It has long been thought that regenerating cells undergo a rejuvenation process to achieve pluripotency. However, a new study in Cell by Sustar and Schubiger challenges this view by showing that regenerating cells in Drosophila melanogaster imaginal discs do not revert to a 'younger', faster cell cycle — but instead have a unique cell cycle profile with characteristics of both younger and older cells.

Imaginal discs are small groups of epithelial cells that become determined late in larval development to form specific structures, such as wings and legs. The process of transdetermination in D. melanogaster imaginal discs is a well–characterized model system for cell plasticity. Transdetermination can be induced by disc injury ('disc fragmentation') or activation of the Wnt-family wingless (wg) gene, which causes a change in cell fate — for example, from leg to wing — of a small subset of regenerating cells in a region of the disc known as the 'weak point'.

To find out whether regenerating cells in fragmented imaginal discs rejuvenate, Sustar and Schubiger analysed their cell cycle profile and doubling time. The cells did not revert to a 'younger' cell cycle with a shorter doubling time, and maintained a profile that was similar to that of unfragmented disc cells of the same age.

Induction of the Wg signalling pathway activates the enhancer of the vestigial (vg) selector gene, which is necessary for wing development and the induction of leg-to-wing transdetermination. To visualize leg-to-wing transdetermination, the authors used the vg regulatory element to make a fluorescent reporter construct. Following Wg overexpression, disc cells initially divided asynchronously, but after ∼2 days, cells in S phase were exclusively localized in the weak point region. This change in the cell cycle occurred before the reporter gene was visibly expressed and, therefore, before transdetermination. In the early phase of transdetermination, a greater proportion of transdetermining cells were in S phase compared with non-transdetermining cells and later, the cell cycle profile reverted to that of non-transdetermining cells. Strikingly, the cell cycle profile in the early phase of transdetermination was unique and did not match that of any specific developmental stage.

So, Wg induction seems to trigger an alteration in the cell cycle of transdetermining cells in the weak point. In addition, these cells are initially considerably larger than non-regenerating cells. So, might cell cycle induction or the activation of cell growth be sufficient to induce transdetermination? Sustar and Schubiger overexpressed several cell cycle and cell growth genes. They found that overexpression of the insulin receptor gene or Ras, but not other genes, mimicked the effects of Wg induction. The authors propose, however, that transdetermination probably requires more than a growth signal, and they suggest that Wg expression in the weak point has multiple functions — growth activation and an increase in developmental plasticity.