Polarity of the mouse embryo is anticipated before implantation

In most species, the polarity of an embryo underlies the future body plan a nd is determined from that of the zygote, However, mammals are thought to b e an exception to this in the mouse, polarity is generally thought to devel op significantly later, only after implantation. It has not been possible, however, to relate the polarity of the preimplantation mouse embryo to that of the later conceptus due to the lack of markers that endure long enough to follow lineages through inplantation. To test whether early developmenta l events could provide cues that predict the axes of the postimplantation e mbryo, me have used the strategy of injecting mRNA encoding an enduring mar ker to trace the progeny of inner cell mass cells into the postimplantation visceral endoderm, This tissue, although it has an extraembryonic fate, pl ays a role in axis determination in adjacent embryonic tissue. We found tha t visceral endoderm cells that originated near the polar body (a marker of the blastocyst axis of symmetry) generally became distal as the egg cylinde r formed, while those that originated opposite the polar body tended to bec ome proximal, It follows that, in normal development, bilateral symmetry of the mouse blastocyst anticipates the polarity of the later conceptus. More over, our results show that transformation of the blastocyst axis of symmet ry into the axes of the postimplantation conceptus involves asymmetric visc eral endoderm cell movement. Therefore, even if the definitive axes of the mouse embryo become irreversibly established only after implantation, this polarity can be traced back to events before implantation.