Microtubules (green) are sufficient to induce an actin cleavage furrow (red).

To partition cellular components into dividing daughter cells, the spindle apparatus induces the formation of a cleavage furrow, but results conflict about which specific components are required for this process. By physically dissecting the spindle apparatus in grasshopper spermatocytes, Alsop and Zhang now show on page 383 that bundled microtubules are the only structural constituent of the spindle apparatus required for inducing cleavage.Previous work has pointed indirectly to asters, the central spindle, and chromosomal passenger proteins as essential inducers of the cleavage furrow. In the new study, the authors used micromanipulation to create pockets of cell membrane containing specific components of the spindle apparatus. As long as functional microtubules are present in these artificial cells, cleavage furrows are induced and ingress while the microtubules undergo reorganizations characteristic of metaphase, anaphase, and telophase. Alsop and Zhang stress that other components of the spindle apparatus undoubtedly influence furrow formation when they are present, but only microtubules are essential.

The authors are now introducing various degrees of asymmetry in microtubule distribution in the artificial cells. The results suggest that shifting the zone where the plus-ends of spindle microtubules overlap redefines the location of the cleavage furrow. ▪