Posterior spindle displacement is delayed until metaphase by an anterior tether.

In asymmetric divisions, the cell alters the normal process that centers the spindle in the mother cell, but how it regulates the timing of spindle movement is unclear. Now, on page 245, Labbé et al. describe a tethering system that resists premature movement of the mitotic spindle during asymmetric cell divisions in C. elegans embryogenesis.

It was known that when researchers cut spindle microtubules with a laser, both centrosomes moved toward their respective cell poles, but the posterior one moved more quickly. To find out what establishes these uneven pulling forces and when the asymmetry first arises, Labbé and his colleagues used a laser to sever microtubules at different times throughout the first cell cycle. If they destroyed the anterior centrosome in prophase, the posterior centrosome moved posteriorly. But after destruction of the posterior centrosome the anterior one stayed centered, suggesting that there is a force pulling toward the tail of the embryo and something resisting it—but not actively pulling—on the anterior end.

When the team ablated microtubules near the anterior centrosome but on the cortical side, they found that the whole spindle moved toward the posterior pole. They hypothesize that the cut releases a tether that anchors the anterior centrosome to the anterior cortex and resists the posterior pulling forces. By metaphase, the tether releases and the spindle moves to the posterior of the embryo in preparation for the asymmetric division.

Because microtubules are known to be more stable at the anterior cortex compared with the posterior, the researchers speculate that such asymmetry might contribute to the tether. They are exploring this hypothesis by monitoring changes in microtubule dynamics at the cell cortex throughout the cell cycle.