Formation of the first meiotic spindle (M I) in mouse oocytes

Abstract

During meiosis, two successive divisions occur without any intermediate S phase, to produce haploid gametes. The first meiotic division is unique in that homologous chromosomes are segregated while the cohesion between sister chromatids is maintained, resulting in a reductional division. Moreover, the duration of the first meiotic M phase is usually prolonged when compared to mitotic M phases, lasting 8 hours in mouse oocytes from germinal vesicle breakdown to the extrusion of the first polar body. We investigated the spindle assembly pathway and its role in the progression of the first meiotic M phase in mouse oocyte. During the first 4 hours, a bipolar spindle forms and the chromosomes congress near the equatorial plane of the spindle without stable kinetochore- microtubule end interactions. This late prometaphase spindle is then maintained for 4 hours with chromosomes oscillating 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 hours after entry in M phase). This event allows the final alignment of the chromosomes and the exit from metaphase. The continuous presence of the prometaphase spindle is not required for progression of the first meiotic M-phase. Thus, in contrast to mitosis, the time devoted to spindle 253 formation does not strictly determine the duration of M phase. Finally, the ability of kinetochores to interact with microtubules, likely controlled by post-translational modifications, is acquired at the end of the first meiotic M phase and determines the timing of polar body extrusion (Brunet, S., Santa-Maria, A, Guillaud, I’., Dujardin, D., Kubiak, J.Z. & Maro, B. 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. J. Cell Biol. , in press (1999)). In somatic cells the bipolarity of spindle is predetermined by the existence of two centrosomes in prophase. Interactions between the microtubules nucleated by centrosomes and chromosomal kinetochores enable the formation of the spindle. We investigated the role of the chromosomes in spindle formation by bisecting maturing oocytes and creating cytoplasts devoid of chromosomes. After bisection, oocyte fragments were cultured. Perfect bipolar spindles were observed in many cytoplasts similar to the spindles present in nucleated halves (Brunet, S., Polanski, Z., Verlhac, M.-H., Kubiak, J.Z. & Maro, B. Bipolar meiotic spindle formation without chromatin. Current Biology 8, 1231-1234 (1998)). Thus a bipolar spindle can form in vivo in the absence of any chromatin, demonstrating that this ability is an intrinsic property of the microtubule network. 3e Colloque SFp, Paris-Sud ‘99 icole Polytechnique, Palaiseau, 28 juin-2 juillet 1999