When a mammalian zygote divides, the two cells follow distinct fates. The first to undergo a second division contributes to the embryonic part of the blastocyst, whereas the other cell contributes to the abembryonic portion. Earlier, it was noted that the cleavage plane for the initial cell division event correlated with the location of the fertilization cone that formed after sperm entry into the oocyte. Piotrowska and Zernicka-Goetz have examined embryos that had never been fertilized and those from which cytoplasm at the site of sperm entry had been removed. In these embryos, the two blastomeres that formed in the initial cleavage event did not differ in their proportional contributions to the embryonic and abembryonic parts of the blastocyst. Hence, sperm is needed not only for its genetic content but also as a spatial cue for embryo patterning. — BAP
Development129, 5803 (2002).
