The Male Gamete
Chemicals present in inhaled tobacco smoke reach the male reproductive
system and the concentration of substances such as cadmium, cotinine, lead, malondialdehyde and protein carbonyls are significantly higher in the seminal plasma of smokers, in some cases in proportion to the levels seen in serum.[4,5] The effect of tobacco constituents on classical sperm parameters and fertility has been under investigation for decades. The scope of this section is the review of the impact of tobacco exposure on sperm function in IVF cycles.
Biochemical Alterations in Semen From Smokers & Their Physiological/structural Consequences for Spermatozoa
Cigarette smoking is associated with increased levels of seminal reactive oxygen species. Increased leukocyte concentration in the ejaculate of smokers seems to be, at least in part, responsible for this.[6,7] The scavenging capacity of the antioxidant seminal defense system is limited. Indeed, the concentration of ascorbic acid (the main extracellular water-soluble antioxidant) and the activity of other components of the antioxidant defense (glutathione S-transferase and reduced glutathione) are significantly reduced in the ejaculate of smokers.[3,8] An imbalance between their concentration and that of reactive oxygen species leads to cellular damage (i.e., oxidative stress [OS]).
Oxidative stress has been linked to a number of physiological and structural abnormalities in human sperm. Cigarette smoking and OS have been associated with reduced fertilizing capacity through failure to extrude residual spermatozoon cytoplasm and diminished acrosin activity.[9,10] During spermiogenesis, cytoplasm reduction takes place. Maintenance of residual cytoplasm is increased in smokers and is reported to impair sperm function, including IVF capacity.[11,12,13] Reduced acrosin activity has also been reported in sperm under OS. Sperm plasma membrane is rich in polyunsaturated fatty acids, which makes it particularly susceptible to OS. Tobacco consumption was shown to lead to an increased incidence of alterations in sperm plasma membrane usually seen in apoptotic cells.
Increased OS may lead to damage of all cellular constituents (lipids, carbohydrates, proteins and DNA). Electron microscopy of the sperm tail showed that a number of ultrastructural abnormalities are more frequently seen in smokers: absence of one or more fiber doublets, central fibers and coarse outer fibers. The DNA damage caused by reactive oxygen species is documented and possibly constitutes the worst effect of OS in sperm.
Cotinine has been targeted as one relevant molecule on the determination of compromised sperm fertilizing capacity in vitro. Sperm cultured with different concentrations of this nicotine metabolite display a significant impairment in motility. When cotinine concentration in culture media is 400 ng/ml or higher, the percentage of hyperactivated spermatozoa, the sperm membrane function evaluated by the hypo-osmotic swelling test and the capacity of penetrating zona-free hamster oocytes are all negatively affected. Interestingly, the capacity of sperm to activate hamster oocytes by ooplasmic injection is maintained. It is important to emphasize that cotinine concentration of 400 ng/ml or higher is frequently seen in the serum and seminal plasma of smokers. The processes through which cotinine alters spermatozoa physiology are not established. It is possible that other molecular interactions independent of OS may also be relevant.
Genetic Alterations in Spermatozoa From Smokers
Oxidative stress determined by cigarette smoke was demonstrated to lead to structural alterations in spermatozoa DNA. Polycyclic aromatic hydrocarbons metabolites link to nitrogenated base guanosine and form premutational/carcinogenic lesions known as DNA adducts.[20,21] The incidence of DNA adducts is significantly higher in spermatozoa from smokers. High concentrations of DNA adducts are associated with reduced sperm motility and leukospermia in infertile men, findings that are known to be related to OS.[22,23] The concentration of DNA adducts in spermatozoa was also inversely associated with the ability to conceive in 1 year.
The risk of transmission of DNA premutations or mutations to descendants is very worrisome. The increase in germ-line heritable DNA sequence mutations in individuals exposed to tobacco smoke was studied in mice. Exposed animals had a significantly higher mutation frequency in spermatogonial stem cells. Data suggested that mutations accumulate with extended exposure. This kind of mutation may be associated with an increased incidence of genetic diseases among the nonsmoking descendants of male smokers. In humans, paternal smoking habit was associated with an increased rate of DNA adducts in embryos on day 3 of their in vitro development and an increased incidence of childhood cancer, a condition known to be linked to genetic mutations.
Apart from the formation of adducts, OS seen in spermatozoa from smokers also increases the incidence of DNA fragmentation. Different groups studied spermatozoa DNA fragmentation in smoking and nonsmoking men from infertile couples.[6,28,29] An increased rate of fragmentation in smokers was found in pre-[6,29] and postswim-up samples. This finding is associated with reduced fertilization rate and embryo cleavage rate in IVF cycles.[23,30] Interestingly, young, healthy, fertile, smoking donors were reported not to display a significant increase in DNA fragmentation in spermatozoa.[15,31] This may be the consequence of a more efficient protection against OS or (less likely) a more efficient DNA repairing machinery.
Increased incidence of aneuploid spermatozoa was also related to cigarette smoking, although results of studies published on this issue are not uniform. In general, it seems that tobacco consumption increases the rate of aneuploidy in spermatozoa for some chromosome pairs (e.g., chromosomes 1 and 13).[32,33] In other words, susceptibility to tobacco-induced nondisjunction during male meiosis would vary among chromosome pairs. A marked interindividual variability in smoking-induced sperm aneuploidy is also observed.[32,33] Even within the population of individuals with severe oligozoospermia (a set of patients known to have, as a whole, an increased rate of aneuploid spermatozoa), a smoking habit was reported to determine a higher aneuploidy rate, specifically for chromosome 21.
Based on data mentioned previously, studies attempted to confirm that male tobacco consumption worsens the prognosis of IVF cycles. Couples with a male smoker were reported to have a reduced fertilization rate, implantation rate, pregnancy rate[36,37] and a diminished probability of achieving a 12-week pregnancy. It is of note that the analysis of the interaction between male age and smoking status showed that only for smokers did an increasing age determine a significant reduction in the probability of achieving a 12-week pregnancy.
The identification of a number of abnormalities in the genetic material of spermatozoa produced by smokers indicates that, even if intracytoplasmic sperm injection (ICSI) overcomes many of the drawbacks in sperm function determined by tobacco exposure, the presence of DNA adducts, DNA fragmentation and an increased aneuploidy rate is likely to impair IVF cycle prognosis. Indeed, a significantly lower pregnancy rate with conventional IVF and ICSI was reported in couples with a smoking husband. In addition, the presence of sperm DNA adducts was shown to be related to increased day 2 and 3 embryo fragmentation, poorer embryo cell division and blastocyst
The probability of achieving a clinical pregnancy by performing testicular biopsy in cases of azoospermia was also shown to be related to a smoking habit. Smoking azoospermic men had a significantly reduced probability of a clinical pregnancy when compared with nonsmokers (17.4 vs 43.7%) and the live-birth rate was also significantly reduced (20 vs 43.2%). The dramatic contrast of these figures reinforces the possibility that the impairment in testicular function determined by tobacco depends highly on the quality of primary testicular physiology. This possibility is already raised by the observation that young, fertile, smoking sperm donors do not have increased sperm DNA fragmentation.
In spite of all the findings of IVF cycles performed on couples with a male smoker, it is difficult to differentiate between a direct sperm effect of tobacco consumption and all the possible effects of passive smoking on the female partner. This difficulty is clearly illustrated by the report of reduced number of oocytes retrieved in IVF cycles performed to partners of male smokers. Animal models allow an absolute control over confounding variables that are difficult to be eliminated in clinical studies, such as passive female smoking and exposure to other toxicants (e.g., caffeine and alcohol). In rats, male exposure to cigarette smoke results in a secretory deficiency of Sertoli and Leydig cells and impaired epididymal maturation. Dramatic reductions of fertility in vivo and in vitro fertilizing capacity with conventional IVF are observed. ICSI is capable of overcoming the in vitro limitation in fertilization and early embryo development is apparently normal, but a reduced implantation potential of embryos obtained from 'smoking' male rats is observed, regardless of the insemination technique used (conventional IVF or ICSI). It is noteworthy that studies in humans led to a similar conclusion.
In conclusion, a strong body of evidence demonstrates that exposure to tobacco constituents affects sperm function and integrity. Some of the drawbacks identified seem to be overcome by ICSI, but genetic damage would still be a problem in semen from smokers, regardless of the insemination technique used.
Expert Rev of Obstet Gynecol. 2008;3(4):555-563. © 2008 Expert Reviews Ltd.
Cite this: Cigarette Smoking and IVF - Medscape - Jul 01, 2008.