Ramifications of kinetochore-associated microtubule mesh for the mechanism of spindle assembly. (A) Plus end–directed motor CenpE facilitates formation of end-on attachments by transporting short microtubules along the kinetochore surface that delivers the plus ends into the kinetochore. As a result of end-on attachments, the minus ends of attached microtubules protrude from the kinetochore. (B) Astral microtubules approaching kinetochores may connect directly to the kinetochore surface or the minus ends of protruding short microtubules. Although either connection leads to a poleward force because of dynein activity, the force is generated at distinctly different locales: kinetochore proper versus distal ends of attached microtubules. A dense mesh of microtubules forming at early stages of spindle assembly increases the probability of the latter type of interaction.