Activation of sperm swimming movements requires stimulation of the propulsion machinery of the sperm. Sperm are activated in a series of steps. First, contact with diffusible molecules contained in egg jelly initiates chemotaxis. Chemotaxis is the process by which a migrating (or swimming) cell moves toward a higher concentration of a diffusuble or substrate-bound guidance molecule. Second, physical contact with egg jelly occurs. This causes the acrosome reaction, which is triggered a rise in internal calcium concentration inside the sperm head.


Image courtesy of Thomas Gensch, Forschungszentrum Jülich.


The small peptide RESACT acts as a chemoattractant for sea urchin sperm. When receptors on the surface of the sperm bind RESACT, a signal transduction cascade involving cyclic GMP (cGMP) is set in motion, triggering faster sperm swimming. In this movie, inactive RESACT is added to a field of sperm. A flash of light causes photolytic activation of the RESACT, resulting in rapid movement of the sperm toward the source of the RESACT. See if you can see where the RESACT was added!

Courtesy of Thomas Gensch, Forschungszentrum Jülich.
sperm chemotaxis (4.8 Mb)
sperm chemotaxis (4.8 Mb)