Role for sperm in spatial patterning of the early mouse embryo

Despite an apparent lack of determinants that specify cell fate, spatial pa tterning of the mouse embryo is evident early in development. The axis of t he post-implantation egg cylinder can be traced back to organization of the pre-implantation blastocyst(1). This in turn reflects the organization of the cleavage-stage embryo and the animal-vegetal axis of the zygote(2,3). T hese findings suggest that the cleavage pattern of normal development may b e involved in specifying the future embryonic axis; however, how and when t his pattern becomes established is unclear. In many animal eggs, the sperm entry position provides a cue for embryonic patterning(4-6), but until now no such role has been found in mammals. Here we show that the sperm entry p osition predicts the plane of initial cleavage of the mouse egg and can def ine 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 tim ing of the early cleavages, these events together shape the blastocyst whos e organization will become translated into axial patterning after implantat ion. We present a model for axial development that accommodates these findi ngs with the regulative nature of mouse embryos.