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Asymmetric Divisions of Embryonic Neuroblasts

Cell division in the gastrulating epithelial cells is invariably symmetric, producing two daughters of equal size. Embryonic neuroblast cells, however, delaminate from the epithelial sheet and undergo an asymmetric division to generate two cells of different size and of different developmental fates: a large neuroblast cell, and a smaller ganglion mother cell (GMC) [29, 30]. In most asymmetric divisions, the mitotic spindle is positioned in the cell through interactions between astral micro-tubules and specific cues at the cell cortex [31, 32]. These interactions align the spindle with information in the cortex, and they usually lead to the displacement of the metaphase spindle within the cell. As a result, division produces two cells of unequal size, and cortically placed developmental cues are differentially segregated between the two daughter cells. In Drosophila neuroblasts, the spindle aligns with cortical cues but it remains symmetrically localized within the cell during metaphase [33]. Only during anaphase does an asymmetry become apparent, as the mi-crotubule aster in the large nascent neuroblast cell becomes much larger than that in the smaller nascent GMC. The large aster appears to "push" the cleavage furrow away, toward the small aster, making the GMC smaller than the neuroblast.

During these asymmetric neuroblast divisions, the centrosomal proteins y-tubu-lin, CP190, and CP60 are symmetrically distributed on the centrosomes in metaphase [33]. By anaphase, however, all these proteins are present at higher levels on the centrosome in the large nascent neuroblast, presumably explaining why it nucleates a larger array of astral microtubules. Thus, the basis for this asymmetric division appears to be the development of an asymmetry between the two centrosomes during anaphase. Many of the proteins involved in setting up this asymmetric division in flies have now been identified [32]. Of particular importance is the localization to the apical cortex of two protein complexes, one containing Par6, Bazooka/Par3, and an atypical protein kinase C (aPKC) and the other containing Partner of Inscutable (Pins) and the Ga subunit of a heterotrimeric G protein [34]. Both of these complexes are highly conserved and play an important part in organizing various aspects of cell polarity in many cell types [35]. It remains a major challenge to understand how these cortically localized complexes influence both the positioning of the spindle within the cell and the recruitment of proteins to the centrosome in anaphase.

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