The Aromatase–Estrogen System in the Testes of Non-Mammalian Vertebrates

Abstract

Estrogens are important physiological regulators of testicular activity in vertebrates. Estrogen levels depend on the activity of P450 aromatase, the enzyme responsible for the irreversible conversion of testosterone into 17β-estradiol. Therefore, P450 aromatase is the key player in the aromatase–estrogen system. The present review offers a comparative overview of P450 aromatase activity in male gonads of amphibians, reptiles, and birds, with a particular emphasis on the functions of the aromatase–estrogen system in these organisms during their developmental and adult stages. The aromatase–estrogen system appears to be crucial for the sex differentiation of gonads in vertebrates. Administration of aromatase inhibitors prior to sexual differentiation of gonads results in the development of males rather than females. In adults, both aromatase and estrogen receptors are expressed in somatic cells, Leydig and Sertoli cells, as well as germ cells, with certain differences among different species. In seasonal breeding species, the aromatase–estrogen system serves as an “on/off” switch for spermatogenesis. In some amphibian and reptilian species, increased estrogen levels in post-reproductive testes are responsible for blocking spermatogenesis, whereas, in some species of birds, estrogens function synergistically with testosterone to promote spermatogenesis. Recent evidence indicates that the production of the aromatase enzyme in excessive amounts reduces the reproductive performance in avian species of commercial interest. The use of aromatase inhibitors to improve fertility has yielded suitable positive results. Therefore, it appears that the role of the aromatase–estrogen system in regulating the testicular activity differs not only among the different classes of vertebrates but also among different species within the same class.

Diverse effect of in ovo treatment with metaloestrogen: selenium on chick testis histology and estrogen signaling

“…In addition, these results indicate possible Se effects on GPER and ERα protein and/or on Se binding to these receptors but not ERβ. Studies of mammalian and non-mammalian vertebrate species, including birds, revealed that the metabolism of androgens to estrogens, so-called aromatization, is crucial for male reproduction and fertility and plays a key role in gonadal sex differentiation during development (41,48). Moreover, any change in the balance of steroid hormones within the seminiferous tubule can result in altered sperm parameters as previously reported earlier by the group of Carreau (50) and our own (31).…”

Could Exposure to Glyphosate Pose a Risk to the Survival of Wild Animals? A Case Study on the Field Lizard Podarcis siculus

“…This finding was supported by immunocytochemical investigations, which showed a higher presence and wider distribution of ERs in Gly-treated testis compared to the control, with a dominant effect of Erβ [ 54 ]. These analyses also showed that Gly treatment did not affect the distribution of aromatase, a key enzyme in the conversion of testosterone into 17-β estradiol, essential for the regular course of spermatogenesis [ 62 , 63 ]. From these data, we conclude that Gly acts as an endocrine disruptor by targeting ERs, influencing their expression and synthesis [ 59 ].…”

The Diverse Roles of 17β-Estradiol in Non-Gonadal Tissues and Its Consequential Impact on Reproduction in Laying and Broiler Breeder Hens

“…Estrogens have long been implicated in the feminization of the reproductive axis during sexual differentiation, with early exposure to estrogen critical for the normal development of the ovary (Elbrecht and Smith, 1992). Thus, female embryos of turkeys, quail, and chickens have higher circulating E 2 concentrations compared to their male counterparts (Woods and Brazzill, 1981;Schumacher et al, 1988;Abdelnabi et al, 2001;Ottinger et al, 2001;Sechman et al, 2011;Rosati et al, 2021). Recent evidence demonstrates that JUN is a critical regulator of sexual differentiation, regulating DMRT1, SOX9, and FOXL2, the genes previously hypothesized to be the determinants of differentiation in avian species.…”