Assisted fertilization and embryonic axis formation in higher primates

In naturally fertilized embryos of various organisms, the spermatozoon provides a localized cue to initiate early embryonic patterning. In mice, the sperm entry point (SEP) may reorient the first cleavage division, which separates the zygote into two halves that follow distinct fates. However, it is unknown whether the mechanical injection of spermatozoa into an oocyte by intracytoplasmic sperm injection (ICSI), a technique commonly used in human assisted reproduction, possesses such a role. Rhesus macaque embryos fertilized by ICSI were examined in order to determine the consequences of placing the spermatozoon at specific positions in the ooplasm and whether this can provide new information about patterning in mammalian eggs. The SEP specified by the injected spermatozoa was most often localized near the first cleavage plane and was mainly distributed along the boundary zone that separates the embryonic and abembryonic parts of the monkey blastocyst. Moreover, the ICSI data, when compared with naturally fertilized mouse embryos, showed a similar outcome in terms of cleavage axes and first embryonic axis specification. As there are no studies to date regarding sperm entry in human oocytes and its influence on embryonic development, this investigation using the rhesus macaque as a clinical model is noteworthy.