Spindle microtubule organizing centres − Rakyat Biologi

Division apparatus in animal female meiosis differs greatly from the canonical mitotic one and even from division apparatus in spermatocytes. It owes its uniqueness to the absence of centrosomes (Hertig and Dams, 1967; Szollosi, 1972; Gard, 1992; Therkauf and Hawley, 1992; Manandhar et al., 2005). Centrosome elimination occurs during oogenesis, but its mechanism is poorly understood (Mahandhar et al., 2005). Centrosomes are restored after fertilization (Mahandhar, et al., 2000). In the absence of centrosomes during meiosis, chromosomes play a key role in spindle microtubule assembly (see 1.1.2.2. and 1.3.). After NEB, spindle microtubules nucleate in the vicinity of chromosomes and then, progressively, become organized into a bipolar structure (Dumont and Desai, 2012). This indicates that chromosomes themselves may be sites of MTOC location (see 1.1.2.1.). Alternatively, MTOCs present in the cytoplasm could participate in the spindle formation responding to chromosomal signals (see 1.3.1.).

In mouse oocytes, lacking centrioles, multiple MTOCs scattered in the cytoplasm have been observed (Fig.7; Schuh and Ellenberg, 2007). They have been suggested to originate after NEB from pre-existing cytoplasmic microtubule network. Initially, small MTOC asters are formed, which then are proposed to accumulate PCM (see 1.1.2.1.) and coalesce to produce an a-polar microtubule network in the area of chromosomes. Later, the MTOCs become sorted and organize intermediate multipolar spindle. Eventually, a bipolar barrel-shaped spindle with MTOCs at the poles is formed. MTOCs contain typical components of PCM, such as pericentrin and γ-tubulin (Gueth-Hallonet et al., 1993; Carabatsos et al., 2000). Nevertheless, the exact nature of the MTOCs is not understood.

To the contrary, in Drosophila oocytes, no prominent MTOC has been observed (Fig.7.; Therkauf and Hawley, 1992; Matthies et al., 1996). After NEB, microtubules start to assemble in the area of chromosomes and the free microtubules have been proposed to be captured by the chromosomes and then organized into a well tapered spindle structure (see 1.2.2.; Therkauf and Hawley, 1992; McKim and Hawley, 1995; Matthies et al., 1996; Skold et al., 2005; Cullen and Ohkura, 2001). Typical PCM components, including γ-tubulin and pericentrin, have not been found at the spindle poles (Therkauf and Hawley, 1992; Matthies et al. 1996; Tavosanis et al., 1997). γ-tubulin, however, has been shown located throughout the spindle, enriched towards the poles and at the central spindle (see 1.1.2.; Endow and Hallen, 2011; Hughes et al., 2011). γ-tubulin has been reported to be required for spindle assembly by Tavosanis et al. (1997) and Hughes et al., (2011), although this has not been confirmed by Wilson and Borisy (1998) or Endow and Hallen (2011). The exact microtubule nucleation site in Drosophila oocytes has not been identified.

In C.elegans oocytes, MTOC-like structures also have not been identified. After NEB, a diffuse cloud of microtubules progressively assembles into dense microtubule bundles, which eventually form a spindle (Mc Nally et al., 2006). γ-tubulin has been observed to initially concentrate at the nuclear envelope and then form a diffused cloud in the area of nucleus location before NEB, suggestive of its role in microtubule nucleation.

Overall, microtubules can assemble at distance from chromosomes, but eventually form a single structure at the area of chromosomes. Later this structure is organized into a bipolar spindle. Independently of whether the MTOCs are visible or not, the process likely requires γ-tubulin in all above systems.