Fig. 3. The pronuclei fail to congress in fue zygotes. By labelling the nuclei of zygotes, fixed at consecutive time points after in vitro fertilization, we have documented the following sequence of pronuclear events. (A,D) Congression of pronuclei around 12 minutes post-fertilization, in wild-type (A) and fue (D) zygotes. (B,E) Around 14-20 minutes post-fertilization the pronuclei fuse in wild type (B) but are separate in fue zygotes (E). (C,F) Around 22 minutes post-fertilization one zygotic nucleus is present in wild type (C) and two pronuclei are observed in fue (F) zygotes. Eggs fertilized with in vivo BrdU-labelled sperm, were fixed, and nuclear BrdU was immunodetected at the two-cell stage. In wild-type embryos (G), two BrdU-labelled nuclei (arrowheads) are detected, in contrast to fue embryos (H) where only one BrdU-labelled nucleus (arrowhead) is detected, although two nuclei are always present in both fue (J) and wild-type embryos (I). This confirms that fue zygotes have a defect in pronuclear congression, and therefore fusion of the pronuclei does not occur as opposed to wild-type zygotes. Scale bars: 10 µm.