(a) Live imaging and tracking of KTs. Maximum z-projection images of 2mEGFP-CENP-C (KTs, green) and H2B-mCherry (chromosomes, red) in the oocytes of young (2 months old) and aged (16 months old) BDF1 mice. KT signals are peak-enhanced and background-subtracted. The 3D plots show KT positions (spheres) and their tracks (lines). Same colours were used for homologous KTs. The aged oocyte underwent balanced predivision of sister chromatids (arrowheads and insets). Time after NEBD (h:mm). Scale bar, 5 μm. See also Supplementary Movie 2. (b) Distinct types of segregation errors during MI. Red spheres and lines indicate KTs that have segregated abnormally. See also Supplementary Fig. 1. (c) Balanced and unbalanced predivision are predominant over nondisjunction. The rates of segregation errors during MI in the oocytes from young BDF1, aged BDF1, and aged CD-1 (11 months old) mice. Fisher’s exact test was performed. ***P<0.001.