Abstract
Total regeneration of experimentally excised lens from the dorsal part of the iris-pigmented epithelium of newts has been a key model of tissue regeneration via cells originating from a foreign tissue. Due to the strict spatial restriction of the lens origin in the newt iris, it has often been assumed that only the dorsal iris cells are endowed with an intrinsic potential to give rise to lens tissues. However, our reinvestigation of the process revealed completely different mechanisms underlying lens regeneration and its spatial restriction, comprising the following two steps: (i) Fibroblast growth factor (FGF) 2-dependent proliferation of iris-pigmented epithelium and activation of early lens genes (Pax6, Sox2, MafB) over the entire circumference of the iris; and (ii) dorsal iris-restricted activation of the canonical Wnt signals (involving Wnt2b and Frizzeld4) that leads to localized expression of late lens genes (Prox1, Sox1, beta-crystallin). Injection of FGF2 into normal eyes specifically elicited the second lens development from the dorsal iris, and the administration of recombinant Wnt3a to the cultured iris-pigmented epithelium caused even ventral iris-derived lens development. Thus, it is concluded that the regulation of FGF2 and Wnt signals is a determinative of the iris-derived lens regeneration in the newt eye.
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