'Everyone knows that fingernails keep growing, but no one really knows why', said lead author Dr Mayumi Ito, assistant professor of dermatology at New York University School of Medicine. Her study is the first to suggest that mammals may be able to regrow parts of their fingers thanks to pathways similar to those allowing salamanders to regrow entire limbs.
The researchers used cell-labelling techniques on genetically manipulated mice and identified a family of self-renewing stem cells in the nail bed which are responsible for nail growth. The stem cells are able to attract nerves to the area, a process that triggers further tissue regeneration through a protein called FGF2.
In one experiment a partially amputated mouse toe tip regrew after chemical stimulation of the nail stem cells.
The researchers also identified that the 'Wnt' signalling network – a molecular pathway responsible for nail and tissue growth in mammals and for limb regrowth in salamanders - was involved. 'I was amazed by the similarities', says Dr Ito. 'It suggests that we partly retain the regeneration mechanisms that operate in amphibians'.
'This is encouraging because the similarities give us hope that we will be able to induce human regeneration in the not-too-distant future', Dr Ken Muneoka, a molecular biologist at Tulane University in New Orleans, who was not involved in the study, told Nature News.
However, Dr Ashley Seifert, a regeneration biologist at the University of Kentucky, reminded the same publication that the parallels between mammal and salamander regeneration pathways are limited. Amphibians are able to regenerate a limb at any level of amputation and have no nail beds. Mammals, says Dr Seifert 'independently evolved the ability to regenerate digit tips through a mechanism dependent on the nail organ'.
Furthermore, in Dr Ito's experiments simply activating the Wnt pathway where there was too little tissue immediately below the nail bed (the nail epithelium) meant that the toe tip did not regrow.
'If the mechanism that they outlined was important', Dr Seifert suggested to Nature News, 'one would expect that the result of this experiment would be at least a partial stimulation of regeneration'.
Dr Ito's team plan to investigate the molecular mechanisms linking the Wnt signalling pathway to nail stem cells and bone and nail regrowth in future experiments.
Their current study was published in Nature.