|Title of host publication||Encyclopedia of Reproduction|
|Subtitle of host publication||Volume 6: Comparative Reproduction|
|Editors||Penny Swanson, Michael K. Skinner|
|Place of Publication||Oxford UK|
|Number of pages||11|
|Publication status||Published - 1 Jan 2018|
In vertebrates, sperm structure is highly conserved with the head containing the hereditary information (DNA) neatly packaged into the nucleus and a neck or connecting piece usually attaching the head to the sperm tail or flagellum. The sperm tail is usually subdivided into a midpiece containing the mitochondria - the power generators of the cell - and a principal piece, the longest part of the spermatozoon, that uses energy from the mitochondria to move the tail. Distinguishing structural specialisations in the different vertebrate groups include: a helical head shape in sharks; side fins in bony fish sperm tails producing enhanced motility; an undulating tail membrane in amphibians; a larger mid-piece, well-developed long fibrous sheath and linear mitochondrial cristae in reptiles; helical head and mitochondrial helix in birds; and a more complex sperm maturation process in the epididymis, and tail modifications and sperm aggregations that influence motility in mammals. There is a close relationship between the evolution of structural and functional specialisations of vertebrate spermatozoa and the need to ensure successful fertilization. For example, evidence from comparative studies suggests that longer spermatozoa swim faster. Sperm motility ensures that the spermatozoon makes contact with the oocyte during fertilization and enzymes released from the acrosome on the sperm head, in addition to sperm motility, assist the fertilizing spermatozoon to reach the oocyte and incorporate its nucleus into the oocyte cytoplasm. Fusion of sperm and egg membranes is a prerequisite for fertilization and for the sperm nucleus to enter the cytoplasm of the oocyte. Other factors like mode of fertilisation, chemical characteristics of the seminal fluid, milieu of the female reproductive tract and sperm storage requirements may have also influenced the evolution of sperm specialisations.