Setting the boundaries for sperm

The sperm plasma membrane is compartmentalized, including specific regions that interact with the egg zona pellucida and subsequently fuse with the oolemma. Trafficking mechanisms that maintain membrane compartments in other cells do not operate in sperm, and diffusion barriers may instead be responsible. To investigate these, Roy Jones and co-workers have used various advanced fluorescence microscopy techniques (FRAP, FLIP and SPFI) to examine the mobility of lipids within the sperm membrane (see p. 6485). Labelling boar sperm with the lipid reporter DiC₁₆, they track it through the three major sperm-head compartments: the anterior acrosome, the equatorial segment and the post-acrosomal region. The authors find that individual DiC₁₆ molecules diffuse freely over the sperm head. By contrast, diffusion of DiC₁₆ that has aggregated to form 200-nm particles is restricted: the particles cannot move between the equatorial segment and the post-acrosomal region (but can move in and out of the anterior acrosome). Jones and co-workers propose that a molecular barrier separates the equatorial segment and the post-acrosomal region. This could comprise an array of transmembrane `picket' proteins and might require assemblies such as lipid rafts to disassemble before they can cross it.