During development, it is just as crucial to have the right number of cells as it is to specify their fates correctly. Extracellular cues have long been thought to regulate cell number, but what are these cues? A paper in Cell now reveals that, in the Drosophila melanogaster eye, activation of the epidermal growth factor receptor (EGFR) triggers division and that this same signal regulates cell survival.

Baker and Yu focused on a group of cells that form late in the eye disc. By forming mutant clones within a wild-type tissue, they found that spitz, the ligand for EGFR, is important for division. Looking at different markers for cells that have entered the cell cycle, they then showed that the cells were arrested in G2.

This led them to wonder whether EGFR is required for G2/M progression. As suspected, EGFR mutant clones did not enter mitosis. Using EGFR overexpression, they then confirmed that autonomous EGFR is sufficient to trigger division.

Where does the trigger for EGFR activation come from? The authors showed that neighbouring cells provide this signal — a mechanism that might ensure coordination between the number of cell types in the tissue.

Once the cells are formed, the authors showed, EGFR mediates their survival — a role previously suspected and which is distinct from its function in G2/M progression. So it seems that EGFR has a dual function — ensuring that enough cells form and that, once formed, they do not die.