Vertebrate limb development.

The recent identification of Wnt-7a as a signalling molecule in dorsal/ventral patterning means that we now have a known signal for control of each of the three limb axes. Fibroblast growth factors can allow proximal/distal patterning and Sonic hedgehog gene expression signals anterior/posterior patterning. Networks of these signals not only coordinate cell responses, but also mutually maintain each other. A positive feedback loop is established which coordinates expression of Sonic hedgehog in mesenchyme cells of the polarizing region and Fgf-4 expression in overlying apical ridge ectoderm. Wnt-7a expression in dorsal ectoderm also influences Sonic hedgehog expression in the polarizing region. Initiation of development of a complete limb can be achieved with just one signal, a growth factor.