Microtubule translocation and polymerisation during cortical rotation in Xenopus eggs

Abstract

The development of dorsal axial structures in frogs depends on a process of cortical rotation in which the cortex of the fertilised egg becomes displaced with respect to the cytoplasm. An array of aligned microtubules that develops between the vegetal cortex and cytoplasm is implicated in generating movement. Rhodamine-tubulin was injected into eggs to allow patterns of microtubule movement and polymerisation in the vegetal array to be examined. Time-lapse video microscopy of living eggs showed that mo st of these microtubules move with the vegetal cytoplasm relative to the cortex, at the same speed as cytoplasmic pigment granules. This implies that movement is generated between the microtubules of the vegetal array and the cortex. A few microtubules we re also detected that appeared immobile with respect to the cortex. Rhodamine-tubulin became incorporated into vegetal microtubules when injected at any time during the period of cortical rotation. The newly formed microtubules connected the vegetal array and internal cytoplasm. This indicates that local outward-direct ed polymerisation continues in this region as the cortex translocates. Experiments with low doses of nocodazole showed that this continuing polymerisation does not contribute to the rotation. Concentrations of the drug that prevented tubulin polymerisatio n had no effect on the speed of rotation if applied after the vegetal array had formed. The same doses prevented movement if applied early enough to prevent the formation of the array. These observations support the idea that mechanochemical enzymes assoc iated with the vegetal microtubules translocate the cortex along microtubules anchored in the subcortical cytoplasm.