Abstract
[Introduction] The first mitotic cleavage plane correlates with the sperm entry position fertilization cone, or the second polar body (2pb) (Gardner et al., 1997; Karolina et al., 2002). Recently, Hiiragi et al. (2004) have shown that the first mitotic cleavage plane of the mouse egg is not predetermined but is rather defined by the topology of the two apposing pronuclei. [Methods] Mouse zygotes were produced by ICSI using oocytes and sperm from B6D2F1 mice. First, sperm heads were injected into oocytes at different positions at angles to the line of the metaphase II site and the center of egg (45° to 180°). Then male and female pronuclear position were observed 2 h interval until the early 2-cell embryo. Second, in order to examine the effects of the microtubule network on the migration of male and female pronuclei, eggs 2 h after ICSI were treated with 0.02 μg/ml demecolcine or 5 μg/ml cytochalasin B. Then the positions of male and female pronuclei were examined at 12 or 16 h after ICSI. Finally, the positions of the spindle, male and female chromosomes to the 2pb during the first cleavage were observed. [Results] The female pronucleus was always located in the space side of the 2pb and this tie is caused by a tie-microtubule cluster between the 2pb and female pronucleus. The male pronucleus migrated to the center of zygote and usually made angles 90 to 135° to the line of the female nucleus and the 2pb. Additionally, the final position of the male pronucleus was independent of the sperm injection site. In conclusions: The first mitotic cleavage plane of the mouse zygote is defined by the tie of the female pronucleus and the 2pb through a tie-microtubule cluster.
