Robust growth termination is essential to ensure that organs reach their correct size and grow no further. The precise mechanism of growth termination and the relative contributions of reduced cell proliferation and increased cell differentiation are elusive, and it is not known to what extent these mechanisms may be conserved in different evolutionary contexts. In this issue (p. ), Dagmar Iber, Fernando Casares and colleagues combine quantitative three-dimensional measurements with mathematical modelling to investigate growth dynamics in the Drosophila eye disc. The authors show that, much as in other organs and species, the growth rate declines continuously in the eye disc. Moreover, they computationally evaluate how well different candidate growth laws fit with the observed kinetics of organ growth and differentiation, and find that both an exponential and an area-dependent decline in the growth rate fit the data, although the latter offers the most parsimonious explanation. By testing this model prediction in a Drosophila strain with smaller eyes, they confirm experimentally that the area growth rate declines in inverse proportion to the total eye disc area, even when the growth rates and relative areas are very different. The area-dependent growth mechanism proposed by the authors is an alternative model to explain the still unresolved issue of how organs know when to stop, and to stop consistently.
