The complex dynamic network of microtubule and microfilament cytasters of the leech zygote

The organization of the cytoskeleton in the early first interphase zygote a nd its involvement in organelle redistribution were studied in the glossiph oniid leech Theromyzon trizonare by confocal and electron microscopy, immun ofluorescence, and time-lapse video imaging after microinjection of labeled tubulin and/or actin and loading with a mitotracker. The cytoskeleton cons ists of an inner or endoplasmic and an outer or ectoplasmic domain. The inn er domain consists of a monaster whose fibers retract from the zygote perip hery by the end of the early first interphase. The outer domain is built up on a network of microtubules and microfilaments cytasters. Short pulses of microinjected labeled actin or tubulin and Taxol treatment demonstrate that cytasters are centers of microtubule and microfilament nucleation. Immunos taining with anti-centrophilin, anti-BX-63, and anti-AH-6 indicates that th e network of cytasters includes centrosomal antigens. Cytasters move in an orderly fashion at speeds of 0.5-2 mum/min, in an energy-dependent process retarded and finally blocked by the ATP analogue AMP-PNP and high concentra tions of Taxol. Colliding cytasters fuse and form larger cytoskeletal nucle ation centers. The leech zygote is a highly compartmentalized cell whose cy tasters function as articulated components of a very dynamic cytoskeletal s ystem engaged in bulk transportation of organelles during ooplasmic segrega tion. (C) 2000 Academic Press.