Midbody formation triggers asymmetric fate specification in neural stem cells.

Fate determinants are segregated during asymmetric cell division (ACD) to generate distinct sibling cell fates, but determinants must not be deployed until fate can be specified asymmetrically. Determinants could be deployed after division, once sibling cell cytoplasms are separated, but potentially long after mitotic exit, when the cell state is uniquely plastic. Here we show that the midbody impedes fate determinant diffusion across the cytokinetic pore, allowing asymmetric fate specification to begin before cell division completes. Fate determinants sequestered at the membrane during mitosis are released immediately following nuclear division via the cell cycle phosphatase String. Our results identify the midbody as a key facilitator of ACD that allows fate determinants to be deployed before division, when cell state can be readily influenced.