Limb-type-specific expression of Tbx5/Tbx4 plays a key role in drawing distinction between forelimb and hindlimb. We showed that some conditions to regulate the Tbx expressions, in order to get insights into specification and determination during commitment of limb-type identity during morphogenesis. Transplantation experiments revealed that axial structures medial to the lateral plate mesoderm at the level of wing region are capable of transforming the leg identity to the wing one, suggesting that soluble factor(s) secreted from the median tissues is involved in the limb-type determination. We propose a model for limb-type identification in which the inhibitory regulation is involved in restricting one Tbx gene expression by masking the other Tbx expression there. Emerging developmental studies contribute to our understanding of vertebrate evolution because changes in the developmental process and the genes responsible for such changes provide a unique way to evaluate the evolution morphology.
Limbs, the locomotor organs that are unique to vertebrates, are often involved in the fields of paleontology and phylogeny because their structure is highly visible and their bony pattern is easily preserved in the fossil records. On the basis of the latest knowledge and viewpoints, we examined the evolution of vertebrate limb development with using embryos of cartilaginous fishes. We obtained some embryos of species of skate and shark, and cloned some PCR fragments for Tbx5/Tbx4 genes. Tbx5 is only expressed in the pectoral fin bud in skate and shark embryos whereas Tbx4 is exclusively expressed in the pelvic fin bud, suggesting that these species have distinct identity between pectoral fin and pelvic fin recognized by the specific expression of Tbx genes.