With a complement, the sperm gets the egg

When a sperm fuses with an egg, a specific reaction takes place that exposes the inner membrane of the spermatozoa, where a protein called the complement regulator membrane cofactor protein (MCP) is expressed. MCP is an immune regulatory molecule and its presence in reproductive tissues was unclear.

In a study appearing online on April 14 in advance of the print publication of the May issue of the Journal of Clinical Investigation, John Atkinson and colleagues from the University of Washington assessed the function of MCP on spermatozoa.

The authors have looked at the activation of the complement cascade, a cell-signaling pathway that is activated during immune reactions, but that was not known to contribute to normal physiological processes. They show that a novel form of restricted complement activation occurs on the surface of viable sperm upon exposure to follicular fluid.

The data suggest that this localized complement activation aids in the fusion process between the spermatozoa and egg. In addition, this targeted and restricted form of complement activation on cell surfaces may have implications for other cell-cell interactions.