Axial rotation in rat embryos: morphological analysis and microsurgical study on the role of the allantois.

In mouse and rat embryos, the embryonic disc develops within a cup-shaped "egg cylinder" and consists of an inner layer of ectoderm and an outer layer of endoderm. Because of this configuration, the embryo first develops in a dorsally flexed position and then undergoes "axial rotation" to a ventrally flexed position. In the present study, we first analyzed the morphological process of axial rotation in rat embryos using novel reference axes set in the egg cylinder that remained invariant during the process. Our new perspective allowed us to demonstrate that the process consists of three movements which start at different stages of development: twisting of the upper body at stage 12/s7-8, twisting of the middle body at stage 13/s11-12, and twisting of the lower body (so called "tail") at stage 14/s15-16. Axial rotation is an interesting developmental event not only because it is such a dynamic process but also because it is one of the earliest morphological signs of body asymmetry. This asymmetry is strongly biased in that the tail almost always finishes up on the right side of the embryo for reasons that are still unknown. In the second part of the study, we performed microsurgical experiments to extend our previous finding that removal of the allantois results in random determination of tail sidedness. We demonstrated that an allantois transplanted from another embryo can prevent this abnormal sidedness in an embryos whose allantois had been removed and that transecting the allantois did not lead to abnormal tail sidedness. A possible explanation is that the allantois produces a chemical factor that controls tail sidedness.