In echindoerms such as sea urchins, the head of the sperm is enveloped by an actin-based structure formed by the fertilized egg known as the fertilization cone. The fertilization cone helps to draw the sperm head deep within the egg. Mammals do not produce such a dramatic structure. The two images below show the fertilization cone clearly, using scanning electron microscopy (top) and rhodamine phalloidin injection (bottom).

Scanning elecron micrograph of a sea urchin sperm penetrating an egg. The fertilization cone is visible to the right of the sperm head. Image courtesy of Gerald Schatten, Univ. of Pittsburgh.

Fertilization cone visualization in Lytechinus. Actin has been visualized using rhodmaine phalloidin. Courtesy of Mark Terasaki, Univ. of Connecticut.

Click on the thumbnail to see a movie of fertilization cone formation, courtesy of Mark Terasaki, Univ. of Connecticut.
fertilzation cone visualized via phalloidin (1.2 Mb)

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

Pronuclear migration in L. variegatus (5.7 Mb)
Migration reversibly blocked by colcemid (5.7 Mb)

Imaging of pronuclear migration can also be performed by labeling membranous structures within the egg with a dye, as the image below from Mark Terasaki, Univ. of Connecticut, shows.

Imaging of pronuclear migration in Lytechinus. The egg's contents have been labeled using a liophilic dye (DiI). Courtesy of Mark Terasaki, Univ. of Connecticut.

Click on the thumnail to see the corresponding movie...

pronuclear migration imaged using DiI (0.3 Mb)