Disorganization, Meiotic Spindle

The spindle is a cytoskeletal structure present in every eukaryotic cell that must form before cell division in order to properly separate chromosomes between daughter cells [Prosser and Pelletier, 2017]. The spindle organizes itself in a bipolar configuration within the cell prior to cell division. Several hundred proteins are required to assemble a functioning spindle, and microtubules are the most abundant components of the machinery. Although the function of the spindle is similar between mitotic and meiotic cells, spindle formation occurs via distinct mechanisms in female germ cells with respect to other cell types (including male germ cells) [Dumont and Desai, 2012]. This is because spindle formation is generally driven by centrioles, which are lacking in eggs [Szollosi et al., 1972; Manandhar et al., 2005]. The processes in somatic cells and male germ cells versus those operating in oocytes are briefly described below. In this key event, a bipolar spindle configuration is not achieved. Alternatively, there may be some spindle fibers that are not of the appropriate length, shape or structure to ensure that chromosomes can be properly aligned at metaphase and equally distributed between daughter cells.

Somatic cells and male germ cells:

The spindle of mitotic cells and that of male germ cells is organized by the centrosome which is composed by a pair of centrioles surrounded by an amorphous pericentriolar material containing more than 100 proteins [Andersen et al., 2003]. Many proteins that are involved in regulating microtubule dynamics and spindle assembly checkpoint (SAC) are contained in the centrosome. The centrosome is the principal microtubule-organizing center (MTOC) in mammalian cells and plays a major role in controlling microtubule dynamics, nucleation, and kinetochore–microtubule attachments [Conduit et al., 2015]. Errors in these processes lead to structural and functional abnormalities in the mitotic spindle [Rivera-Rivera and Saavedra, 2016].

Centriole and centrosome duplication are tightly coordinated with DNA replication, mitosis, and cytokinesis and play key roles in regulating transitions through the cell cycle [Chan, 2011]. The centrioles, cylindrical particles composed by nine triplet microtubules [Gogendeau et al., 2015], duplicate by forming daughter centrioles oriented at right angles with respect to the parent centrioles and then become surrounded by separate pericentriolar material during S-phase [Bettencourt-Dias and Glover, 2007]. Before mitosis, the newly formed centrosomes move to the opposite site of the nucleus and originate the two poles of the mitotic spindle [Kellog, 1989; Paintrand et a.l, 1992; Chavali et al., 2012]. Microtubules begin to radiate away from the centrosome and move toward the metaphase plate forming the mitotic spindle. During the assembly of the mitotic spindle, some microtubule fibers attach to the kinetochores on chromosomes, some radiate from the spindle poles toward the cell cortex and others extend past the metaphase plate forming a region of overlap with spindle fibers originating from the opposite centrosome [Cassimeris and Skibbens, 2003; Prosser and Pelletier, 2017]. Although a bipolar spindle can be formed in the absence of centrosomes, having too many centrosomes can result in a morphologically abnormal spindle and increase the chance of chromosome missegration [Hinchcliffe, 2014; Nigg and Holland, 2018].

Oocytes: In mammalian oocytes, centrioles and centrosomes are absent [Manandhar et al.. 2005] and the meiotic spindle starts its growth from several MTOCs that substitute for the conventional centrosome pair. A mouse oocyte can have up to 80 of these MTOCs [Dumont and Desai, 2012]. These MTOCs gradually coalesce and surround the chromosomes [Schuh and Ellenberg, 2007]. Then, microtubules elongate forming a barrel-shape bipolar spindle. Recent data suggest that MTOCs undergo a three-step decondensation and fragmentation process that facilitate their equal distribution to the spindle poles [Clift and Schuh, 2015]. In addition, recent evidence has shown the presence of actin fibers in the mammalian oocyte spindle that are important for ensuring proper chromosome segregation [Mogessie and Schuh, 2017]. Evidence is also emerging about differences in spindle assembly between rodent and human oocytes. Specifically, human oocytes may lack MTOCs and spindle assembly is mediated by chromosomes and the small guanosine triphosphate Ran [Holubcová et al., 2015].

Spindle abnormalities in its structure and shape that can be recorded are: reduction of microtubule density, loss of barrel shape, monopolar or multipolar spindle, reduced distance between the poles [Ibanez et al., 2003; Shen et al., 2005; Eichenlaub-Ritter et al., 2007; Xu et al., 2012]. In addition, the use of enhanced polarizing microscope (Polscope/SpindleViewTM) allows the detection of reduction in the birefringency and reduced light retardance of the spindle, which are indicators of loss of organization, at doses below which spindle abnormalities are detected with more conventional immunofluorescence methods [Shen et al., 2005].

Spindle organization is generally assessed by fluorescent immunodetection of its components and confocal microscopy [Ibanez et al., 2003; Shen et al., 2005; Eichenlaub-Ritter et al., 2007; Xu et al., 2012]. Localization of proteins with a known role in spindle function is also assessed [Tong et al., 2002; Yao et al., 2004; Cao et al., 2005]. 3D live imaging of cells expressing fluorescent-tagged proteins provides the possibility to follow spindle function at high resolution, and to describe and measure abnormal parameters (e.g., spindle morphology, altered distance between the two poles, mono- or multipolarity) [Schuh and Ellenberg, 2007]. Enhanced polarizing microscope has also been used to assess spindle integrity in human oocytes during in vitro fertilization techniques [Wang et al., 2001a,b; Keefe et al., 2003; Staessen et al., 1997].

All eukaryotic cells possess a spindle that must be properly organized for normal cellular division. Thus, this key event, although typically measured in mouse and human cells, is theoretically relevant to any eukaryotic cell type.

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