Quantitative approaches for the study of microtubule aster motion in large eggs

The proper positioning of microtubule (MT) asters underlies fundamental processes such as nuclear centration, cell polarity, division positioning, and embryogenesis. In large eggs and early blastomeres, MT asters may exhibit long range motions with atypical speed and precision to target their functional position. The biophysical mechanisms regulating such motions remain however largely unknown. The centration of sperm asters in sea urchin embryos is a stereotypical example of such aster long range motion. In this chapter, we describe methods developed in this system to (1) quantify sperm aster 3-D motion with confocal microscopy and automated image analysis and (2) severe a portion of astral MTs with a UV laser. These methods may serve as a template to dissect the generic mechanisms of aster motion and force production in other embryos and cell types.