Blunt or flared, sheet like or C shaped – growing microtubule plus ends have been attributed a number of different morphologies. But which of the morphologies observed in multiple systems in vitro reflect the situation in vivo? On page , Johanna Höög, Claude Antony, Damian Brunner and colleagues describe a new approach for assessing microtubule morphology in Saccharomyces pombe, and use this approach to determine what growing microtubules look like in 3D inside the cell. They combine light microscopy with electron tomography to observe the re-growth of microtubules following wash-out of the depolymerising drug methyl benzimidazol-2-yl-carbamate – with a surprising result. Whereas previous studies reported growing plus ends to be mostly blunt or in sheets, the authors show that, in vivo, 90% of them have a flared morphology. By identifying and categorising growing microtubule ends, they also demonstrate that sheets are, in fact, the rarest of the end structures found. Based on their results, the authors propose a new model for microtubule growth that challenges the way this cellular process and its regulation is currently perceived: in contrast to the previously suggested ‘closing-sheet model’, they conclude that microtubule plus-end growth in S. pombe occurs through closure of a funnel-like structure.
