Correlative changes during early morphogenesis of the sacroiliac complex in squamate reptiles

We tested the "limb bud" hypothesis, which explains morphogenetic mechanisms of the formation of the sacroiliac skeletal complex in tetrapods. The hypothesis assumes that: 1) the destruction of the embryonal sacral myomeres and the appearance in their place of a sacral gap filled in with mesenchymal cells favor the development of the sacroiliac complex; and 2) the destruction of myomeres takes place under the influence of limb buds. We studied serial hystological sections of embryos from squamate reptiles with large limb buds (sand lizard, Lacerta agilis L.), small and short-living limb buds (slow worm, Anguis fragilis L.) and without limb buds (adder, Vipera berus (L.)). In embryos of the sand lizard, the hypaxial part of the second sacral myomere degenerated, whereas that of the first one survived in its cranial part. Thus, a large sacral gap was formed where two sacral ribs expanded later. They stretched in a manner similar to the sacral gap across the longitudinal axis of the body, the large ilium lying opposite them. In embryos of the slow worm, the sacral gap was of reduced size and was significantly beveled caudally. The only sacral rib and the upper part of the ilium, which lie within the sacral gap, were beveled in the same manner. In embryos of the adder, myomere destruction was not observed, and sacral ribs and the pelvic girdle did not arise. The obtained results generally agree with the limb bud hypothesis; therefore, it can be said that tetrapods possess a simple and effective morphogenetic mechanism by which the hind limbs create their own support on the axial skeleton.