Role for sperm in spatial patterning of the early mouse embryo

Abstract

Despite an apparent lack of determinants that specify cell fate, spatial patterning of the mouse embryo is evident early in development. The axis of the post-implantation egg cylinder can be traced back to organization of the pre-implantation blastocyst. This in turn reflects the organization of the cleavage-stage embryo and the animal-vegetal axis of the zygote. These findings suggest that the cleavage pattern of normal development may be involved in specifying the future embryonic axis; however, how and when this pattern becomes established is unclear. In many animal eggs, the sperm entry position provides a cue for embryonic patterning, but until now no such role has been found in mammals. Here we show that the sperm entry position predicts the plane of initial cleavage of the mouse egg and can define embryonic and abembryonic halves of the future blastocyst. In addition, the cell inheriting the sperm entry position acquires a division advantage and tends to cleave ahead of its sister. As cell identity reflects the timing of the early cleavages, these events together shape the blastocyst whose organization will become translated into axial patterning after implantation. We present a model for axial development that accommodates these findings with the regulative nature of mouse embryos.

Effect of sperm entry on blastocyst development after in vitro fertilization and intracytoplasmic sperm injection — mouse model

“…1). These results are consistent with previous studies in ICSI-derived monkey embryos [38] and mouse embryos fertilized in vivo [19,36,39]. Most importantly, the presented data suggest that the relationship established between an egg and a sperm in early embryo development is not altered by the method of spermatozoon introduction.…”

“…In NM mouse eggs there is a reduced tendency to be penetrated by sperm near the animal pole, which is marked by the second polar body [13], whereas the most preferred place for sperm attachment is the equatorial region of the egg [19,36]. Our observations showed that both NM and IVF oocytes are most often approached by sperm at the equatorial site.…”

“…In NM embryos, the fertilization cone (FC), which forms above male chromatin and indicates the sperm entry position, was marked by a small bead as previously described [36]. In order to mark the sperm entry site in IVF and ICSI embryos, a new labeling technique was developed.…”

“…Since the position of the incoming sperm has been linked to the embryonic-abembryonic axis formation in the blastocyst [36][37][38][39], we investigated this relationship in eggs where sperm entered the cytoplasm in different ways. We used the SEP provided by NM, IVF, or ICSI as a point of reference during preimplantation embryo development.…”

“…We used the SEP provided by NM, IVF, or ICSI as a point of reference during preimplantation embryo development. To follow this entry site, each embryo was labeled and then scored according to the SEP at the two-cell and blastocyst stage [36,37].…”