Microtubules (MTs) are fundamental components of the cytoskeleton, which can be categorised as centrosomal or non-centrosomal MTs. However, little is known about the regulation of non-centrosomal MTs. In their latest study, Masatoshi Takeichi and colleagues investigate the function of CAMSAP3, which is involved in non-centrosomal MT assembly. Using a genetic loss-of-function mouse model, the authors show that CAMSAP3-deficient brains have increased fusion and stenosis that narrows the lateral ventricles. Imaging and labelling techniques reveal that CAMSAP3 mutants have fewer neural stem cells in the ventricular-subventricular zone (adjacent to the ventricles). Furthermore, ependymal cells, which line the ventricles, are fewer in number and have reduced apical surface areas. The researchers show that, in ependymal cells, the apical domain is regulated by mTORC1 located on lysosomes, and both mTORC1 signalling and lysosome numbers are reduced at the apical domain of ependymal cells in the CAMSAP3 mutants. Finally, they show that the most apical portion of the MT network is depleted in CAMSAP3-deficient cells. Together, these data indicate a mechanism whereby CAMSAP3 activity is required to produce the apical-most MT network in ependymal cells, which is necessary for trafficking lysosomes to the apical domain. Here, mTORC1 signalling regulates apical domain size and, subsequently, morphology of the lateral ventricles.