Organ growth is actively controlled and coordinated, both during development and during postembryonic life. This scaling is known to involve systemic signals that couple nutritional status to growth. Here, Clara Becker and colleagues characterise how one such signal, insulin-like growth factor (IGF), controls retinal growth in medaka fish (Oryzias latipes), which display life-long postembryonic growth. They first show that IGF ligands and receptors are expressed in the retinal ciliary marginal zone (CMZ), which functions as a stem cell niche. Using a chemical inhibitor that targets the IGF receptor Igf1r, they further report that IGF signalling regulates proliferation in the CMZ. In line with this, the authors demonstrate that constitutive activation of Igf1r in the CMZ results in increased neuroretinal size without affecting the stereotypical structure of the neuroretina, and without affecting the sizes of other body parts. Finally, the researchers reveal that Igf1r activation decreases cell cycle length in retinal progenitor cells, thereby specifically expanding this progenitor population, but not stem cells; this ultimately leads to increased neuronal cell numbers and hence increased retinal size. Together, these findings highlight that retinal growth can be uncoupled from overall body growth, and uncover a key role for IGF signalling in controlling retinal size, morphology and cell type composition.
