Mechanism of Dorso-Ventral Axis Specification in Nuclear Transplanted Eggs of Xenopus laevis: (dorso-ventral axis/nuclear transplantation/subcortical rotation/gray crescent/Xenopus laevis).

Abstract

The rotation of subcortical cytoplasm relative to egg surface ("subcortical rotation", 28) is known to specify the dorso-ventral axis of fertilized Xenopus eggs. However, except for brief descriptions (14, 15), no information has been provided on the process of the dorso-ventral axis specification in nuclear transplanted Xenopus eggs. This study shows that (i), among the events that occur in association with dorso-ventral axis specification in fertilized Xenopus eggs, only the subcortical rotation occurs in a normal manner in both nuclear transplanted and non-nuclear transplanted (enucleated) Xenopus eggs, (ii), in these eggs, there is no factor that appears to affect the direction of subcortical rotation as the sperm entry point does in fertilized eggs: neither the site of pricking for activation nor that of injection for nuclear transplantation affects the direction of the subcortical rotation in those eggs and (iii) the subcortical rotation in nuclear transplanted eggs predicts very precisely the direction of their dorso-ventral axis as it does in fertilized eggs. These results suggest that matured Xenopus oocytes have a cytoplasmic potential to induce subcortical rotation along a particular meridian in the absence of a sperm aster and that the subcortical rotation induced by this cytoplasmic potential specifies the dorso-ventral axis in nuclear transplanted Xenopus eggs as in fertilized eggs.