Echinoderms - Fertilization

Pronuclear migration in L. variegatus. The male pronucleus is colorized in blue; female in pink. Frames from a movie courtesy of Shinya Inoue, Marine Biollgical Laboratory, Woods Hole.

Pronuclear migration is a dramatic process in many species, including sea urchins. Microtubules, nucleated by the sperm's centrosome, serve as guides for the movement of the female pronucleus, a process mediated by the microtubule motor cytoplasmic dynein.

Microtubules during pronuclear migration in L. pictus. The small male pronlucleus and the larger female pronucleus are stained blue with a DNA dye; microtubules are immunostained in green. Courtesy of Jon Holy, Univ. of Minnesota-Duluth.

In the classic footage below, courtesy of Shinya Inoue, Marine Biological Laboratory, Woods Hole, the migration of the female pronucleus towards the male pronucleus can be seen. Because it is a microtubule-dependent process, pronuclear migration can be reversibly blocked by microtubule depolymerizing drugs, such as colcemid. When the drug is washed out, microtubules can reform, and migration can resume.