REGENERATION OF WING BUD STUMPS OF CHICK-EMBRYOS AND REACTIVATION OF MSX-1 AND SHH EXPRESSION IN RESPONSE TO FGF-4 AND RIDGE SIGNALS

We examined systematically the ability of chick limb bud stumps to reg enerate distal structures when fibroblast growth factor (FGF)-4 is app lied. When amputations were made within 600 mu m of the tip and FGF-4 applied either posteriorly or both apically and posteriorly, outgrowth of stump tissues occurred and a virtually complete skeleton developed . 'Regeneration' of distal structures was correlated with reactivation of Msx-1 and Shh expression. At proximal amputation levels where FGF- 4 did not lead to 'regeneration', neither Msx-1 nor Shh expression was induced. We also grafted cells from progressively more proximal level s of mouse limb buds to chick wing bud tips beneath the apical ridge a nd the pattern of reactivation of Msx-1 expression along the proximal distal axis of the limb buds was similar to that found in chick limb b uds.