Sperm entry position provides a surface marker for the first cleavage plane of the mouse zygote.

The sperm entry position (SEP) of the mouse egg, labelled by placing a bead at the fertilisation cone, tends to be associated with the first cleavage plane (Piotrowska and Zernicka-Goetz: Nature 409:517-521, 2001). Nevertheless, in up to one-fourth of embryos the cleavage furrow did not pass close to the bead, and following the division the bead marked the cleavage plane in only 60% of cases. This raised the question of whether such variability arose from the labelling itself or had a biological basis. The zona pellucida was not responsible for this effect because similar results were obtained in its presence or absence. However, this variability could be attributable to the large size of the fertilisation cone relative to the SEP. Therefore, we have developed a means of fluorescently labelling sperm that can record the exact site of its penetration when the label transfers to the egg surface. This approach indicates that the SEP marks the first cleavage in the great majority (88%) of embryos. In conclusion, direct sperm labelling shows precisely the correlation between the SEP and the first cleavage, although there is natural variability in this process.