Abstract
Besides the well-known function of thyroid hormones (THs) for regulating metabolism, it has recently been discovered that THs are also involved in testicular development in mammalian and non-mammalian species. THs, in combination with follicle stimulating hormone, lead to androgen synthesis in Danio rerio, which results in the onset of spermatogenesis in the testis, potentially relating the hypothalamic–pituitary–thyroid (HPT) gland to the hypothalamic–pituitary–gonadal (HPG) axes. Furthermore, studies in non-mammalian species have suggested that by stimulating the thyroid-stimulating hormone (TSH), THs can be induced by corticotropin-releasing hormone. This suggests that the hypothalamic–pituitary–adrenal/interrenal gland (HPA) axis might influence the HPT axis. Additionally, it was shown that hormones pertaining to both HPT and HPA could also influence the HPG endocrine axis. For example, high levels of androgens were observed in the testis in Odonthestes bonariensis during a period of stress-induced sex-determination, which suggests that stress hormones influence the gonadal fate toward masculinization. Thus, this review highlights the hormonal interactions observed between the HPT, HPA, and HPG axes using a comparative approach in order to better understand how these endocrine systems could interact with each other to influence the development of testes.
Highlights
Thyroid hormones (THs) have been implicated in a plethora of physiologic actions, such as metabolism, development, growth, and reproduction [reviewed in Ref. [1,2,3,4,5]]
These findings suggest that thyrotropin-releasing hormone (TRH) is not a major thyroid-stimulating hormone (TSH)-releasing factor in fish
Aforementioned, gonadotropin-releasing hormone (GnRH) treatment increased TSH and T4 secretions in fish and amphibians [17, 33, 44, 45, 99]; no changes in T3 were observed in C. auratus [35]
Summary
Thyroid hormones (THs) have been implicated in a plethora of physiologic actions, such as metabolism, development, growth, and reproduction [reviewed in Ref. [1,2,3,4,5]]. This paper provides a comprehensive review of existing literature on the effects of THs in testicular development in non-mammalian species, highlights the interaction of the hypothalamic–pituitary–thyroid (HPT) gland, –adrenal/interrenal (HPA), and –gonadal (HPG) axes (Table 1), and identifies key areas for future research. The hypothalamic tripeptide thyrotropin-releasing hormone (TRH) stimulates the anterior pituitary to synthesize and secrete the thyroid-stimulating hormone (TSH; Figure 1). Teleost fish have no portal systems that connect the CNS and the pituitary, in which hypothalamic neurons terminate very close to adenohypophysial cells [79]. These findings suggest that TRH is not a major TSH-releasing factor in fish. Crh transcripts start being detected earlier than crh-r2, i.e., during www.frontiersin.org
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