MORPHOLOGICAL DIVERSITY OF THE AVIAN FOOT IS RELATED WITH THE PATTERNOF MSX GENE-EXPRESSION IN THE DEVELOPING AUTOPOD

The formation of the digits in amniota embryos is accompanied by apopt otic cell death of the interdigital mesoderm triggered through BMP sig naling. Differences in the intensity of this apoptotic process account for the establishment of the different morphological types of feet ob served in amniota (i.e., free-digits, webbed digits, lobulated digits) . The molecular basis accounting for the differential pattern of inter digital cell death remains uncertain since the reduction of cell death in species with webbed digits is not accompanied by a parallel reduct ion in the pattern of expression of bmp genes in the interdigital regi ons. In this study we show that the duck interdigital web mesoderm exh ibits an attenuated response to both BMP-induced apoptosis and TGF bet a-induced chondrogenesis in comparison with species with free digits. The attenuated response to these signals is accompanied by a reduced p attern of expression of msx-1 and msx-2 genes. Local application of FG F in the duck interdigit expands the domain of msx-2 expression but no t the domain of msx-1 expression. This change in the expression of msx -2 is followed by a parallel increase in spontaneous and exogenous BMP -induced interdigital cell death, while the chondrogenic response to T GF beta s is unchanged. The regression of AER, as deduced by the patte rn of extinction of fgf-8 expression, takes place in a similar fashion in the chick and duck regardless of the differences in interdigital c ell death and msx gene expression. Implantation of BMP-beads in the di stal limb mesoderm induces AER regression in both the chick and duck. This finding suggests an additional role for BMPs in the physiological regression of the AER. It is proposed that the formation of webbed vs free-digit feet in amniota results from a premature differentiation o f the interdigital mesoderm into connective tissue caused by a reduced expression of msx genes in the developing autopod. (C) 1998 Academic Press.