In most animal cells, the cytoplasm is organized by a radial array of microtubules — known as aster — that spans the whole cytoplasm. The minus ends of aster microtubules are anchored at the centrosome; the microtubules polymerize outwards, from their plus ends, which undergo dynamic instability. Ishihara et al. found that in very large cells, microtubule nucleation occurs away from centrosomes to enable asters to fill the whole cell. They carried out quantitative imaging on a cell-free system derived from Xenopus laevis eggs that mimics aster growth in interphase zygotes. In this system, new microtubule plus ends (displaying classic dynamic instability) were generated in the aster at locations far away from the organizing centre. Thus, aster growth in large cells seems to be sustained by microtubule nucleation away from the centrosome, which is presumably stimulated by pre-existing microtubules.