Hypertonicity-induced projections reflect cell polarity in mouse metaphase II oocytes: involvement of microtubules, microfilaments, and chromosomes.

A previous study showed that with hypertonic sucrose treatment, a projection is formed in mouse metaphase II (MII) oocytes in proximity to the spindle and chromosomes, where a polarized cortical domain is located. However, little is known about the mechanisms involved in this process. Here, we designed a series of experiments to test the hypothesis that hypertonicity is the induction factor for the formation of projections in mouse MII oocytes. Our hypothesis was supported by the following evidence: 1) different concentrations of sucrose affected the formation and shape of projections, whereas serum or basic media had little effect; 2) other hypertonic sugar solutions could also induce projection formation; and 3) projections could also be induced by hypertonic NaCl solution. We then tested the hypothesis that the cytoskeleton was involved in the formation of hypertonicity-induced projections. This was investigated by culturing MII- and germinal vesicle-stage mouse oocytes in the presence or absence of cytoskeletal inhibitors, including cytochalasin B (disruption of actin filaments), nocodazole (disruption of microtubules), and taxol (polymerization of tubulin molecules). We found that none of the cytoskeletal inhibitors alone could prevent hypertonicity-induced projection formation, whereas the combination of cytochalasin B with nocodazole or with taxol blocked the formation of these projections in most matured oocytes. When immature oocytes were incubated in cytochalasin B, but not in nocodazole or taxol, the formation of an actin-rich domain and the peripheral positioning of the spindle were blocked during maturation; hence, no projections were formed, even after hypertonic sucrose treatment. Based on these observations, we propose that three components are necessary for projection formation: 1) a polarized cortical patch (e.g., an actin-rich domain), 2) rigid submembrane structures (e.g., a spindle and/or chromosomes), and 3) solid connections between the above. Any disturbance of one of these factors will affect the hypertonicity-induced projection formation. Hypertonicity-induced projection in mouse oocytes thus provides an experimental model for studies regarding cell polarity and the interaction between membrane and submembrane components.