Sperm Transport During ejaculation, mature spermatozoa are released from the vas deferens along with ﬂuid from the prostate, seminal vesicles, and bulbourethral glands. The released semen is a gelatinous mixture of spermatozoa and seminal plasma; however, this thins out 20 to 30 minutes after ejaculation. This process, called liquefaction, is the direct result of proteolytic enzymes within the prostatic ﬂuid (30). Following ejaculation, the released spermatozoa must undergo capacitation to become competent to fertilize the oocyte. Capacitation occurs within the cervical mucus and involves removal of inhibitory mediators such as cholesterol from the sperm surface, tyrosine phosphorylation, and calcium ion inﬂux, all of which allow the sperm to recognize additional fertilization cues during travel through the female reproductive tract. When the sperm reach the tubal isthmus they are slowly released into the ampulla, further reducing the number of sperm that reach the oocyte (31). Sperm transport from the posterior vaginal fornix to the fallopian tubes occurs within 2 minutes during the follicular phase of the menstrual cycle (32).
Fertilization As the capacitated sperm near and pass through cumulus cells surrounding the oocyte, hydrolytic enzymes are released from the acrosome via exocytosis in a process called the acrosome reaction. Both capacitation and the acrosome reaction can be induced in vitro (28,31). Following the acrosome reaction, the sperm binds to and penetrates the zona pellucida (the extracellular coat surrounding the oocyte). This allows the sperm to fuse with the plasma membrane of the oocyte, an event that promotes changes in the oocyte and prevent entry by additional sperm (31). As the first sperm penetrates the zona pellucida, cortical granules are released (the cortical reaction) from the oocyte into the perivitelline space. This stops the oocyte’s zona pellucida from binding new sperm and inhibits penetration by previously bound sperm, further reducing the possibility of polyspermy (33).
Table 32.2 Drugs that Can Impair Male Fertility
Erectile dysfunction β-blockers, thiazide diuretics, metoclopramide
Drugs of misuse Anabolic steroids, cannabis, heroin, cocaine
From Hirsh A. Male infertility. BMJ 2003;327:669–672, with permission.
Sperm Sensitivity to Toxins Decreased sperm concentration and motility have been noted in areas of the United States with heavy agriculture and pesticide use, but occupational exposures have not been linked to infertility (24,34). Higher intake of food containing soy is associated with lower sperm concentrations (35). Alcoholism negatively affects all semen analysis parameters, and either smoked or chewed tobacco is associated with decreased density and motility (36–39). Marijuana inhibits motility and the acrosome reaction in vitro, and cocaine inhibits sperm motility and is associated with male infertility (40–42). Certain drugs may reduce sperm numbers or function or may cause ejaculatory dysfunction (Table 32.2). Vaginal lubricants such as Astroglide, KY Jelly, saliva, and olive oil inhibit sperm motility in vitro, while no adverse effects are seen with hydroxyethylcellulose (Pre-Seed), mineral oil, or canola oil (32).