Unlike mammals, adult bony fish and some amphibians can regenerate amputated limbs. This `facultative' regeneration involves the Fgf-dependent formation and maintenance of a blastema, a mass of undifferentiated,proliferative mesenchymal cells. Now, on , Wills and co-workers reveal that the developmental machinery that regenerates amputated fins in zebrafish is also involved in homeostatic regeneration, the regular replacement of cells lost through daily wear and tear. They show that transgenic inhibition of Fgf receptors in uninjured zebrafish causes severe fin atrophy within 2 months. Furthermore, markers of blastema-based regeneration are expressed at low levels at the tips of uninjured fins, and mutations in other factors that are crucial for the regeneration of amputated limbs (for example, the kinase Mps1 and the ligand Fgf20a) cause the rapid,progressive loss of fin structures in uninjured fish. The researchers speculate, therefore, that the high facultative regenerative capacity of some organisms may be an evolutionary consequence of a crucial role for homeostatic regeneration in their tissues.
