Microsurgical study on the mechanisms determining sidedness of axial rotation in rat embryos

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Abstract
Sidedness of left/right asymmetric body structures is strongly biased in most animals by mechanisms that are not well understood. In rat embryos, axial rotation starts at the 9–10 somite stage and is almost completed at the 17–18 somite stage. As a result, the ventrally flexed tail (caudal part of the body) and chorioallantoic placenta on the yolk sac take up their position normally on the right side of the embryo. Because the tail and chorion become connected via the allantois around the time when axial rotation takes place, we hypothesized that the allantois and possibly its connection to the chorion is important in determining sidedness of the tail. In the present study, we tested this hypothesis by surgically removing either the allantois or chorion before axial rotation started. Embryos were explanted at 8 am on Day 9 of gestation (presomite stage), and either the allantois or chorion was removed using microforceps. Embryos were then cultured in rotating bottles, and sidedness of the tail, chorioallantoic placenta, and bulboventricular loop (heart) was determined after 50 hours (∼25–26 somite stage). Removal of the allantois (n = 55) resulted in absence of the umbilical cord and a 49.1% incidence of inverted tail; a chorioallantoic placenta‐like structure developed on the yolk sac in the normal position. Removal of the chorion (n = 114) resulted in absence of the chorioallantoic placenta and a 20.2% incidence of inverted tail; the allantois either floated in the extraembryonic coelomic cavity or reached the yolk sac but did not form a chorioallantoic placenta‐like structure. Sidedness of the heart was not affected in either group. These results indicate that removal of the allantois results in complete loss of control of tail sidedness, whereas removal of the chorion results in only a partial loss of control. An incidental finding was that an avascular area developed on the yolk sac around the chorioallantoic placenta. This area became larger when the allantois was removed but disappeared when the chorion was removed.