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Asymmetric cell division: microtubule dynamics and spindle asymmetry

Julia A. Kaltschmidt^* and Andrea H. Brand

Wellcome/Cancer Research UK Institute and Department of Genetics, University of Cambridge, Tennis Court Road, Cambridge CB2 1QR, UK
^* Present address: HHMI, Department of Biochemistry and Molecular Biophysics, Center for Neurobiology and Behavior, Columbia University, 701 West 168th Street, New York, NY 10032, USA

View larger version (20K): [in a new window]   Fig. 1. Chocolate cake with strawberry icing (A,B) and epithelial cell versus neuroblast division (C,D). (A) Symmetric division: a vertical slice divides the cake into two pieces of equal size and content, both containing the same amount of chocolate cake and strawberry icing. (B) Asymmetric division: a horizontal slice yields two pieces of unequal size and content, a large piece of chocolate cake and a small piece of cake with all of the icing. (C) Epithelial cells (red) divide symmetrically, giving rise to two equal cells. Cell fate determinants (green) localised at the basolateral cortex are partitioned equally to both daughter cells. (D) Neuroblast (blue) divisions are intrinsically asymmetric: the two daughter cells differ in cell size, mitotic potential and cell fate. Asymmetrically localised cell fate determinants, such as proteins or mRNAs (green), are segregated asymmetrically into the basal GMC. Apical is up and basal is down.

View larger version (21K): [in a new window]   Fig. 2. Three models explaining cleavage plane induction. (A) According to the astral relaxation model the asters signal to the cell cortex near the poles, inducing it to relax. The additional two models, equatorial stimulation through the asters (B) and equatorial stimulation through the spindle midzone microtubules (C), are based on induction of the equatorial region of the cell, either by signals from the asters (B) or from the overlapping microtubules of the spindle midzone (C). White arrows indicate stimuli and are pointed towards the induced area of the cell cortex (red). Microtubules are indicated as grey lines and centrosomes as filled grey circles (see also || footnote on p. 2258).

View larger version (32K): [in a new window]   Fig. 3. Possible models by which an unequal pulling force could be generated. (A) The force generated by the pushing apart of the overlapping midzone microtubules is unequal. (B) The force of the pulling astral microtubules is unequal. The microtubules on one side of the cell cortex are either destabilised (red lines in blue area in C) or stabilised (red lines in green area in D). Microtubules are indicated as black lines and centrosomes as filled black circles (see also || footnote on p. 2258).

View larger version (37K): [in a new window]   Fig. 4. Generation of eccentrically placed cleavage plane in different model organisms. (A) In the C. elegans zygote, the mitotic spindle is initially positioned symmetrically along the anterior-posterior axis. Anterior is left and posterior is right. The centrosome-pronucleus complex (pronuclei are represented as empty circles, centrosomes as small filled circles) is oriented along the anterior-posterior axis. During anaphase the anterior aster is stationary (black lines) while the posterior aster oscillates (red lines changing to pink) to a position closer to the cell cortex. At telophase (grey ovals represent telophase DNA) the spindle is asymmetrically positioned with the posterior elongated centrosome closer to the cell wall. Similarly, in the clam Spisula zygote, one aster is stationary while the other oscillates to a position closer to the cell cortex. (B) In vegetal cells of sea urchin embryos, lateral migration of the nucleus (empty circle changing from red to pink) led by a centrosome (small filled circle changing from red to pink) prior to spindle formation gives rise to an asymmetrically placed spindle. At telophase (grey ovals represent telophase DNA) the micromere aster (red) is fattened while the macromere aster (black) is spherical. (C) In the Drosophila neuroblast, the spindle forms symmetrically between the two spindle poles. Apical is up and basal is down. At the onset of anaphase, the microtubules appear to shorten on the basal side of the cell and elongate/enrich on the apical side. At telophase (grey ovals represent telophase DNA) the centrosome of the basal aster is smaller than that of the apical aster.