Fig. 5.

Dynamic Wnt expression is stem-cell-independent. (A) In situ hybridizations of wntP-1 and wntP-2 in normal animals (“wild type”) or animals treated with 6,000 rads of γ irradiation 4 days (“irradiated”) before surgical removal of heads, trunks, and tails and fixation at time points (h, hours) after amputation. (Upper) wntP-1 expression was detected at posterior-facing wounds in irradiated head fragments at 18 h but not at 96 h. (Lower) wntP-2 expression was detected in the posterior of 96-h heads. Anterior, top. Arrows, new expression of wntP-1 or wntP-2. Panels represent ≥4 of 5 animals. (Scale bars: 200 μm.) (B) Model of regeneration polarity in head fragments. wntP-1 is induced at any wound. Posterior wntP-1 activates wntP-2 through β-catenin-1, possibly acting in parallel with other genes or resulting in activation of other genes that contribute to the polarity decision (gray arrows). The polarity decision is stem-cell-independent, but tail formation subsequently requires neoblast stem cells. Purple spots, wntP-1 expression. Green gradient, wntP-2 expression.