Body wall morphogenesis: limb-genesis interferes with body wall-genesis via its influence on the abaxial somite derivatives.

The vertebrate body wall is regionalized into thoracic and lumbosacral/abdominal regions that differ in their morphology and developmental origin. The thoracic body wall has ribs and intercostal muscles, which develops from thoracic somites, whereas the abdominal wall has abdominal muscles, which develops from lumbosacral somites without ribs cage. To examine whether limb-genesis interferes with body wall-genesis, and to test the possibility that limb generation leads to the regional differentiation, an ectopic limb was induced in the thoracic region by transplanting prospective limb somatopleural mesoderm of Japanese quail between the ectoderm and somatopleural mesoderm of the chick prospective thoracic region. This ectopic limb generation induced the somitic cells to migrate into the ectopic limb mesenchyme to become its muscles and caused the loss of distal thoracic body wall (sterno-distal rib and distal intercostal muscle), without causing any significant effect on the more proximal region (proximal rib, vertebro-distal rib and proximal intercostal muscle). According to a new primaxial-abaxial classification, the proximal region is classified as primaxial and the distal region, as well as limb, is classified as abaxial. We demonstrated that ectopic limb development interfered with body wall development via its influence on the abaxial somite derivatives. The present study supports the idea that the somitic cells give rise to the primaxial derivatives keeping their own identity and fate, whereas they produce the abaxial derivatives responding to the lateral plate mesoderm.