Determined by the ectoderm. The dorsal-ventral axis in the distal digits is upside down. Wnt7a gene is expressed in dorsal ectoderm of the limb. Loss of function mutant for Wnt7a has solepads on both surfaces. Activity of the Lmx1 protein specifies dorsal by activating dorsal limb genes.
Cell death occurs by activation of cell death genes.
BMP4 is present in the interdigital spaces in chick limbs. If expression of BMP4 is prevented, no apoptosis occurs.