Endocrine disrupting chemicals and reproductive disorders in women, men, and animal models

Few human studies have assessed the relationship between EDCs and male fertility rates. However, studies have examined the relationship between exposure to EDCs and markers of male reproductive health that collectively make up testicular dysgenesis syndrome (TDS), including semen quality, testicular cancer, cryptorchidism, hypospadias, low testosterone levels, and decreased anogenital distance ( Skakkebaek et al., 2016 ). Of particular concern is sperm count, which has been declining in Western men over the past century ( Swan, Elkin, & Fenster, 2000 ). From 1973 to 2011, sperm count and sperm concentration have declined over 50%, with no sign of leveling off ( Levine et al., 2017 ). Evidence from humans, animals, and wildlife suggests that the mechanisms through which EDCs impact male fertility are mediated by the development of symptoms of TDS ( Edwards et al., 2006 ). Human studies indicate that prenatal phthalate exposure is associated with decreased anogenital distance, one known marker of decreased testosterone during development ( Radke, Braun, Meeker, & Cooper, 2018 ). Extensive animal literature demonstrates that multiple phthalates exert anti-androgenic action, leading to TDS ( Latini, Del Vecchio, Massaro, Verrotti, & De Felice, 2006 ). Associations between male reproductive health and PCBs have been widely studied, identifying significant effects on sperm motility ( Meeker & Hauser, 2010 ). Polybrominated diphenyl ethers (PBDEs) in sera were negatively associated with sperm quality in young men in Japan ( Akutsu et al., 2008 ) and in men recruited from a fertility clinic in Canada ( Abdelouahab, Ainmelk, & Takser, 2011 ). However, studies on adult exposure to dioxins and HCB have not identified associations with sperm parameters or hormone levels ( Vested, Giwercman, Bonde, & Toft, 2014 ). In rodents, developmental exposures to PCBs have been shown to alter anogenital distance and testosterone levels in males ( Ulbrich & Stahlmann, 2004 ). Epidemiological studies show associations between occupational pesticide exposure and male infertility, especially semen quality and TTP ( Bretveld, Brouwers, Ebisch, & Roeleveld, 2007 ). In the 1970s, pesticide workers exposed to dibromochloropropane were reported to have oligospermia and azoospermia due to suppression of spermatogenesis ( Whorton, Krauss, Marshall, & Milby, 1977 ). Later studies on the same population following cessation of exposure found that some of the cases of azoospermia were permanent ( Bretveld et al., 2007 ; Sheiner, Sheiner, Hammel, Potashnik, & Carel, 2003 ). Other pesticides associated with adverse male reproductive outcomes in men include ethylene dibromide, chlordecone, carbaryl, and methyl parathion, as well as mixtures of pesticides ( Bretveld et al., 2007 ). In animal studies, vinclozolin and MXC have been shown to disrupt male reproductive function transgenerationally via epigenetic mechanisms ( Anway & Skinner, 2006 ). Cryptorchidism is defined as undescended testes, in which one or both testes fail to descend into the scrotal sac. The normal descent of the testis has been described in a two stage model consisting of the transabdominal phase and the inguinoscrotal phase ( Foresta, Zuccarello, Garolla, & Ferlin, 2008 ; Hughes & Acerini, 2008 ; Hutson et al., 2013 ; Kurpisz, Havryluk, Nakonechnyj, Chopyak, & Kamieniczna, 2010 ; Virtanen & Adamsson, 2012 ). Testicular descent is controlled mainly by testosterone, anti-Müllerian hormone (AMH), and insulin-like factor 3 (INSL3) ( Hughes & Acerini, 2008 ; Klonisch, Fowler, & Hombach-Klonisch, 2004 ). INSL3 has been shown to control the transabdominal phase, and AMH and testosterone have been shown to control the inguinoscrotal phase ( Bay, Main, Toppari, & Skakkebaek, 2011 ; Foresta et al., 2008 ; Hughes & Acerini, 2008 ; Hutson et al., 2013 ; Mamoulakis, Antypas, Sofras, Takenaka, & Sofikitis, 2015 ). Cryptorchidism causes many negative effects on male reproduction. Without normal descent into the scrotal sac, the testes do not function at a lower ambient temperature, often leading to compromised spermatogenesis and/or subfertility/infertility. In addition to sperm alterations and infertility, cryptorchidism has been strongly associated with testicular cancer ( Bay, Asklund, Skakkebaek, & Andersson, 2006 ; Bay et al., 2011 ; Foresta et al., 2008 ; Hughes & Acerini, 2008 ; Hutson et al., 2009 , 2013 ; Klonisch et al., 2004 ; Kurpisz et al., 2010 ; Vigneswaran & Abern, 2018 ). Although few studies have evaluated associations between phthalate exposure and cryptorchidism in humans, a French study revealed a positive correlation between self-reported exposure to phthalates and cryptorchidism ( Wagner-Mahler et al., 2011 ). In contrast, studies from Copenhagen and Australia found no association between phthalate exposure and cryptorchidism ( Hart et al., 2018 ; Jensen et al., 2015 ). Additionally, phthalates have been shown to increase the incidence of cryptorchidism in rodents ( Chen et al., 2015 ; Ema & Miyawaki, 2001 ; Ema, Miyawaki, Hirose, & Kamata, 2003 ; Ema, Miyawaki, & Kawashima, 2000 ; Fisher, Macpherson, Marchetti, & Sharpe, 2003 ; Imajima, Shono, Zakaria, & Suita, 1997 ; Jiang, Ma, Yuan, Wang, & Zhang, 2007 ; Kim et al., 2010 ; Shen et al., 2017 ; Shono, Shima, Kondo, & Suita, 2005 ). Very few studies have examined exposure to bisphenols and cryptorchidism in human and animal models. Human studies indicate that BPA concentrations in placenta, maternal sera, and sera from cryptorchid boys are higher than in boys without cryptorchidism ( Fernandez et al., 2016 ; Fisher et al., 2020 ; Komarowska et al., 2015 ). Additionally, prenatal exposure to BPA increased the incidence of undescended testes in mice in the F1 and F2 generations, without affecting adult reproductive structures or functions later in life ( Tyl et al., 2008 ). Human studies have not been consistent in relating persistent environmental contaminants with cryptorchidism. For example, some studies have found associations between maternal exposure to polychlorinated dibenzo-p-dioxins, PCDFs, and PCBs and cryptorchidism ( Brucker-Davis et al., 2008 ; Koskenniemi et al., 2015 ). In contrast, other epidemiological studies have found no associations between in utero exposure to PCBs, dioxins, or HCB and cryptorchidism ( Axelsson et al., 2020 ; McGlynn et al., 2009 ; Virtanen et al., 2012 ). Human studies on the associations of pesticide exposure and cryptorchidism in boys have been inconsistent. For example, many studies have examined pesticide exposure in mothers that mostly work in greenhouses or gardens and found positive associations with the occurrence of cryptorchidism in boys ( Agopian, Lupo, Canfield, & Langlois, 2013 ; Andersen et al., 2008 ; Brucker-Davis et al., 2008 ; Carbone et al., 2006 ; Damgaard et al., 2006 ; Gabel et al., 2011 ; Weidner, Møller, Jensen, & Skakkebaek, 1998 ). However, other human studies have found no associations between pesticide exposure and cryptorchidism ( Gaspari et al., 2012 ; Pierik, Klebanoff, Brock, & Longnecker, 2007 ; Rouget et al., 2020 ; Saiyed et al., 2003 ; Trabert, Longnecker, Brock, Klebanoff, & McGlynn, 2012 ; Waliszewski et al., 2005 ). Some animal studies have found that exposure to vinclozolin increased the incidence of undescended testes compared to control treated rodents ( Shono et al., 2004a , 2004b ; Wolf, LeBlanc, Ostby, & Gray, 2000 ). Finally, other EDCs have been associated with cryptorchidism in both human and animal studies. Parabens, used as preservatives in personal care products, have been associated with cryptorchidism in boys in Spain ( Fernandez et al., 2016 ). Additionally, prenatal exposure to DES has been associated with the occurrence of cryptorchidism in both the United States and Spain ( Palmer et al., 2009 ; Tournaire et al., 2018 ). Animal studies have shown that DES exposure negatively affects the proper development of the gubernaculum (a structure involved in testis descent) and can lead to undescended testes ( Emmen et al., 2000 ; Nomura & Kanzaki, 1977 ; Zhang, Li, Ma, Huang, & Jiang, 2012 ; Zhang et al., 2009 ). Additionally, DES exposure in rodents decreased protein and mRNA expression of INSL3 and down regulated steroidogenic factor 1, suggesting a possible mechanism for cryptorchidism ( Emmen et al., 2000 ; Zhang et al., 2009 ). Hypospadias is a condition in which the urethral opening develops on the ventral side of the penis. It is one of the most common congenital malformations of the male reproductive tract, occurring in ~0.5% of live births in North America, and requires surgery to correct ( Springer, van den Heijkant, & Baumann, 2016 ). Hypospadias is thought to be caused by both genetic and environmental factors; however, for the majority of cases, the etiology is unknown ( Shih & Graham, 2014 ). In humans, the male reproductive tract develops between 8 and 16 weeks gestation in a process tightly controlled by genetics and sex steroid hormone signaling. Thus, the gestational peroid is an important exposure window in the development of male sex organs. Although hypospadias can occur in conjunction with cryptorchidism or other congenital conditions, it most commonly occurs alone and in mild form ( Carmichael, Shaw, & Lammer, 2012 ). Pharmaceuticals and environmental chemicals have been implicated in the development of hypospadias in human and animal studies and generally follow either an estrogenic or anti-androgenic mode of action ( Cunha, Sinclair, Risbridger, Hutson, & Baskin, 2015 ). An early indication that environmental chemicals could be a factor in development of hypospadias came from studies of DES, an estrogen mimic that was given to pregnant women in the mid-20th century without proper testing for teratogenicity. Multiple studies of the sons of women exposed to DES in utero indicated statistically significant increases in hypospadias development of up to 20-fold higher than in the general population ( Brouwers et al., 2006 ; Klip et al., 2002 ). Additional studies have found increased hypospadias development in grandsons of DES-exposed women, suggesting an epigenetic mode of action ( Kalfa, Paris, Soyer-Gobillard, Daures, & Sultan, 2011 ). In rodents, prenatal and neonatal DES treatment induced development of hypospadias in male pups ( Cunha et al., 2015 ; Sinclair, Cao, Baskin, & Cunha, 2015 ). Other pharmaceuticals, including progestins, loratadine, and clomiphene, have been associated with hypospadias in human and animal studies ( Carmichael et al., 2005 ; Given et al., 2016 ; Meijer, De Jong-Van Den Berg, Van Den Berg, Verheij, & De Walle, 2006 ; Willingham, Agras, Vilela, & Baskin, 2006 ). Although human evidence for associations between phthalates and hypospadias in male infants is mixed, rodent studies provide significant causal evidence. Occupational maternal exposure to phthalates is significantly associated with increased risk of hypospadias when exposure is approximated using a job matrix in multiple studies worldwide ( Haraux et al., 2017 ; Morales-Surez-Varela et al., 2011 ; Nassar, Abeywardana, Barker, & Bower, 2010 ; Ormond et al., 2009 ). However, few studies find associations between phthalate metabolites in maternal urine or amniotic fluid and hypospadias ( Anand-Ivell et al., 2018 ; Jensen et al., 2015 ; Raghavan, Romano, Karagas, & Penna, 2018 ). In rats, hypospadias has been induced following gestational exposure to a wide range of chemicals, including di-n-butyl phthalate, benzylbutyl phthalate, and DEHP ( Gray Jr et al., 2000 , 2004 ). Phthalates induced hypospadias through anti-androgenic modes of action by interfering with fetal Leydig cell differentiation and function, resulting in decreased androgen production during critical windows of development ( Foster, 2006 ). Non-pesticide persistent organic pollutants have been investigated in human studies for associations between maternal biomarkers of exposure and hypospadias incidence, with no associations found for PCBs or PBDEs ( Raghavan et al., 2018 ). Although persistent pollutants such as PCBs, PBDEs, and dioxins are reproductive toxicants, their putative mechanism of action is through the aryl hydrocarbon receptor, and they did not induce hypospadias in animal models ( Gray et al., 2004 ). Numerous pesticides that bind to the androgen receptor and act through anti-androgen mechanisms have been implicated in the development of hypospadias in human and animal studies. A meta-analysis of parental occupational exposure to pesticides indicated a modest elevated risk ( Rocheleau, Romitti, & Dennis, 2009 ). A large study that analyzed maternal levels of DDT and DDT metabolites found associations with hypospadias in offspring, although smaller studies found no statistically significant effects ( Raghavan et al., 2018 ). Furthermore, epidemiological studies have identified associations with atrazine and HCB ( Agopian et al., 2013 ; Giordano et al., 2010 ). In rodents, prenatal treatment with anti-androgenic pesticides, including vinclozolin, procymidone, DDT and its metabolites, linuron, and flutamide, caused hypospadias and other male reproductive tract abnormalities ( Gray et al., 2004 ). Although hypospadias is not directly translatable to many non-mammalian species, wildlife exposed to pesticides exhibit TDS ( Edwards et al., 2006 ). For example, juvenile alligators exposed to DDT via a chemical spill in Lake Apopka in Florida exhibited reduced penis length compared to unexposed populations ( Guillette, Pickford, Crain, Rooney, & Percival, 1996 ).

The female reproductive system is a primary target for EDCs. Exposures to pharmaceuticals, chemicals used in consumer goods and industrial practices, as well as pesticides and herbicides can have an immediate impact on female fertility and in some cases, affect the fertility of future generations ( Gore et al., 2015 ). Because the mammalian female is born with a finite number of oocytes, exposure to EDCs at any point in life can affect female fertility as an adult ( Hannon & Flaws, 2015 ). Furthermore, the development of the reproductive tract is profoundly influenced by steroid hormones during development, making the developmental period a vulnerable time for EDC effects ( Yao, Lopez-Tello, & Sferruzzi-Perri, 2020 ). Perhaps one of the most tragic histories of an EDC and its effect on reproduction is that of diethylstilbestrol (DES), a synthetic form of the female hormone estrogen, prescribed beginning in 1940 to prevent complications of pregnancy. Although a study published in 1953 reported that DES did not improve pregnancy complications ( Dieckmann, Davis, Rynkiewicz, & Pottinger, 1953 ), it was prescribed for two more decades. Several studies clearly indicate that prenatal exposure to DES reduces fertility in the female offspring in humans and animal models ( Goldberg & Falcone, 1999 ). Plasticizers, such as phthalates and bisphenol A (BPA), found in plastics are synthetic chemicals that have adverse effects on female fertility. Phthalates were introduced in commercial products in the 1920s and are still used in many consumer goods, including plastic food containers, blood bags and medical tubing, as well as personal care products. Prospective clinical studies indicate that higher phthalate concentrations in urine were associated with longer time to pregnancy (TTP) ( Jukic et al., 2016 ; Thomsen et al., 2017 ). Recent studies of medically assisted pregnancy outcomes reported that higher urinary phthalates were associated with lower probabilities of implantation, clinical pregnancy, and live birth ( Al-Saleh et al., 2019 ; Messerlian et al., 2016 ; Mínguez-Alarcón et al., 2019 ). Basic science studies support the conclusion that exposure to phthalates decreases fertility for females exposed to the chemical as well as females of subsequent generations ( Chiang & Flaws, 2019 ; Patiño-García, Cruz-Fernandes, Buñay, Palomino, & Moreno, 2018 ; Rattan, Brehm, Gao, & Flaws, 2018 ; Yang, Arcanjo, & Nowak, 2020 ; Zhou et al., 2017a , 2017b ). BPA is found in a number of consumer products such as plastic bags, bottles, and packaging, including food and drink cans. BPA concentrations were found to be significantly higher in infertile patients compared with fertile subjects ( Caserta et al., 2013 ). Additionally, BPA was associated with a reduction in fertility and fecundability, as well as with increased embryo implantation failure and decreased estrogen, fertilized oocytes, and oocyte counts in patients undergoing in vitro fertilization (IVF) ( Ehrlich et al., 2012 ; Radwan et al., 2020 ; Wang et al., 2018 ). However, some human studies report a lack of association of BPA exposure with reproductive endpoints ( Jukic et al., 2016 ; Mínguez-Alarcón et al., 2019 ; Vélez et al., 2015a ; Yeum et al., 2019 ). Studies in laboratory animals clearly indicate an impairment of female reproductive capacity with BPA as well as with BPA alternatives, bisphenol S and bisphenol E ( Acevedo, Rubin, Schaeberle, & Soto, 2018 ; Shi, Whorton, Sekulovski, MacLean, & Hayashi, 2019 ; Srivastava & Dhagga, 2019 ). EDCs found in personal care products and cosmetics, such as parabens and glycol ethers, may be associated with poor fertility outcomes. Parabens are a class of preservatives widely used in the majority of personal care products, cosmetics, medicines, and food products. The Longitudinal Investigation of Fertility and the Environment (LIFE) Study found that female urinary methyl paraben and ethyl paraben levels were associated with an increase in TTP, whereas a variety of parabens were associated with decreases in ovarian antral follicle counts and estradiol levels (Jurewicz et al., 2020; Smarr, Sundaram, Honda, Kannan, & Louis, 2017 ). However, parabens were not associated with adverse IVF outcomes ( Mínguez-Alarcón et al., 2019 ). In animal studies, developmental exposure to n-butylparaben reduced fertility compared to controls in rats ( Maske, Dighe, & Vanage, 2018 ). Several heavy metals termed metalloestrogens have been shown to modulate estrogen signaling and affect female fertility ( Darbre, 2006 ). Lead exposure was associated with an increase in spontaneous abortions and infertility in women ( Borja-Aburto et al., 1999 ; Rahman, Fatima, Chowdhury, & Rahman, 2013 ). The Study of Metals in Assisted Reproductive Technologies found that both mercury and cadmium were associated with decreases in clinical and biochemical pregnancies, and the LIFE study showed that cadmium was associated with reduced odds of fertility ( Bloom et al., 2012 ; Buck Louis et al., 2012). Additionally, mercury exposure was associated with reduced fecundability ( Cole et al., 2006 ). However, another study showed no association between mercury and fertility ( Wright et al., 2015 ). A meta-analysis revealed that arsenic was associated with stillbirth and spontaneous abortion ( Quansah et al., 2015 ). Copper was associated with negative IVF outcomes ( Kumar et al., 2018 ). Several EDCs that are products or byproducts of commercial and industrial practices have long half-lives in our environment. Many of these chemicals, such as dioxins, PCBsand PFCs negatively impact female fertility. The most well studied dioxin, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), was associated with longer TTP ( Eskenazi et al., 2010 ). Rat studies have shown that in utero exposure to TCDD reduced fertility in the exposed rats as well as in three subsequent generations, suggesting that TCDD exerts effects in an epigenetic manner ( Bruner-Tran & Osteen, 2011 ). PCBs have been associated with longer TTP ( Björvang et al., 2020 ; Chevrier et al., 2013 ). Exposure to a PCB mixture reduced fertility with a significant increase in atretic ovarian follicles in mice ( Pocar et al., 2012 ). Perfluorononanoic acid (PFNA) has been associated with longer TTP ( Jørgensen et al., 2014 ). Perfluorooctanoic acid (PFOA) and perfluorohexane sulfonate were associated with reduced fertility and fecundity ( Vélez et al., 2015b ). Other studies, however, report no association between PFCs and fecundity ( Bach et al., 2015 ; Crawford et al., 2017 ). A number of pesticides and herbicides have been shown to act as EDCs and negatively impact female fertility. Discontinued in the US in 1972, dichloro-diphenyl-trichloroethane (DDT) was withdrawn as a pesticide due to increased evidence of declining benefits and in public response to the 1962 publication Rachel Carson’s Silent Spring , which drew concern to the dangers of pesticide use. DDT is still used in many countries and persists in the environment. Based on laboratory studies, DDT has long been suspected of negatively impacting female fertility ( Tiemann, 2008 ). Subsequent studies found that DDT was associated with implantation failure in IVF patients, and females exposed in utero to DDT experienced reduced fertility ( Al-Hussaini et al., 2018 ; Cohn et al., 2003 ). Methoxychlor (MXC), an insecticide developed to replace DDT, has an unexpected affinity for the estrogen receptor (ER), and it is toxic to ovarian follicles and causes implantation failure in laboratory rodents ( Cummings & Gray, 1989 ; Paulose, Tannenbaum, Borgeest, & Flaws, 2012 ). A phenolic compound with antibacterial properties, triclosan, has been used for over 40years in consumer products. Little is known about the effects of this endocrine disrupter on fertility. The LIFE study showed no association between triclosan and TTP ( Smarr et al., 2017 ). However, the Maternal-Infant Research on Environmental Chemicals study showed an association between triclosan in first trimester urine and longer TTP ( Vélez et al., 2015a ) and the Shanghai Birth Cohort Study reported longer TTP with higher levels of urine triclosan in preconception urine ( Zhu et al., 2019 ). The reproductive capacity of females is determined by many different factors, including regular reproductive cycles, fluctuation in hormones, and number of viable and healthy oocytes. The reproductive cycle of women is the menstrual cycle that consists of the proliferative phase, the secretory phase, and menses, with the average cycle lasting for 28 days ( Nowak, 2018 ). The reproductive cycle of rodents is the estrous cycle. It consists of proestrus, estrus, metestrus, and diestrus, and the estrous cycle typically lasts 4–5 days ( Nowak, 2018 ). Changes in reproductive cycles depend on the cyclic changes in the hypothalamic-pituitary-ovarian (HPO) axis, specifically fluctuations in estrogen, progesterone, luteinizing hormone, and follicle-stimulating hormone (FSH). In addition to reproductive cycles and hormonal changes, the health and quantity of the ovarian follicle pool is vital for successful reproduction. Females are born with a finite primordial follicle pool and a natural decline of follicles occurs mainly by atresia, but some primordial follicles develop into more mature follicle types for possible ovulation ( Hannon & Flaws, 2015 ). Primordial follicles grow into primary, preantral, and eventually antral follicles, and antral follicles are important for successful reproduction and the production of sex steroid hormones, specifically estradiol ( Hannon & Flaws, 2015 ). The natural decline in cycle lengths, dysregulation of the HPO axis, and decline in the ovarian follicle pool occurs in all females, leading to reproductive aging, or menopause in women. However, the occurrence of irregular cyclicity, early menopause, and premature or primary ovarian insufficiency (POI) can negatively affect the health of females. Irregular cycles can be due to hormonal abnormalities in the hypothalamus, pituitary, or ovary. Irregular cyclicity is considered a biomarker of reproductive aging, or menopause, and early menopause can cause many detrimental effects on overall female health ( Barbieri, 2014 ; Burger, Hale, Robertson, & Dennerstein, 2007 ; Cauley, 2015 ; dos Santos, da Silva, Ribeiro, & Stefanon, 2014 ; Toffoletto, Lanzenberger, Gingnell, Sundstrom-Poromaa, & Comasco, 2014 ). Menopause consists of dysregulation of the hormones in the HPO axis, irregular cyclicity, and a decreased follicle pool, which together lead to decreased fertility. Early menopause can have negative effects on reproductive health by decreasing time for the possibility of pregnancy, and it can cause negative effects on non-reproductive health such as heart health, bone health, and cognitive function ( Barbieri, 2014 ; Burger et al., 2007 ; Cauley, 2015 ; dos Santos et al., 2014 ; Toffoletto et al., 2014 ). In addition to early menopause, many young women under the age of 40 experience POI. POI is defined as a loss of ovarian function, leading to decreased estradiol levels, increased FSH levels, premature loss of follicles, amenorrhea (absence of menses), and subfertility or infertility ( De Vos, Devroey, & Fauser, 2010 ; Rudnicka et al., 2018 ; Vabre et al., 2017 ; Wesevich, Kellen, & Pal, 2020 ). Exposure to EDCs can lead to irregular cycles, early menopause, and POI. In human studies, a high sum of phthalate metabolites was associated with an increased risk of ever experiencing hot flashes, which is a biomarker of menopause ( Ziv-Gal et al., 2016 ). Additionally, studies have shown that phthalates and/or phthalate metabolites have been associated with increased odds of POI in women ( Cao et al., 2020 ; Özel et al., 2019 ). Further, animal studies have shown that prenatal exposure and adult exposure to both single phthalates and phthalate mixtures altered estrous cyclicity ( Brehm & Flaws, 2019 ; Brehm, Zhou, Gao, & Flaws, 2020 ; Chiang & Flaws, 2019 ; Chiang, Lewis, Borkowski, & Flaws, 2020 ; Hannon, Niermann, & Flaws, 2016 ; Zhou, Chiang, Brehm, & Flaws, 2017 ; Zhou et al., 2017a , 2017b ). Further, phthalate exposure has been shown to accelerate biomarkers of reproductive aging in rodents ( Brehm & Flaws, 2019 , Brehm et al., 2020 ; Chiang & Flaws, 2019 ; Chiang et al., 2020 ; Hannon et al., 2016 ). Finally, phthalates have been shown to affect estrous cyclicity and reproductive aging in both multigenerational and transgenerational manners, suggesting possible epigenetic mechanisms underlie the effects of phthalates on these endpoints (( Brehm & Flaws, 2019 ; Rattan et al., 2019 ). Bisphenols and other plasticizers have been shown to affect cyclicity, reproductive aging, and POI. BPA has been associated with POI in women ( Özel et al., 2019 ), possibly by downregulating microRNAs associated with decreased sex steroid hormones and POI ( Ziv-Gal & Flaws, 2016 ). In rodent studies, BPA has been shown to alter estrous cyclicity, most likely via alterations in the regulation of estrogens ( Brehm & Flaws, 2019 ; Rattan et al., 2017 ; Viguie et al., 2018 ; Ziv-Gal & Flaws, 2016 ). TCDD is an environmental contaminant that has been shown to increase the risk of early menopause in women ( Eskenazi et al., 2005 ; Rattan et al., 2017 ). Additionally PCBs and polychlorinated dibenzofurans (PCDFs) have been associated with abnormal menstruation in adolescent girls ( Yang et al., 2005 ), and dioxin-like PCBs have been significantly associated with POI in women ( Pan et al., 2019 ). Animal studies have shown that exposure to TCDD and PCB altered estrous cyclicity ( Rattan et al., 2017 ; Zhang, Ji, et al., 2018 ) and TCDD alone caused early reproductive aging in female rodents ( Franczak, Nynca, Valdez, Mizinga, & Petroff, 2006 ). Increased body burdens of pesticides have been associated with an early age at menopause ( Akkina, Reif, Keefe, & Bachand, 2004 ; Cooper, Savitz, Millikan, & Chiu Kit, 2002 ; Rattan et al., 2017 ). Additionally, pesticide exposure in girls hired as farmworkers has been associated with irregular menstrual cycles ( Varnell et al., 2020 ). Further, insecticides such as DDT and pyrethroids have been associated with POI in women ( Li et al., 2018 ; Pan et al., 2019 ). Pesticide exposure in rodents has been shown to cause irregular estrous cyclicity and advance the onset of reproductive aging ( Armenti, Zama, Passantino, & Uzumcu, 2008 ; Gore, Walker, Zama, Armenti, & Uzumcu, 2011 ; Pascotto et al., 2015 ; Rattan et al., 2017 ). Moreover, studies suggest that pesticide exposure may alter neuroendocrine gene expression, DNA methylation, ovarian gene expression, and folliculogenesis, contributing to reproductive aging in female rodents ( Armenti et al., 2008 ; Gore et al., 2011 ). A few studies have examined other EDCs such as parabens, DES, nonylphenols, and triclosan and their associations with abnormal cyclicity or early menopause. A study examining triclosan effects found that high levels of triclosan were associated with an increased risk of abnormal menstruation in women planning to conceive ( Zhu et al., 2019 ). In rodent studies, DES, nonylphenols, parabens, and triclosan have been shown to cause irregular estrous cyclicity ( Bitencourt, Fortunato, Panis, Amorim, & de Arruda Amorim, 2019 ; Rattan et al., 2017 ; Zhou, Chiang, et al., 2017 ). Polycystic ovarian syndrome (PCOS) is an endocrine disorder affecting nearly 10% of women of childbearing age ( Neven, Laven, Teede, & Boyle, 2018 ). Although the exact etiology of PCOS is unknown, it is characterized by hyperandrogenism, polycystic ovaries, and anovulation ( Neven et al., 2018 ). Furthermore, a subset of women diagnosed with PCOS present with peripheral insulin resistance, impaired glucose tolerance, and increased risk for the development of type 2 diabetes and metabolic syndrome ( Diamanti-Kandarakis & Dunaif, 2012 ; Moran, Misso, Wild, & Norman, 2010 ). In addition to metabolic disorders, PCOS is linked with infertility. PCOS patients account for 80% of women diagnosed with anovulatory infertility ( Balen et al., 2016 ). Women with PCOS undergoing IVF often produce poor quality oocytes, leading to lower fertilization, cleavage, and implantation rates and a higher miscarriage rate compared to women without PCOS (35.8% vs 23.6%, respectively) ( Balen et al., 2016 ; Qiao & Feng, 2011 ). Moreover, PCOS patients that do conceive have a higher risk of pregnancy complications, including gestational diabetes, hypertension, pre-eclampsia, preterm birth, and perinatal mortality ( Boomsma et al., 2006 ). Considering the associated health and fertility complications, it is critical to understand what factors contribute to the manifestation of PCOS. Recent studies have suggested a link between exposure to EDCs and PCOS. Epidemiological studies have shown that BPA is elevated in serum, urine, and follicular fluid samples from women diagnosed with PCOS compared to women without PCOS ( Kandaraki et al., 2011 ; Konieczna et al., 2018 ; Tarantino et al., 2013 ; Wang et al., 2017 ). Although one study reported no difference in serum BPA concentrations, higher serum levels of the BPA replacement chemical BPS in women with PCOS was found compared to women without PCOS (Jurewicz et al., 2020). Moreover, higher concentrations of BPA have been detected in serum and urine samples from adolescents with PCOS compared to those without PCOS, suggesting that BPA exposure plays an early role in the pathogenesis of PCOS ( Akgül et al., 2019 ; Akın et al., 2015 ). In rodent-based studies, animals exposed neonatally to BPA had higher testosterone levels and exhibited polycystic ovaries compared to vehicle treated animals ( Fernández, Bourguignon, Lux-Lantos, & Libertun, 2010 ; Yang et al., 2019 ). In epidemiological studies, PCOS patients exhibited levels of phthalates and phthalate metabolites in blood and urine that are less than or equivalent to those seen in control samples ( Akın et al., 2020 ; Vagi et al., 2014 ). One study identified a negative association between monoethyl phthalate levels in maternal antenatal blood and the occurrence of polycystic ovaries in the daughters ( Hart et al., 2014 ). Conversely, di(2-ethylhexyl) phthalate (DEHP) levels were higher in follicular fluid collected from women with PCOS compared to non-PCOS women ( Jin, Zhang, Pan, Wang, & Qu, 2019 ). Moreover, primary human granulosa cells exposed to DEHP in vitro exhibited increased androgen synthesis, increased cytochrome P450 17A1 (CYP17A1) expression, and decreased aromatase expression ( Jin et al., 2019 ). In rodent studies, exposures to phthalates and phthalate mixtures have been shown to increase the occurrence of ovarian cysts compared to vehicle treated animals ( Brehm, Rattan, Gao, & Flaws, 2018 ; Zhang,Mu, et al., 2018 ; Zhou et al., 2017a , 2017b ). Interestingly, exposing pregnant mice to phthalates or mixtures containing phthalates and BPA increased cyst formation in F1-F3 generations, indicating these plasticizers have transgenerational effects ( Brehm et al., 2018 ; Manikkam, Tracey, Guerrero-Bosagna, & Skinner, 2013 ; Zhou et al., 2017a , 2017b ). PFCs such as PFOA and perfluorooctanesulfonic acid (PFOS) are chemicals used in the manufacturing of textiles and non-stick cookware. Epidemiological studies show elevated levels of PFOS and PFOA in sera from women with PCOS compared to non-PCOS women and an increased risk of the development of PCOS in women with high serum concentrations of PFAS and PFOS ( Heffernan et al., 2018 ; Vagi et al., 2014 ). Furthermore, PFOS levels were positively associated with irregular menstrual cycles in both the PCOS and control populations ( Heffernan et al., 2018 ). Pesticides are another group of environmental chemicals that have been linked to PCOS. Organochlorine pesticides such as DDT and dichlorodiphenyldichloroethylene (DDE) were present in higher concentrations in sera from PCOS patients ( Guo et al., 2017 ; Yang et al., 2015 ). Moreover, serum DDT levels were positively correlated with testosterone and negatively correlated with FSH and sex hormone-binding globulin levels ( Guo et al., 2017 ). In rodents, gestational exposure to the fungicide vinclozolin or pesticides such as permethrin and N, N-diethyl-meta-toluamide (DEET) promoted the development of polycystic ovaries in a transgenerational manner ( Guerrero-Bosagna et al., 2012 ; Nilsson et al., 2012 ). Endometriosis is a reproductive disorder characterized by the formation of endometrial-like lesions on pelvic tissues, likely caused by retrograde menstruation of blood and endometrial cells into the abdomen ( Bulun et al., 2019 ). Women with endometriosis often experience chronic pelvic pain, painful periods, and infertility ( Bulun et al., 2019 ). Approximately 10% of women suffer from endometriosis, and 30–50% of these women are infertile ( Bulletti, Coccia, Battistoni, & Borini, 2010 ). Endometriosis is associated with reduced ovarian reserve, poor oocyte quality, impaired embryo implantation, and reduced live birth rate ( Bulletti et al., 2010 ; Macer & Taylor, 2012 ). Although the exact molecular mechanism driving endometriosis is unknown, endometriotic lesions exhibit reduced apoptosis, impaired stromal cell decidualization, and increased expression of factors involved in inflammation, angiogenesis, and tissue remodeling ( Bulun et al., 2019 ; Kim, Kurita, & Bulun, 2013 ). Moreover, excessive estrogen production and progesterone resistance are common features in endometriosis ( Kim et al., 2013 ). Given the hormone sensitivity of endometriosis, it is important to investigate potential links between EDCs and its pathogenesis. Epidemiological studies show a positive correlation between urinary BPA levels and endometriosis ( Peinado et al., 2020 ; Simonelli et al., 2017 ; Upson et al., 2014 ; Wen et al., 2020 ). Similarly, in an experimentally induced mouse model of endometriosis, BPA exposure induced endometrial ERβ expression and increased the size and number of endometriotic lesions compared to vehicle treated mice ( Xue et al., 2020 ). Interestingly, co-administration of an ERβ antagonist blunted the effects of BPA on lesion number and volume, suggesting that BPA promotes endometriotic lesion formation and growth in an ERβ-dependent manner ( Xue et al., 2020 ). Additionally, the BPA substitute, bisphenol AF, induced lesion growth in a mouse endometriosis model and exhibited a greater effect than BPA ( Jones, Lang, Kendziorski, Greene, & Burns, 2018 ). Several epidemiological studies show a positive association between blood and urinary phthalate/phthalate metabolite levels, including DEHP and mono-n-butyl phthalate, and incidence of endometriosis ( Buck Louis et al., 2013 ; Chou et al., 2020 ; Cobellis et al., 2003 ; Huang et al., 2010 ; Kim et al., 2011 , 2015 ; Reddy, Rozati, Reddy, & Raman, 2006 ). Interestingly, others report an inverse association between urinary mono-(2-ethylhexyl) phthalate, a metabolite of DEHP, and endometriosis ( Upson, Sathyanarayana, et al., 2013 ; Weuve, Hauser, Calafat, Missmer, & Wise, 2010 ). In vitro studies using isolated human endometrial cells show that exposure to DEHP increased expression of factors involved in tissue remodeling and progesterone metabolism ( Kim et al., 2015 ; Kim, Kim, Kim, & Cho, 2017 ). Moreover, human endometrial tissue implanted into immunocompromised mice exposed to DEHP showed greater growth, proliferation, and expression of matrix metalloproteinases 2 and 9 compared to vehicle treated mice ( Kim et al., 2015 ). Higher serum levels of PFOS, PFOA, and PFNA were reported in women with endometriosis compared to those without endometriosis ( Campbell, Raza, & Pollack, 2016 ). Moreover, PFOA exposure was associated with both increased risk of endometriosis development and increased severity of the condition ( Louis et al., 2012 ). Many pesticides exhibit endocrine disrupting actions that could contribute to the development of endometriosis. In humans, serum, urine, and fat levels of organochlorine pesticides and pesticide byproducts, such as lindane, chlorpyrifos, and hexachlorobenzene (HCB), were positively associated with endometriosis (Buck Louis et al., 2012; Cooney, Buck Louis, Hediger, Vexler, & Kostyniak, 2010 ; Li, Chen, Lin, Buck Louis, & Kannan, 2020 ; Ploteau et al., 2017 ; Upson, De Roos, et al., 2013 ). In rats, HCB exposure increased lesion size and vascularization compared to vehicle treatment in an experimentally induced endometriosis model ( Chiappini et al., 2019 ). Elevated blood and adipose tissue levels of several PCB congeners, including anti-estrogenic, dioxin-like, and non-dioxin-like PCBs, as well as dioxins such as 1,2,3,7,8-pentachlorodibenzo-p-dioxin and TCDD were associated with endometriosis in women ( Louis et al., 2005 ; Martínez-Zamora et al., 2015 ; Ploteau et al., 2017 ; Porpora et al., 2006 , 2009 ). Rhesus monkeys chronically exposed to TCDD for 4 years showed an increased incidence of endometriosis 10 years after termination of exposure compared to control monkeys ( Rier, Martin, Bowman, Dmowski, & Becker, 1993 ). In mice, exposure to dioxins and dioxin-like PCBs enhanced growth of endometriotic lesions in an experimentally induced model, whereas non-dioxin-like PCBs did not affect lesion growth ( Huang et al., 2017 ; Johnson, Cummings, & Birnbaum, 1997 ). Uterine fibroids or leiomyomas are steroid hormone sensitive benign tumors that form in the myometrial layer of the uterus and are classified as subserosal, intramural, submucosal, or pedunculated, based on the location of the fibroid ( Ikhena & Bulun, 2018 ; McWilliams & Chennathukuzhi, 2017 ). Uterine fibroids are the most common gynecologic tumors and are present in up to 80% of women by the age of 50 ( Ikhena & Bulun, 2018 ). Approximately 15–30% of women with fibroids present with serious symptoms, including heavy and prolonged bleeding, chronic pelvic pain, and urinary incontinence ( Zimmermann, Bernuit, Gerlinger, Schaefers, & Geppert, 2012 ). Further, symptomatic women experience pregnancy (reduced implantation rate, spontaneous abortion, infertility) and obstetric (preterm labor, placental abruption, postpartum hemorrhage) complications ( Cook, Ezzati, Segars, & McCarthy, 2010 ). Uterine fibroids contribute to 30–50% of hysterectomies and the annual economic burden is estimated to be $34.4 billion, exceeding that of breast and ovarian cancer ( Al-Hendy, Myers, & Stewart, 2017 ). Given their prevalence and financial burden, it is critical to understand the role of EDCs in the pathogenesis of uterine fibroids. Epidemiological studies have shown higher concentrations of the plasticizer BPA in uterine fibroid tissue and urine from women with fibroids compared to women without fibroids ( Othman, Al-Adly, Elgamal, Ghandour, & El-Sharkawy, 2016 ; Pollack et al., 2015 ). Interestingly, no differences in serum BPA levels were detected with the presence or absence of uterine fibroids ( Han et al., 2011 ; Jeong et al., 2013 ). In vitro, BPA promoted cell cycle progression and proliferation in human leiomyoma cells ( Li, Lu, Ding, Xu, & Shen, 2019 ; Wang, Kao, Chang, Lin, & Kuo, 2013 ; Yu et al., 2019 ). In rodents, the presence of leiomyomas was observed in rats neonatally exposed to BPA while no leiomyomas were detected in vehicle treated rats ( Newbold, Jefferson, & Padilla-Banks, 2007 ). In adult rats, BPA exposure increased myometrial proliferation and thickness, although no leiomyomas were observed ( Othman et al., 2016 ). This suggests timing of exposure is a critical component in fibroid development. Several epidemiological studies have reported greater urine concentrations of phthalate metabolites in women with fibroids and positive associations between phthalate metabolites and fibroids ( Huang et al., 2014 , 2010 ; Kim et al., 2016 ; Lee et al., 2020 ; Sun et al., 2016 ). In human leiomyoma cells, in vitro exposure to DEHP decreased apoptosis and increased cell viability, proliferation, and collagen production ( Kim, 2018 ; Kim, Kim, Oh, et al., 2017 ). Similarly, human fibroid tissue xenografted into immunodeficient mice that were fed DEHP showed greater proliferation, tumor growth, and collagen production than fibroid tissue implanted in vehicle fed mice ( Kim et al., 2020 ). Organochlorine pesticides DDT and DDE were both positively associated with fibroid development ( Saxena et al., 1987 ; Trabert et al., 2015 ). Additionally, uterine fibroids were observed in rhesus monkeys chronically exposed to DDT ( Takayama, Sieber, Dalgard, Thorgeirsson, & Adamson, 1999 ). In vitro, other organochlorine pesticides, such as kepone and endosulfan, stimulated proliferation of rat leiomyoma cells in an ER dependent manner ( Hodges, Bergerson, Hunter, & Walker, 2000 ). Another study demonstrated that the pyrethroid insecticide fenvalerate promoted cell proliferation, cell cycle progression, collagen production, and suppressed apoptosis in both human leiomyoma and uterine smooth muscle cells ( Gao et al., 2010 ). Other EDCs have been associated with uterine fibroids in epidemiological studies. Cadmium is a heavy metal present in cigarette smoke that has been shown to have detrimental effects on female reproduction ( Henson & Chedrese, 2004 ). Elevated blood cadmium levels were positively associated with uterine fibroid development ( Johnstone et al., 2014 ; Ye et al., 2017 ). Additionally, DES, a synthetic estrogen once used to support pregnancy, was associated with higher incidence of uterine fibroids in women prenatally exposed to DES compared to women whose mothers did not take DES ( Mahalingaiah et al., 2014 ).

EDCs are an integral part of our lives. These chemicals are used in plastics, personal care products, industrial practices, herbicides, and pesticides. EDCs can contribute to male and female reproductive disorders. As EDCs are identified and removed from everyday products, other chemicals will fill the functional void. Thus, more studies on the effects of current and emerging/replacement EDCs on reproductive health are essential, ideally before a chemical is mass produced and used in everyday items and applications.

Endocrine disrupting chemicals (EDCs) are exogenous agents that can interfere with the synthesis, storage, release, transport, metabolism, binding, action, and/or elimination of endogenous hormones. EDCs are encountered in everyday life as they are present in plastic food containers, water bottles, personal care products, air pollution, and pesticides. Substantial evidence shows that EDCs impact human and animal reproductive health as well as contribute to reproductive disorders ( Sifakis, Androutsopoulos, Tsatsakis, & Spandidos, 2017 ). In the female, EDCs are associated with subfertility, aberrations in the reproductive cycle, polycystic ovarian syndrome, endometriosis, and uterine fibroids ( Fig. 1 ). In the male, EDCs have been implicated in male subfertility, cryptorchidism, and hypospadias ( Fig. 2 ). The EDCs reviewed in this chapter are those that are the most studied including, but not limited to, bisphenols, phthalates, polychlorinated biphenyls (PCBs), perfluorochemicals (PFCs), polybrominated diphenyl ethers, dioxins, and pesticides. Furthermore, the EDCs reviewed in this chapter are those that have been linked to reproductive disorders such as subfertility/infertility, acyclicity, polycystic ovarian syndrome, endometriosis, and uterine fibroids in women and subfertility, cryptorchidism, and hypospadias in men. Finally, some of the EDCs reviewed in this chapter have been shown to cause reproductive disorders in mammalian animal models.