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.