In most species, the polarity of an embryo underlies the future body plan and is determined from that of the zygote. However, mammals are thought to be an exception to this; in the mouse, polarity is generally thought to develop 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 implantation. To test whether early developmental events could provide cues that predict the axes of the postimplantation embryo, we have used the strategy of injecting mRNA encoding an enduring marker to trace the progeny of inner cell mass cells into the postimplantation visceral endoderm. This tissue, although it has an extraembryonic fate, plays a role in axis determination in adjacent embryonic tissue. We found that visceral endoderm cells that originated near the polar body (a marker of the blastocyst axis of symmetry) generally became distal as the egg cylinder formed, while those that originated opposite the polar body tended to become proximal. It follows that, in normal development, bilateral symmetry of the mouse blastocyst anticipates the polarity of the later conceptus. Moreover, our results show that transformation of the blastocyst axis of symmetry into the axes of the postimplantation conceptus involves asymmetric visceral 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.

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