Thesis advisor: David Burgess / After fertilization, centration of the nucleus is a necessary first step for ensuring proper cell division and embryonic development in many proliferating cells such as the sea urchin. In order for the nucleus to migrate to the cell center, the sperm aster must first capture the female pronucleus for fusion. While microtubules (MTs) are known to be necessary for centration, the precise mechanisms for both capture and centration remain undetermined. Therefore, the purpose of this research was to investigate the role of MTs in nuclear centration. Fertilized sea urchin cells were treated with the pharmacological agent, urethane (ethyl carbamate), in order to induce MT catastrophe and shrink MT asters during pronuclear capture and centration. It was discovered that proper MT length and proximity are required for pronuclear capture, since diminished asters could not interact with the female pronucleus at opposite ends of the cell. Furthermore, shrinking MTs prevented nuclear centration in a dose-dependent manner, suggesting a role for cortical and/or cytoplasmic factors in force generation on the nucleus. Lastly, the result of an off-centered nucleus was the failure to center the mitotic apparatus and properly establish the division plane. The result of diminished MTs was asymmetric cell division, furrowing defects, and abnormal embryonic development. These results suggest that the role of the sperm aster is to capture the female pronucleus, migrate towards the cell center, and to potentially localize determinants necessary for embryonic development. / Thesis (MS) — Boston College, 2016. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Biology.