Similar experiments using a filter to separate the tissues showed that these inductions only work if cell processes can extend through the filter and directly contact the responding cells. Vertebrate limb development Vertebrate limbs develop from limb buds. The vertebrate limb bud consists of a core of l oose mesenchymal mesoderm covered by an epithelial ectodermal layer . Cells within the progress zone rapidly divide, and differentiation only occurs once cells have left the progress zone . The apical ectodermal ridge at tip of limb bud induces the formation of the progress zone .
Pattern formation organizes cell types into their proper locations based on positional information.
The current model is that proximal-distal pattern formation is regulated by the amount of time a cell spends in the progress zone. Dorsal-ventral patterning is controlled by the overlying ectoderm. Pattern formation is regulated by the same signals in both limbs, although these signals are interpreted differently . Misexpression of Tbx4 in the forelimb region leads to leg-like structures in this region. Misexpression of Tbx5 in the hindlimb region leads to wing-like structures in this region.