Fig. 8. Role of Pitx genes in limb bud development. (A) Model for role Pitx1 and Pitx2 genes in hindlimb bud formation. The early co-expression of Pitx genes in the mesoderm of the limb bud field appears to be required for growth of bud mesenchyme in response to signals such as Fgf10. At this time, we do not have specific evidence to implicate Fgf10 more than other signals, except for the early expression of Pitx genes. This model is consistent with relatively normal signaling in Pitx mutant embryos. (B) As double null Pitx mutants could not be studied extensively, it cannot be excluded that Pitx genes are required for expression of Fgf10 and that they control the growth capacity of hindlimb bud mesenchyme in this way. (C) Differential control of limb bud outgrowth and specification by Pitx and Tbx genes. Previous work suggested that Tbx5 is a determinant for specification of forelimb identity whereas Pitx1 and the downstream Tbx4 gene both contribute to specification of hindlimb identity. The present work shows that Pitx1 and Pitx2 genes are required for hindlimb bud outgrowth but this function does not appear to require mouse Tbx4 (V. Papaioannou, personal communication). By contrast, Tbx5 appears to fulfill in forelimb buds an outgrowth function similar to that fulfilled by Pitx genes in hindlimb buds (Agarwal et al., 2003) (M. Logan, personal communication).