the appearance of the first skeletal condensation but is retained on the digital tips, though in a moderately regressed condition, almost till the time of the appearance of all the phalangeal condensations. These studies on the mesopodium differ with most studies on reptilian and avian mesopodia in favoring the view that very few skeletal condensations fuse or disappear during the development. They thus raise important issues concerning the ontogeny and phylogeny of the pentadactyl limb. While the AER has a substantial role in the limb morphogenesis, it most probably is not responsible for the information to mesoderm regarding the number, size, shape and relative position of the skeletal elements in the limb.