The cortical rotation breaks the radial symmetry of the amphibian egg, specifying the orientation of the embryonic body axes. The entire outer cortex of the fertilised egg rotates relative to the mass of inner cytoplasm by an angle of about 30° about an axis perpendicular to the primary animal-vegetal axis. As a result of this cortical rotation, 'dorsal determinants', factors able to trigger the formation of the 'organiser' region of the gastrula, are displaced from the vegetal pole region to a more equatorial position where they become activated. The nature of these determinants is not known, however they are thought likely to act in the Wnt signalling pathway. If the cortical rotation is blocked experimentally, the dorsal determinants remain at the vegetal pole, no organiser forms and the embryo develops without dorso-anterior structures such as the notochord, central nervous system and head.

We are trying to understand the mechanism of the cortical rotation and of dorsal determinant translocation. A transient array of aligned, commonly-oriented microtubules that forms beneath the vegetal cortex during the rotation period is clearly implicated both processes. The determinants could be transported along these microtubules by direct association with the moving cortex and/or by association with subcortical particles or vesicles that move rapidly in the same direction by plus end-directed microtubule-based motor proteins. The cortical rotation movement is also thought to be generated by microtubule-based motor proteins rather than by the force of microtubule polymerisation, since polymerisation can be arrested during the rotation period without impairing the rotation (Houliston, 1994; ref).

We have demonstrated an essential requirement for kinesin-related proteins (KRPs) in the cortical rotation by microinjection beneath the vegetal cortex of an anti-peptide antibody recognising multiple Xenopus egg KRPs (Marrari et al, 2000 ; ref ). For more details and to view the movies click here.
We don't know which KRP(s) are involved, although Eg5 probably is not, despite the association of this abundant egg KRP with the subcortical microtubules (Houliston et al, 1994 ref; Chang et al, 1997 ref).

Dynein also plays a role in the cortical rotation, as we have shown by injection of the dynactin subunit dynamitin (Marrari et al, 2004; ref ). For more details and to view the movies click here.

We have developed an in vitro system for the study of the cortical rotation, in which microtubule movement dependent on KRPs and dynein is reactivated on isolated egg cortices. See Marrari et al, 2003 (ref). To view the movies click here.