The centriculum, a membrane that surrounds centrosomes, acts as a microtubule filter.

Centrosomes are the main microtubule-nucleating structures in dividing cells. They are considered membrane-less organelles, but in several cell types they are surrounded by ER-derived membrane. In early embryos, this membrane forms a dense membrane reticulum, named the centriculum, which was shown to affect centrosome structure and consequently microtubule nucleating capacity. The centriculum is adjacent to the centrosome's pericentriolar material (PCM) and to abundant, short pericentrosomal microtubules that surround the centrosome. Here we show that the centriculum serves as a microtubule filter, preventing the extension of a fraction of microtubules due to their collision with the centriculum. Changing microtubule number or stability results in a corresponding change in centriculum size, independent of the PCM, indicative of centriculum-microtubule interactions. Moreover, the porosity of the centriculum correlates with the density of microtubules extending beyond the pericentrosomal region, consistent with the centriculum acting as a microtubule filter. Finally, if microtubule-centriculum collisions result in microtubule catastrophe, the filter function of the centriculum could also explain the high concentration of soluble tubulin at the centrosome.