In vivo inhibition of programmed cell death by local administration of FGF-2 and FGF-4 in the interdigital areas of the embryonic chick leg bud.

The formation of the digits in amniote vertebrates is accompanied by a massive degeneration process that accounts for the disappearance of the interdigital mesenchyme. The establishment of these areas of interdigital cell death (INZs) is concomitant with the flattening of the apical ectodermal ridge (AER), but a possible causal relationship between these processes has not been demonstrated. Recent studies have shown that the function of the AER can be substituted for by implantation of beads bearing either FGF-2 or FGF-4 into the apical mesoderm of the early limb bud. According to these observations, if the onset of INZs is triggered by the cessation of the AER function, local administration of FGFs to the interdigital tissue prior to cell death should delay or inhibit interdigit degeneration. In the present study we have confirmed this prediction. Implanting Affi-gel blue or heparin beads pre-absorbed with either FGF-2 or FGF-4 into the interdigital tissue of the chick leg bud in the stages prior to cell death stimulates cell proliferation and causes the formation of webbed digits. Vital staining with neutral red confirmed an intense temporal inhibition of interdigital cell death after FGF treatment. This inhibition of interdigital cell death was not accompanied by modifications in the pattern of expression of Msx-1 or Msx-2 genes, which in normal development display a domain of expression in the interdigital tissue preceding the onset of degeneration.