Causes and consequences of apoptosis in spermatozoa; contributions to infertility and impacts on development

During early development, apoptosis plays a major role in the ontogeny of the germ line as a means of regulating the germ cell:Sertoli cell ratio. In the adult, apoptosis fulfils another function in removing damaged germ cells from the seminiferous epithelium in response to a wide range of physiological and environmental triggers. These include various forms of electromagnetic radiation, chemotherapeutic agents and commonly encountered toxicants such as phthalate esters, bisphenol A and cadmium. This form of apoptosis can lead to spermatogenic arrest and is predominantly mediated by the Fas/FasL system. In addition, senescent mature spermatozoa can undergo a truncated form of apoptosis in order to ensure their efficient phagocytosis within the male and female reproductive tracts. This apoptotic cascade appears to be triggered by oxidative stress and lipid peroxidation, which leads to activation of mitochondrial reactive oxygen species (ROS) generation in a self-perpetuating redox cycle. The electrophilic aldehydes generated as a result of lipid peroxidation also lead to a rapid loss of sperm motility followed some hours later by caspase activation and phosphatidylserine exposure on the sperm surface. The nuclear DNA suffers oxidative damage during this process but there is no immediate DNA cleavage by endonucleases as there is in somatic cells. The reasons for this deviation from the normal pattern of apoptosis involve the unusual physical architecture of spermatozoa and the limited capacity these cells possess for base-excision repair. These findings have practical implications for the approaches that might be used to detect and prevent DNA damage in spermatozoa.