The Cytoskeleton and Nuclear Disassembly during Germinal Vesicle Breakdown in Starfish Oocytes

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Abstract
In response to maturation-inducing hormone, prophase-arrested oocytes of the starfish Pisaster ochraceus resume meiosis and undergo nuclear disassembly during a process referred to as germinal vesicle breakdown (GVBD). Time-lapse video recordings of maturing oocytes reveal that the nucleus lengthens along the animal-vegetal axis of the oocyte directly prior to GVBD. Neither taxol (10 μM) nor microtubule-depolymerizing agents [colcemid (50 μM), colchicine (250 μM), or nocodazole (1 μM)] prevent the pre-GVBD changes in nuclear shape from occurring, although correlative microscopical studies demonstrate that microtubules are nucleated (taxol) or depolymerized (colcemid, colchicine, nocodazole) at the concentrations listed above. The microtubule-altering drugs also do not affect the time at which GVBD begins or ends. A 10 μM solution of the microfilament-disrupting drug cytochalasin B (CB), on the other hand, essentially eliminates the pre-GVBD elongation of the nucleus. CB also slightly delays the onset of GVBD and significantly lengthens the time required to complete GVBD. Such studies suggest that: (i) drug-sensitive microtubules are not required for GVBD to proceed in a normal fasion; (ii) the pre-GVBD changes in nuclear shape involve microfilament-mediated events; and (iii) cytochalasin-induced depolymerization of microfilaments retards the normal timing of GVBD.