Sperm mitochondria and fertilisation.

Human mitochondrial DNA (mtDNA) is an extranuclear genome that encodes 13 of the polypeptides associated with the process of oxidative phosphorylation (OXPHOS). The role and importance of OXPHOS in sperm motility and function has been debated over the last few years. Here, we argue that sperm OXPHOS is important to sperm function in the light of clinical based evidence in the human where pathogenic mutations have also been described in sperm and are associated with varying degrees of male subfertility. We also discuss the importance of maintaining maternal inheritance of mtDNA and how sperm mtDNA might be eliminated during early embryogenesis in a manner similar to the process which decreases oocyte mtDNA to extremely low levels once it reaches the blastocyst stage of preimplantation development. Finally, we discuss the role of sperm mtDNA replication and why it may be prudent to considerably reduce sperm mtDNA numbers during the transition from spermatogenesis to spermiogenesis.