Our technological environment has increasingly become an estrogenic environment. Many of the endocrine disruptors found in plastics, insecticides, herbicides, perfumes, and hair products act as estrogens. Estradiol (the predominant human estrogen) and most estrogenic compounds bind to the estrogen receptors. These are dormant transcription factors that dimerize when they bind estrogenic compounds. They then migrate into the nucleus and regulate gene transcription. Some of the estrogenic compounds also bind to a membrane-bound G protein-coupled receptor, GPR30, that causes a rapid release of calcium ions from the endoplasmic reticulum (Revankar et al. 2005). We have seen that DES can disrupt reproductive tract development in women. In men, estrogenic endocrine disruptors may also cause severe reproductive tract anomalies, and they have been closely associated with testicular dysgenesis,* a syndrome encompassing low sperm count, poorly formed testes and penis, and testicular tumors (Figure 18.16).
In the past three decades there has been an increase in testicular cancers and a decrease in sperm concentration throughout the industrialized world (Carlsen et al. 1992; Aitken et al. 2004). The sperm count (number of sperm per milliliter) has dropped precipitously throughout much of Europe and the Americas. Skakkebaek and his team at Copenhagen University reviewed 61 international studies done between 1938 and 1992, involving 14,947 men. They found the average sperm count had fallen from 113 million per milliliter in 1940 to 66 million in 1990 (Carlsen et al. 1992). In addition, over the same time period, the number of “normal” sperm (sperm whose shape indicates their potential to fertilize eggs) fell from 60 million per milliliter to 20 million.
Subsequent studies have confirmed and extended Skakkebaek’s findings (Merzenich et al. 2010). A survey of 1,350 sperm donors in Paris found that sperm counts declined by about 2% each year from the 1970s through the mid-1990s (Auger et al. 1995). A recent study (Rolland et al. 2012) shows that this trend has continued through 2005, such that sperm count for 35-year-old French men went from an average of 74 million sperm per milliliter of semen in 1989 to 50 million per milliliter in 2005. A study of Danish men (Jørgensen et al. 2013) found that only one man in four had optimal semen quality. The chance of fertilization diminishes significantly if the sperm concentration is below 40–50 million per milliliter, or if the percentage of normal spermatozoa is below 9% (Guzick et al. 2001), and that was the case for approximately 40% of men in the Danish study. More severe fertility problems are expected when sperm concentration is below 15 million per milliliter and the percentage of normal spermatozoa is less than 5%; these conditions were seen for 15% and 35%, respectively, of men from the general Danish population. In addition to the drop in sperm count documented in these studies, there has also been an increase in testicular cancers over recent decades (see Figure 18.17A).
Sharpe (1994) suggested that testicular dysgenesis syndrome may be due in large part to endocrine disruptors. While no chain of causation has been established completely (see Sharpe and Irvine 2004), there is evidence that the pathologies of this syndrome can be caused by environmentally relevant concentrations of endocrine disruptors. Indeed, all the developmental anomalies (but not the testicular tumors) can be induced by administering phthalate derivatives to pregnant rats (Fisher et al. 2003). Phthalates are ubiquitous in industrialized society and are widely used in plastics and cosmetics (that “new car smell” consists largely of volatilizing phthalates). Among male rats exposed in utero to dibutyl phthalate, more than 60% exhibited cryptorchidism (undescended testicles), hypospadias (misplaced urinary aperture), low sperm count, and testis abnormalities—phenotypes very similar to conditions found in human testicular dysgenesis syndrome. In humans, phthalates have been shown to inhibit testosterone production, alter testes morphology, and change the anatomy of the genital region (Duty et al. 2003; Swan et al. 2005; Desdoit-Lethimonier et al. 2012).
Other endocrine disruptors that adversely affect sperm are dioxins, nonylphenol, bisphenol A, acrylamide, and certain pesticides and herbicides (see Aitken et al. 2004; Newbold et al. 2006). The sunscreen 4-MBC, a camphor derivative, has been found to decrease the size of the testes and prostate glands, and it can delay male puberty in rats (Schlumpf et al. 2004). Pesticides may be critically important in impairing male fertility. The link between pesticides and infertility has been known for a long time (Carson 1962; Colborn et al. 1996). One of the most important endocrine disrupting pesticides is DDT. DDT is an estrogenic compound that has been associated with breast cancer (Cohn et al. 2007), and it breaks down into DDE (dichlorodiphenyldichloroethylene). DDE binds to the androgen receptor, preventing testosterone binding (Xu et al. 2006). In humans, DDT has been linked to pre-term births and immature babies, and it is banned in the United States (Longnecker et al. 2001).
The fungicide vinclozolin (used extensively in grape farming) also works as an anti-androgen, inserting itself into the androgen receptor and preventing testosterone from binding there (Grey et al. 1999; Monosson et al. 1999). Male rats born to mothers injected with vinclozolin late in pregnancy are sterile. The sons of rats injected with vinclozolin during mid-pregnancy were able to reproduce, but their testis cells underwent apoptosis more than usual, their sperm count dropped 20%, and the sperm that remained had significantly lowered motility (Figure 18.17). When affected males were mated with normal females, the male offspring also had this testicular dysgenesis syndrome. Some of the offspring were sterile, and some had reduced fertility. The study (Anway et al. 2005) ended after the fourth generation of males continued to show low sperm count, low sperm motility, prostate disease, and high testicular cell apoptosis. This transgenerational effect is thought to be the result of methylation of genes involved in spermatogenesis (Guerrero-Bosagna et al. 2010).
Some scientists argue that these claims are exaggerated and that their tests on mice indicate that litter size, sperm concentration, and development are not affected by environmentally relevant concentrations of environmental estrogens. However, investigations by Spearow and colleagues (1999) have shown a remarkable genetic difference in sensitivity to estrogen among different mouse strains. The strain that was used for testing environmental estrogens, the CD-1 strain of laboratory mice, is at least 16 times more resistant to endocrine disruption than the most sensitive strains, such as B6. When estrogen-containing pellets were implanted beneath the skin of young male CD-1 mice, very little happened. However, when the same pellets were placed beneath the skin of B6 mice, their testes shrank and the number of sperm seen in the seminiferous tubules dropped dramatically (Figure 18.18). This widespread range of sensitivities has important consequences for determining safety limits for humans.
*Dysgenesis (Greek, “bad beginning”) denotes defects in development.