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.

[The growth and form development of the limb buds in vertebrate animals]. / Rost i stanovlenie formy pochek konechnostei pozvonochnykh zhivotnykh.

The development of the fin and limb buds involves a balance of centrifugal (active) and centripetal (passive) mechanical forces, the first of which acts to move the walls of these structures away from each other and the second holds them together. When the volume of the mesodermal core increases, the generated force meets with the resistance of the basal membrane, and as a result, the limb bud has a tendency to acquire cylindrical shape. Collagen fibers, individual mesenchymal cells, and their groups hold together the dorsal and the ventral wall of the limb bud, prevent the movement of these walls away from each other, and in this way direct bud growth along the proximodistal and the anteroposterior axes. The balance of the forces, which stretch the ectodermal layer, and those, which constrain it, have also been observed in the development of other body parts.

[The developmental patterns of the cartilaginous elements in vertebrate ontogeny]. / Zakonomernosti razvitiia khriashchevykh élementov v ontogeneze pozvonochnykh.

Comparative embryological data presented in the paper support an idea that chondrification of the mesenchyme does not begin until the latter becomes condensated. Size and density of the skeletogenous rudiments are not the same in different vertebrates. As a rule, in animals with much of the mesenchyme (chondrichthyans, amniotes), the prochondral condensations contain more cells and have both greater mass and density. The distribution pattern of the mesenchyme is also significant for the future development of the cartilaginous elements which at the earlier stages grow largely by the recruitment of surrounding mesenchymal cells. Such kind of the growth mode is probably most similar to the cartilage fusion mode: both processed take place in the absence of the perichondrium. The non-skeletal dense structures influence on the development of the cartilaginous skeleton primarily by determining distribution pattern of the mesenchyme, particularly the condensation of skeletogenous cells. THe growing cartilages themselves can influence mechanically on the surrounding organs.