The scaffold role of the fibrous sheath

The fibrous sheath (FS) is a novel structural feature in the principal piece region of the sperm flagellum in marsupial and eutherian mammals. It begins anteriorly at the annulus, is composed of two longitudinal columns connected by circumferential ribs, and surrounds the outer dense fibers in the principal piece. The formation of the FS was described eloquently nearly twenty-five years ago, but the identification of its components has occurred only during the last few years. Most of the genes encoding the known FS components are expressed during the postmeiotic period of spermatogenesis, are expressed only in spermatogenic cells, and are either novel genes or paralogues of genes expressed in somatic cells. The proteins of the FS can be classified in nine categories based on their functional or structural roles. Most of these are resistant to solubilization and remain in the FS during rigorous isolation procedures. Mouse sperm with a knock out of the gene encoding GAPDHS are immotile, which demonstrates that most of the ATP needed for sperm motility is generated by glycolysis. Most of the glycolytic enzymes have been localised to the principal piece and some are tightly associated with the FS. However, the nature of the protein-protein interactions involved in assembly and maintenance of FS are known for only a few of the component proteins.