Human Reproduction Update, human experimental data systematic review, 2019
Overall, the incidence of male reproductive disorders has increased in recent decades. Testicular development during fetal life is crucial for subsequent male reproductive function. Non-genomic factors such as environmental chemicals, pharmaceuticals and lifestyle have been proposed to impact on human fetal testicular development resulting in subsequent effects on male reproductive health. Whilst experimental studies using animal models have provided support for this hypothesis, more recently a number of experimental studies using human tissues and cells have begun to translate these findings to determine direct human relevance.
OBJECTIVE AND RATIONALE
The objective of this systematic review was to provide a comprehensive description of the evidence for effects of prenatal exposure(s) on human fetal testis development and function. We present the effects of environmental, pharmaceutical and lifestyle factors in experimental systems involving exposure of human fetal testis tissues and cells. Comparison is made with existing epidemiological data primarily derived from a recent meta-analysis.
For identification of experimental studies, PubMed and EMBASE were searched for articles published in English between 01/01/1966 and 13/07/2018 using search terms including ‘endocrine disruptor’, ‘human’, ‘fetal’, ‘testis’, ‘germ cells’, ‘testosterone’ and related search terms. Abstracts were screened for selection of full-text articles for further interrogation. Epidemiological studies involving exposure to the same agents were extracted from a recent systematic review and meta-analysis. Additional studies were identified through screening of bibliographies of full-texts of articles identified through the initial searches.
A total of 25 experimental studies and 44 epidemiological studies were included. Consistent effects of analgesic and phthalate exposure on human fetal germ cell development are demonstrated in experimental models, correlating with evidence from epidemiological studies and animal models. Furthermore, analgesic-induced reduction in fetal testosterone production, which predisposes to the development of male reproductive disorders, has been reported in studies involving human tissues, which also supports data from animal and epidemiological studies. However, whilst reduced testosterone production has been demonstrated in animal studies following exposure(s) to a variety of environmental chemicals including phthalates and bisphenol A, these effects are not reproduced in experimental approaches using human fetal testis tissues.
Direct experimental evidence for effects of prenatal exposure(s) on human fetal testis development and function exists. However, for many exposures the data is limited. The increasing use of human-relevant models systems in which to determine the effects of environmental exposure(s) (including mixed exposures) on development and function of human tissues should form an important part of the process for assessment of such exposures by regulatory bodies to take account of animal–human differences in susceptibility.
Development of the male reproductive system and its subsequent function is impacted by events that occur in utero. Perturbations in testicular development or function during fetal life may result in male reproductive disorders that present postnatally. This includes anatomical abnormalities identified at birth, such as cryptorchidism and hypospadias, or disorders presenting in adulthood, including testicular cancer or infertility. These associated disorders are collectively referred to as the testicular dysgenesis syndrome (TDS). The development of TDS has been shown in rats to be influenced by a reduction in androgen production or action during a key period of fetal life, known as the masculinization programming window (MPW). The increasing incidence of TDS disorders over recent decades, highlights the potential importance of environmental impacts in their etiology. Environmental factors that have been proposed to affect fetal testis development and predispose to TDS disorders include environmental chemicals (e.g. plasticizers and pesticides), pharmaceuticals (e.g. analgesics, metformin and diethylstilboestrol) and lifestyle factors (e.g. diet, alcohol and smoking).
Pharmaceuticals : Diethylstilboestrol
Diethylstilboestrol (DES) is a synthetic estrogen that was used clinically to prevent spontaneous miscarriage and pre-term labor from the 1940 s until the early 1970 s. DES was withdrawn from clinical use after the demonstration of a causal role in the development of vaginal carcinoma in girls born to exposed mothers. In addition to the effects on female offspring, an association with structural abnormalities of the male reproductive tract was also described including epididymal cysts, microphallus and testicular hypoplasia.
Animal studies involving in-vitro culture of rat and mouse fetal testis, have reported a reduction in testosterone production following exposure to DES, similar to the results of previous in-vitro studies involving fetal mice and in-vivo studies in rats.
For TDS disorders, which are linked to a reduction in androgen action during fetal life, there is conflicting evidence regarding their association with maternal DES exposure. Three studies have reviewed the literature relating to exogenous estrogen exposure and male reproductive disorders. Whilst early studies reported that hypospadias was significantly associated with DES exposure, it has subsequently been pointed out that this related to urethral abnormalities resulting from exposure to exogenous estrogens (including DES), which may have resulted from abnormalities in penile development rather than an effect on urethral formation as a result of reduced androgen exposure. The meta-analysis of all available evidence revealed a significant association between DES exposure and hypospadias; however, it was concluded that any effect of DES on hypospadias is likely to be small. For cryptorchidism, an increased risk in association with DES exposure is reported; however, this was dependent on the statistical model used and was indicative of heterogeneity. A subsequent cohort study has reported an association between in-utero exposure to DES and an increased risk of cryptorchidism, however, only for those in whom the initial exposure occurred prior to the 11th week of gestation with no significant association following exposure after 11 GWs. Studies have demonstrated no effect of prenatal DES exposure on sperm counts or fertility; however, this is in contrast to a previous study demonstrating an association between prenatal exposure to DES and semen parameters in adult men. Importantly, this study included analysis of men born to a large cohort of mothers who participated in an RCT involving DES exposure during pregnancy.
Experimental evidence from human studies
To date, only two studies have investigated the effect of DES exposure on the human fetal testis (Table VI). In-vitro organ culture of first trimester human fetal testis exposed to DES (10−5 to 10−6 M) for 3 days did not alter testosterone production. Interestingly, this study compared effects of DES exposure in rodent and human fetal testis demonstrating contrasting results between species using an identical experimental system.
In a separate study using the xenograft model, exposure to DES (100μg/kg, three times weekly) for 35 days resulted in no significant difference in testosterone production by second trimester (15–19 GW) testis tissue. Interestingly, host mouse seminal vesicles were significantly increased in weight, which was indicative of increased testosterone production from the xenografted tissue over the entire grafting period. The reason for this unexpected increase in testosterone is unclear.
Whilst rodent studies have indicated a profoundly negative effect of DES exposure on testosterone production by the fetal testis, experimental studies utilizing human fetal testis tissues have failed to identify similar effects, which may relate to the presence of ESR1 in rodent Leydig cells, and the absence of this estrogen receptor in human fetal testis. Epidemiological data suggests that any potential effect of DES exposure on male reproductive development is likely to be of small magnitude. Taken together the results suggest an important species difference in terms of DES effects on fetal testosterone production which may explain why this frequently results in the development of male reproductive disorders in rodents, whilst associations between DES and subsequent male reproductive disorders in humans are rather modest. Whilst DES is unlikely to be used in pregnant women in the future, the findings of this study offer some reassurance regarding the potential of low-level exposure to environmental estrogens to affect human male reproductive development, given their extremely low potency compared with DES and the high exposures that resulted from therapeutic use of DES.
- Full study (free access) : Effect of environmental and pharmaceutical exposures on fetal testis development and function: a systematic review of human experimental data, Human Reproduction Update, doi.org/10.1093/humupd/dmz004, 14 March 2019.
- Featured image dmz004.pdf.