Pulling backwards

One-cell worm embryos pull harder on their posterior centrosome, thus displacing the spindle toward the posterior and creating a smaller posterior cell. Now Stephan Grill, Joe Howard, Anthony Hyman (Max Planck Institute, Dresden, Germany), and colleagues have tracked centrosome fragments and determined that individual force generators at the anterior and posterior pull with equal strength, but that there are more of them at the posterior.

The fragments were liberated by ablating the central, anchoring portion of the centrosome. The fragments from the posterior centrosome flew out toward the cell cortex faster than did those from the anterior centrosome. They did not, however, fly toward a single focal point, which was a feature of some earlier models.

The simple velocity data did not distinguish between more motors or stronger motors. But after a look...