Splanchnic (visceral) mesoderm has limb‐forming ability according to the position along the rostrocaudal axis in chick embryos

Positioning of the limb is one of the important events for limb development. In the early stage of embryogenesis, the lateral plate mesoderm splits into two layers and the dorsal layer (the somatic mesoderm) gives rise to a series of distinct structures along the rostrocaudal axis, including the forelimb bud, flank body wall, and hindlimb bud. To determine whether positional information in the somatic mesoderm for regionalization along the rostrocaudal axis is also inherited by the ventral layer of the lateral plate mesoderm (the splanchnic mesoderm), experiments in which the splanchnic mesoderm was transplanted under the ectoderm in an in ovo chick system were carried out. Transplantation of the wing-, flank-, and leg-level splanchnic mesoderm resulted in the formation of wings, nothing, and legs, respectively. These results suggest that the splanchnic mesoderm possesses the ability to form limbs and that the ability differs according to the position along the rostrocaudal axis. The position-specific ability to form limbs suggests that there are some domains involved in the formation of position-specific structures in the digestive tract derived from the splanchnic mesoderm, and results of cell fate tracing supported this possibility. In contrast, analysis of shh expression suggested that the anteroposterior polarity in the limb region seems not to be inherited by the splanchnic mesoderm. We propose that the positioning of limb buds is specified and determined in the very early stage of development of the lateral plate mesoderm before splitting and that the polarity in a limb bud is established after the splitting of the mesoderm.