Myosin-II puts the squeeze on asymmetric cell division

Abstract

Asymmetric cell division--where two dissimilar daughter cells are produced--relies on asymmetric positioning of the telophase spindle midzone, which specifies the cleavage furrow. Ou et al. (2010) now report in Science a mechanism of asymmetric midzone positioning driven by a polarized cortical distribution of the contractile motor myosin-II.

Figure 1. Comparison of spindle asymmetry establishment mechanisms for unequal cell division

(A) Asymmetric midzone positioning in the C. elegans zygote. The entire spindle is shifted toward the posterior cortex during anaphase by a biased pulling force exerted on astral microtubules (green lines). The resulting asymmetric position of the midzone (orange block) leads to cell division (red line) that generates a large AB blastomere and small P blastomere. (B) Asymmetric cell division in Drosophila embryonic neuroblasts. The metaphase spindle is positioned symmetrically along the apical-basal axis, but during anaphase, more extensive elongation of the apical side of the spindle skews the midzone toward the basal cortex. This results in cell division that generates a large neuroblast (NB) and a small GMC cell. (C) Cell migration and asymmetric divisions of Q neuroblast in C. elegans L1 larva. QL and QR cells migrate (red arrows) and generate QL.a and QL.p, QR.a and QR.p, respectively. After further migration (except for QL.p), each of the four neuroblasts divides asymmetrically (as shown above each) to generate a large cell (orange) and a small cell (grey). (D) Detailed observation of QR.a cell division revealed that the spindle starts out symmetrically positioned, as in Drosophila embryonic neuroblasts, but asymmetric spindle elongation coupled with anterior cortical contraction, driven by a polarized distribution of myosin-II (blue) and posterior expansion, lead to a biased position of the spindle midzone toward the anterior cortex.