Generation of Ccnb3^−/− mice reveals a requirement for cyclin B3 in female meiosis. (A) Targeted mutation of Ccnb3. Top: Exons 6 and 7 are shown as black rectangles. A portion of the sequences of the wild-type allele and em1 mutant allele are shown, with guide RNA position in red and the complement of the protospacer adjacent motif (PAM) sequence in blue. Bottom: Schematic of cyclin B3 protein. (B) Immunoprecipitation and Western blot analysis of adult testis extracts using an anti-cyclin B3 monoclonal antibody. (C) Apparently normal folliculogenesis and oocyte reserves in Ccnb3-deficient females. Left: PFA-fixed, anti-MVH–stained ovary sections from 3-mo-old animals. Zoomed images show presence of primary/primordial follicles indicated by black arrows. Scale bars, 500 µm. Right: Oocyte counts from two 3-mo-old females of each genotype. PFA-fixed ovaries were sectioned completely and stained with anti-MVH. Stained oocytes were counted in every fifth ovary section and summed. Each point is the count for one ovary of one animal; means are 1,114 for Ccnb3^+/− and 987 for Ccnb3^−/−. (D) The scheme illustrates progression through the meiotic divisions until metaphase II arrest in oocytes of the mouse strain used. The graph on the right shows percentages of mature oocytes of the indicated genotypes that underwent GVBD within 90 min in culture after release, and oocytes that extruded PBs. n: total number of oocytes counted in >10 independent experiments (GVBD: 141% Ccnb3^+/−: 90.78%; 119 Ccnb3^−/−: 89.91%; PB extrusion (PBE): 258 Ccnb3^+/−: 78.13%; 342 Ccnb3^−/−: 9.06%).