MICROTUBULE TRANSLOCATION AND POLYMERIZATION DURING CORTICAL ROTATIONIN XENOPUS EGGS

The development of dorsal axial structures in frogs depends on a proce ss of cortical rotation in which the cortex of the fertilised egg beco mes displaced with respect to the cytoplasm. An array of aligned micro tubules that develops between the vegetal cortex and cytoplasm is impl icated in generating movement. Rhodamine-tubulin was injected into egg s to allow patterns of microtubule movement and polymerisation in the vegetal array to be examined. Time-lapse video microscopy of living eg gs showed that most of these microtubules move with the vegetal cytopl asm relative to the cortex, at the same speed as cytoplasmic pigment g ranules. This implies that movement is generated between the microtubu les of the vegetal array and the cortex. A few microtubules were 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 mic rotubules connected the vegetal array and internal cytoplasm. This ind icates that local outward-directed polymerisation continues in this re gion as the cortex translocates. Experiments with low doses of nocodaz ole showed that this continuing polymerisation does not contribute to the rotation. Concentrations of the drug that prevented tubulin polyme risation had no effect on the speed of rotation if applied after the v egetal 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 associated,vith the veg etal microtubules translocate the cortex along microtubules anchored i n the subcortical cytoplasm.