Sonic hedgehog (shh) expression in developing and regenerating axolotl limbs

Sonic hedgehog (shh) expression is detectable in the posterior mesenchyme o f many developing vertebrate limbs. We have isolated an RT-PCR fragment fro m the axolotl, Ambystoma mexicanum, that has high identity to other vertebr ate shh genes. We describe the localization of this transcript during devel opment and regeneration and in response to tissue grafts and retinoic acid (RA) exposure in the axolotl. Even though axolotl digits show a reversed po larity of differentiation (anterior [A] to posterior [P]) when compared to other tetrapods (P to A), shh is nevertheless expressed on the posterior ma rgin of developing and regenerating limb buds. When A cells are grafted adj acent to P cells, an ectopic domain of shh is induced. Exposure to retinoic acid (RA), a molecule known to alter pattern in all three limb axes in uro deles, results in ectopic expression of shh in anterior cells of the regene ration blastema. Prior to this induced expression in response to RA, there is an earlier response by the endogenous domain of shh, which is downregula ted within the first few hours of exposure. J. Exp. Zool. 284:197-206, 1999 . (C) 1999 Wiley-Liss, Inc.