ABSTRACT
Life evolved in a low-oxygen environment in which carbon dioxide and nitrogen dominated, resulting in mild reducing conditions that provided the energy source for early unicellular organisms. Oxygen was a toxic by-product of this aerobic respiration, leading to the oxygenation of the biosphere. This led to a transformation of the environment and a switch to more oxidizing conditions, which arguably necessitated the evolution of aerobic respiration. Within appropriate physiological concentrations, reactive oxygen species are critical mediators of male reproductive function. This includes chromatin condensation in spermatogenesis, epididymal maturation, postejaculation phenomenon such as hyperactivation, capacitation, and acrosome activation, as well as oocyte fusion through spermatozoal zonal binding. Common causes of male infertility include varicocele (unilateral or bilateral), genital tract infections, inflammation, and ductal obstructions. However, a significant proportion (40%–90%) may be classified as idiopathic, which is defined as the inability of a male to impregnate his partner with an otherwise normal routine semen, endocrine, and genetic analysis.
