Developmental mechanisms underlying polydactyly in the mouse mutant Doublefoot

The pre-axial polydactylous mouse mutant Doublefoot has 6-9 digits per limb but lacks anteroposterior polarity (there is no biphalangeal digit 1). It differs from other polydactylous mutants in showing normal Shh expression, but polarizing activity (shown by mouse-chick grafting experiments) and hedgehog signalling activity (shown by expression of Ptc1) are present throughout the distal mesenchyme. The Dbf mutation has not yet been identified. Here we review current understanding of this mutant, and briefly report new results indicating (1) that limb bud expansion is concomitant with ectopic lhh expression and with extension of the posterior high cell proliferation rate into the anterior region, and (2) that the Dbf mutation is epistatic to Shh in the limb.