Kinetochore fibers are not involved in the formation of the first meiotic spindle in mouse oocytes, but control the exit from the first meiotic M phase

During meiosis, two successive divisions occur without any intermediate S p hase to produce haploid gametes. The first meiotic division is unique in th at homologous chromosomes are segregated while the cohesion between sister chromatids is maintained, resulting in a reductional division. Moreover, th e duration of the first meiotic M phase is usually prolonged when compared with mitotic M phases lasting 8 h in mouse oocytes. We investigated the spindle assembly pathway and its role in the progressio n of the first meiotic M phase In mouse oocytes. During the first 4 h, a bi polar spindle forms and the chromosomes congress near the equatorial plane of the spindle without stable kinetochore-microtubule end interactions. Thi s late prometaphase spindle is then maintained for 4 h with chromosomes osc illating in the central region of the spindle. The kinetochore-microtubule end interactions are set up at the end of the first meiotic M phase (8 h af ter entry into M phase). This event allows the final alignment of the chrom osomes and exit from metaphase. The continuous presence of the prometaphase spindle is not required for progression of the first meiotic M phase. Fina lly, the ability of kinetochores to interact with microtubules is acquired at the end of the first meiotic M phase and determines the timing of polar body extrusion.