Mechanism of polar body formation in the mouse oocyte: an interaction between the chromosomes, the cytoskeleton and the plasma membrane*

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Abstract
The influence of mouse oocyte chromosomes on their immediate environment has been investigated following their dispersal by dissolution of the metaphase spindle with nocodazole. Small clusters of chromosomes become redistributed around the egg cortex in a microfilamentdependent process. Each cluster has the capacity, on removal from nocodazole, to organize a spindle that rotates to yield a polar body. In this process of spindle formation, the chromosome clusters are able both to promote tubulin polymerization in their vicinity and to recruit microtubule-organizing centres (MTOCs) which organize the polymerized tubulin into spindles. In addition each oocyte chromosome cluster, as well as the non-dispersed sperm-derived haploid group of chromosomes, induces a focal accumulation of subcortical actin (corresponding to a filamentous area devoid of organelles) and a loss of surface Concanavalin A binding activity (corresponding to a loss of surface microvilli) in the overlying cortex. This induction ceases with the formation of pronuclei whether or not the pronuclei migrate centrally. Pronuclear formation is sensitive to the action of nocodazole for up to 2–4h postinsemination, and pronuclear migration is totally sensitive to the drug. If pronuclei are blocked in a peripheral location by nocodazole they are associated with an elevation in Con A binding activity of the overlying membrane which corresponds to an area of the surface rich in blebby microvilli.