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By reciprocal transplantation experiments with regenerative and nonregenerative Xenopus limbs, we recently demonstrated that the regenerative capacity of a Xenopus limb depends on mesenchymal tissue and we suggested that fgf-10 is likely to be involved in this capacity (Yokoyama et al., 2000, Dev. Biol. 219, 18-29). However, the data obtained in that study are not conclusive evidence that FGF-10 is responsible for the regenerative capacity. We therefore investigated the role of FGF-10 in regenerative capacity by directly introducing FGF-10 protein into nonregenerative Xenopus limb stumps. Exogenously applied FGF-10 successfully stimulated the regenerative capacity, resulting in the reinduction of all gene expressions (including shh, msx-1, and fgf-10) that we examined and the regeneration of well-patterned limb structures. We report here for the first time that a certain molecule activates the regenerative capacity of Xenopus limb, and this finding suggests that FGF-10 could be a key molecule in possible regeneration of nonregenerative limbs in higher vertebrates.
FIG. 1. Regenerative capacity and differentiation of Xenopus limb buds. (A) A complete regenerate from a stage 52 limb bud. Upper inset shows a stage 52 limb bud. Note that only anterior digits (1, 2, 3) have claws. (B) A sample after amputation of stage 56 limbs. No regenerates were formed. Upper inset shows a stage 56 limb. (C) Cartilage staining shows that the cartilage pattern is almost completely formed at stage 56. The line indicates the plane of a section of the stage 56 limb bud shown in (D). (D) Muscle differentiation at the amputation level of a stage 56 limb bud. MF20 staining (green) shows that many well-differentiated muscle cells have already migrated to the amputation site. (E) Phase-contrast photograph of (D). a, anterior; p, posterior; epi, epidermis; msl, muscle cells; fe, femur (cartilage). All arrowheads indicate ampu-tation level (knee level). Bars, 1 mm for (A), (B), and upper inset in (B); 250um for upper inset in (A) and (C);50um for (D) and (E).
Fig. 2. Effect of FGF-10 application on Xenopus limb regeneration. (A) A beads-implanted stump of a stage 56 limb amputated at knee level. Two affi-gel blue beads (arrows) were implanted near anterior and posterior sides of the amputated plane at the distal edge of the femur. (B) A sample after amputation and PBS-soaked beads implantation. Nothing is regenerated. (C and D) Regenerates from FGF-10-applied stage 56 limbs. Note that only anterior digits have claws (arrows). a, anterior; p, posterior. All arrowheads indicate amputation level (knee level). Bars, 1 mm for (B); 250 um for (A).
FIG. 3.Gene expressions in FGF-10-treated blastemas. (A) fgf-8 expression in an FGF-10-applied stage 56 limb 3 days after amputation. (B) shh expression in an FGF-10-applied stage 56 limb 8 days after amputation. (C) Gene expressions in an FGF-10-applied stage 56 limb 8 days after amputation. fgf-10 (C), fgf-8 (D), msx-1 (E), and Hoxa-13 (F) expressions were examined by in situ hybridization in serial sections. (G) Gene expressions in a PBS-applied stage 56 limb 8 days after amputation. fgf-10 (G), fgf-8 (H), msx-1 (I), and Hoxa-13 (J) expressions were not induced by the PBS-soaked beads. a, anterior; p, posterior; d, dorsal; v, ventral. All arrowheads indicate amputation level (knee level). Bars, 100 ﰁm.
