BIPOLAR MEIOTIC SPINDLE FORMATION WITHOUT CHROMATIN

Establishing a bipolar spindle is an early event of mitosis or meiosis . In somatic cells, the bipolarity of the spindle is predetermined by the presence of two centrosomes in prophase. Interactions between the microtubules nucleated by centrosomes and the chromosomal kinetochores enable the formation of the spindle. Non-specific chromatin is suffic ient, however, to promote spindle assembly in Xenopus cell-free extrac ts that contain centrosomes [1,2]. The mouse oocyte represents an exce llent model system in which to study the mechanism of meiotic spindle formation because of its size, transparency and slow development. Thes e cells have no centrioles, and their multiple microtubule-organizing centers (MTOCs) are composed of foci of pericentriolar material [3,4]. The bipolarity of the meiotic spindle emerges from the reorganization of these randomly distributed MTOCs [4]. Regardless of the mechanisms involved in this reorganization, the chromosomes seem to have a major role during spindle formation in promoting microtubule polymerization and directing the appropriate rearrangement of MTOCs to form the two poles [5]. Here, we examined spindle formation in chromosome-free mous e oocyte fragments. We found that a bipolar spindle can form in vivo i n the absence of any chromatin due to the establishment of interaction s between microtubule asters that are progressively stabilized by an i ncrease in the number of microtubules involved, demonstrating that spi ndle formation is an intrinsic property of the microtubule network.