Mouse oocytes are induced by cytochalasin B to undergo ‘pseudo-cleavage’ in vitro into 2 equally sized and separable compartments. This response to the drug is dependent upon the meiotic state of the oocytes, as well as upon the presence of an intact zona pellucida. The resulting 2 cellular compartments can be completely separated from another and cultured in vitro. Each of the compartments possesses characteristic structural features. The most pronounced structural differences include: (i) the presence of a nucleus (germinal vesicle) and nucleolus in one compartment; (ii) the presence of microvilli on the surface of the anucleate, but not the nucleate, compartment; and (iii) the localization (segregation) of mitochondria at the periphery of the anucleate, but not the nucleate, compartment. The results presented suggest that pseudo-cleavage induced by cytochalasin B arises as a consequence of a limited interaction of the drug with the oocyte surface and/or cortex and that it may represent a topographical dissociation of transporting and non-transporting regions of the membrane. These and other features of mouse oocytes treated with cytochalasin B are of interest in view of the involvement of the oocyte zona pellucida and plasma membrane during meiotic maturation, fertilization, and early embryogenesis.

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