Abstract
Sexual reproduction requires the generation of gametes, which are highly specialised for fertilisation. Female reproductive cells, oocytes, grow up to large sizes when they accumulate energy stocks and store proteins as well as mRNAs to enable rapid cell divisions after fertilisation. At the same time, metazoan oocytes eliminate their centrosomes, i.e., major microtubule-organizing centres (MTOCs), during or right after the long growth phases. Centrosome elimination poses two key questions: first, how can the centrosome be re-established after fertilisation? In general, metazoan oocytes exploit sperm components, i.e., the basal body of the sperm flagellum, as a platform to reinitiate centrosome production. Second, how do most metazoan oocytes manage to build up meiotic spindles without centrosomes? Oocytes have evolved mechanisms to assemble bipolar spindles solely around their chromosomes without the guidance of pre-formed MTOCs. Female animal meiosis involves microtubule nucleation and organisation into bipolar microtubule arrays in regulated self-assembly under the control of the Ran system and nuclear transport receptors. This review summarises our current understanding of the molecular mechanism underlying self-assembly of meiotic spindles, its spatio-temporal regulation, and the key players governing this process in animal oocytes.
Highlights
Sexual reproduction requires the generation of gametes, which are highly specialised for fertilisation
As microtubule organisation remains important for oocyte meiosis as well as for pronuclear As microtubule organisation remainselimination important before for oocyte meiosis as leaves well asa for pronuclear migration upon fertilisation, centrosome female meiosis gap, in which migration upon fertilisation, centrosome elimination before female meiosis leaves a gap, which microtubule nucleation and organisation occur in the absence of the canonical animal in
Meiotic spindle self-assembly initiated by Ran-GTP and possibly other chromatin-localised activities highlights an intriguing feature of female reproductive cells: the ovum cytoplasm accumulates moieties required for spindle formation to an extent and to a concentration that allows, once the right trigger is provided, assembly of a functional bipolar spindle without preformed microtubule-organizing centres (MTOCs)
Summary
As microtubule organisation remains important for oocyte meiosis as well as for pronuclear As microtubule organisation remainselimination important before for oocyte meiosis as leaves well asa for pronuclear migration upon fertilisation, centrosome female meiosis gap, in which migration upon fertilisation, centrosome elimination before female meiosis leaves a gap, which microtubule nucleation and organisation occur in the absence of the canonical animal in MTOCs, microtubule nucleation and organisation in the the absence of the canonical. Oocytes of most animal species occur go through two meiotic divisionsanimal without centrosomes, Oocytes of most animal species go through the two meiotic divisions without centrosomes, i.e., i.e., preformed MTOCs, Figures 2 and 3
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