Fig. 2. Symmetric and asymmetric Numb distribution is observed during cortical cell divisions. (A) Illustration of the pair assay. The left panel shows a phase image of a field of single cortical cells 2 hours after plating into Terasaki microwells. The cells were monitored under the microscope and cells that divided were identified: arrows point to pairs of sister cells after 1 day in vitro (DIV). (B-I) Numb staining in cortical progenitor cells. E10-E12 cortical cells were dissociated, plated into Terasaki plates, fixed at different time-points, and stained for Numb. (B-E) Phase-contrast images of the fields shown in F-I. (B,F) A cortical progenitor cell with Numb distributed in a crescent on one side of the cell cortex. (C,G) A pair of sister cells fixed within 10 minutes after mitosis that was in the process of separating. Numb is seen in only one of the daughter cells, demonstrating that it can be asymmetrically segregated during mitosis. Arrow indicates the Numb-negative cell. (D-I) Pairs of sister cells identified 24 hours after plating. Numb may be asymmetrically (H) or symmetrically (I) distributed into the two daughter cells. Scale bars: (A) 100 µm; (B-H) 20 µm.