Lipid heterogeneity and membrane fluidity in a highly polarized cell, the mammalian spermatozoon

The membranes of mammalian spermatozoa are composed of a complex mixture of lipids which is modified during epididymal maturation. Although there are many species-specific changes in the membrane, a common characteristic is the introduction of highly unsaturated phospholipid acyl moieties and an increase in the relative content of ether-linked phospholipids at this time. The predominant phospholipid molecular species is sn-1 ether-linked 16:0/sn-2 ester-linked 22:6-PC and PE. This membrane lipid remodeling is thought to provide the correct infrastructure and fluidity for the membrane-mediated events that lead to fertilization. In the plasma membrane, the lipids are organized into five specialized regions; acrosome (AC), equatorial (EQ), postacrosome (PA), midpiece (MP) and the principle piece (PP). Each region has a distinct lipid composition, as required for associated specialized functions. Experiments suggest that functional barriers to diffusion exist between the regions preventing mixing and randomization of membrane components. There is compelling evidence to support the existence of both lateral and transmembrane lipid heterogeneity in the plasma membrane of spermatozoa.