Plasma membrane structure and remodelling during sperm maturation in the epididymis.

All mammalian spermatozoa are surrounded by a limiting plasma membrane that mediates many of the early events during fertilization. The membrane is derived originally from spermatogonia/spermatocytes in the testis but is then modified considerably during spermiogenesis, epididymal maturation and capacitation. It is characterized by an unusually high proportion of polyunsaturated phospholipids that give it special physical characteristics and compartmentalization of many of its component proteins and lipids into discrete domains on the head and tail. During passage of spermatozoa through the epididymis remodelling of the plasma membrane is commensurate with acquisition of motility and fertilizing capacity. Remodelling processes include uptake of secreted epididymal glycoproteins, removal or utilization of specific phospholipids from the inner leaflet of the bilayer, processing of existing or acquired glycoproteins by endoproteolysis and re-positioning of both protein and lipid components to different membrane domains. These modifications are carefully coordinated at different zones of the epididymis and indirectly affect intracellular membranes, organelles and even nucleoprotein. Maturation changes are described and appraised within the context of current concepts of membrane structure and fertilization.