(a,b) Univalents in human oocytes. Oocytes were stained for KTs (ACA, green) and DNA (Hoechst 33342, red). Oocytes from 28-year old (a) and 42-year-old (b) patients are shown. KT signals are peak-enhanced and background-subtracted. All sister and homologous KT positions were determined in 3D and the number of chromosomes was counted. Projection views from the top and side of the spindle are shown with signal interpolation in z. The outlines of chromosomes (bivalents, blue; univalents, red) and the positions of KTs (circles) are shown. Representative bivalents and univalents are magnified. Scale bars, 2 μm. See also Supplementary Movies 7 and 8. (c) The rate of oocytes with univalents. Separated chromosome units that have one of the homologous KTs and an inter-homologous KT axis with a tilt angle of >25° with respect to the spindle axis were categorized as univalents (n=6, 10). (d,e) Univalents are predisposed to predivision. The inter-sister KT distance (d) and the angle between the inter-sister KT axis and the spindle axis (e) were measured (n=276, 454, 6). (f) Homologous chromosome biorientation. The angle between the inter-homologous KT axis and the spindle axis was measured (n=276, 454). Boxes show the 25th–75th percentiles and whiskers show 1–99 percentiles. Two-tailed, unpaired Student’s t-test was performed. ***P<0.001. y.o., years old.