Regionalization of the axial skeleton derived from the somites: its ontogenesis and phylogenesis

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In the axial skeleton of mammals and birds, the ribs are exclusively in the thoracic region. The most part of the rib is consisuted of the distal rib, which develops depending on the surface ectoderm. Regionalization of the axial skeleton is, thus, considered to be differential development of the distal rib according to its location. An ectopic transplantation of the thoracic somite into the lumbosacral region in a 2-day chick embryo caused ectopic rib, which was shorter than the normal rib. In the course of development of this chimera, we found that the thoracic somite cells stopped their migration when they met the lumbosacral lateral plate mesoderm.On the other hand, we compared the axial skeleton development between chick embryos and mouse embryos and found that order of the ossification was different, which coincide with an idea that the regionalization of the axial skeleton independently occurs in the course of phylogenesis of mammals and birds.