Confocal and video imaging of cytoskeleton dynamics in the leech zygote.

Ooplasmic segregation in the late interphase zygote of the leech Theromyzon trizonare is accomplished by reorganization of an ectoplasmic cytoskeleton formed by polar rings and meridional bands. The dynamic properties of this cytoskeleton were explored by time-lapse confocal and video microscopy. Cytoskeleton assembly was investigated in zygotes pulse-labeled with microinjected fluorophore-tagged or biotin-tagged dimeric tubulin and G-actin. Cytoskeleton disassembly was studied by comparing the linear dimensions of the cytoskeleton at different time points during late interphase. The relative distributions of F- and-G-actin were determined after microinjection of rhodamine-labeled actin and fluorescein-labeled DNase I. Results showed that labeled precursors were readily incorporated into a network of microtubules or actin filaments. Bipolar translocation of the rings and meridional bands was accompanied by the rapid assembly and disassembly of microtubules and actin filaments. Because labeled microtubules and microfilaments gradually decreased, the rate of cytoskeleton disassembly was greater than the rate of cytoskeleton assembly. Hence, ooplasmic segregation was accompanied by the rapid turnover of cytoskeletal components. Co-distribution of F- and-G-actin during mid and late interphase may favor polymer-monomer interchange. We conclude that cytoskeleton reorganization during foundation of cytoplasmic domains can be conveniently studied in the live leech zygote after microinjection of labeled precursors.