Recombinant human insulin-like growth factor I stimulates all stages of 11-ketotestosterone-induced spermatogenesis in the Japanese eel, Anguilla japonica, in vitro

Involvement of sex steroid hormones in the early stages of spermatogenesis in Japanese huchen (Hucho perryi )

In higher vertebrates, considerable progress has been made in understanding the endocrine regulation of puberty; however, in teleosts, the regulatory mechanisms of spermatogenesis during the first annual cycle remain unclear. The present study was conducted to understand the regulatory mechanisms of spermatogenesis throughout the different stages of the first spermatogenic cycle and to check the ability of various steroids and hormones to induce in vitro spermatogonial proliferation in Japanese huchen (Hucho perryi ). The results indicate that the serum level of 11-ketotestosterone (11-KT) was positively associated with germ cell type; the level first began to rise with the appearance of late-type B spermatogonia and continued to increase gradually throughout the active spermatogenic stages and spermiogenesis, reaching a peak value 2 wk before spawning, and then declined. During the spermatogenic stages, the serum concentration of 17alpha,20beta-dihydroxy-4-pregnen-3-one (17alpha,20beta-DP) was undetectable. Only a small peak was detected with the appearance of spermatocytes and spermatids, and at the time of spawning, the level increased dramatically, reaching its maximum value with the onset of milt production. Despite the high variation in serum levels of 17beta-estradiol (E2) both between months and among the individuals, E2 was found during the whole reproductive cycle. From these results, we concluded that 1) 11-KT is necessary for the initiation of spermatogenesis and sperm production, and it probably plays a role in spermiation, 2) 17alpha,20beta-DP is essential for the final maturation stage, could play a significant role in the mitosis phase and meiosis process, and probably participates in the regulation of spawning behavior, and 3) estrogen is an indispensable male hormone that plays a physiological role in some aspects of testicular functions, especially during the mitotic phase. The three steroids were also able to induce DNA synthesis, spermatogonial renewal, and/or spermatogonial proliferation in vitro.

Mammalian follicle-stimulating hormone and insulin-like growth factor I (IGF-I) up-regulate IGF-I gene expression in organ culture of newt testis

We previously showed that porcine follicle-stimulating hormone (pFSH) and human recombinant insulin-like growth factor (rhIGF-I) promote the differentiation of secondary spermatogonia into primary spermatocytes in organ cultures of newt testes, respectively. To elucidate the molecular action of FSH and IGF-I, we cloned cDNAs for newt IGF-I and IGF-I receptor (IGF-IR), and examined their mRNA expression in organ culture during newt spermatogenesis. Northern blot and reverse transcription-polymerase chain reaction (RT-PCR) analyses revealed that IGF-I mRNA was highly expressed in somatic cells (mostly Sertoli cells) at the secondary spermatogonial stage but barely in germ cells, and that IGF-IR mRNA was expressed in both germ and somatic cells at all stages examined. The addition of pFSH to newt testis markedly increased IGF-I mRNA expression. Also, rhIGF-I increased IGF-I mRNA expression, whereas IGF-IR mRNA expression declined slightly. These results suggest that the ability of FSH to promote the differentiation of secondary spermatogonia is at least partly mediated by somatic cell-derived IGF-I, and that IGF-I mRNA expression in somatic cells is auto-upregulated.

Gonadotropins, their receptors, and the regulation of testicular functions in fish

The pituitary gonadotropins luteinizing hormone (LH) and follicle-stimulating hormone (FSH) regulate steroidogenesis and spermatogenesis by activating receptors expressed by Leydig cells (LH receptor) and Sertoli cells (FSH receptor), respectively. This concept is also valid in fish, although the piscine receptors may be less discriminatory than their mammalian counterparts. The main biological activity of LH is to regulate Leydig-cell steroid production. Steroidogenesis is moreover modulated in an autoregulatory manner by androgens. The male sex steroids (testosterone in higher vertebrates, 11-ketotestosterone in fish) are required for spermatogenesis, but their mode of action has remained obscure. While piscine FSH also appears to have steroidogenic activity, specific roles have not been described yet in the testis. The feedback of androgens on gonadotrophs presents a complex pattern. Aromatizable androgens/estrogens stimulate LH synthesis in juvenile fish; this effect fades out during maturation. This positive feedback on LH synthesis is balanced by a negative feedback on LH release, which may involve GnRH neurones. While the role of GnRH as LH secretagogue is evident, we have found no indication in adult male African catfish for a direct, GnRH-mediated stimulation of LH synthesis. The limited available information at present precludes a generalized view on the testicular feedback on FSH.

Steroid hormones stimulate gonadotrophs in juvenile male African catfish (Clarias gariepinus)

In juvenile African catfish (Clarias gariepinus), the pituitary LH content strongly increased after the beginning of spermatogonial proliferation. We hypothesized that a signal of testicular origin is involved in stimulating the gonadotrophs. We investigated the effects of castration and sex steroid treatment on gonadotrophs in juvenile males by quantifying LH production and release and LH subunit transcript levels and by examining gonadotroph morphology and proliferation. Castration reduced but did not abolish the maturation-associated elevation in pituitary LH content. Treatment with testosterone but not with 11-ketotestosterone, an otherwise potent androgen in fish, reversed the castration-induced decrease of pituitary LH levels. An increased pituitary LH content was accompanied by an increased number of cytologically mature gonadotrophs. However, no evidence was found for gonadotroph proliferation, so that quiescent gonadotrophs may have become activated. Although 11-ketotestosterone treatments had no effect in castrated males, this androgen attenuated gonadotroph activation in intact males. Because androgen production in juvenile catfish is downregulated by treatment with 11-ketotestosterone, its inhibitory effects on gonadotrophs in gonad-intact males may be due to suppression of Leydig cell testosterone production, which appears to be a limiting factor for the activation of catfish gonadotrophs. Aromatizable androgens may have opposite effects on fish (stimulatory) and mammalian (inhibitory) gonadotrophs.

Insulin-like growth factor receptors and their ligands in gonads of a hermaphroditic species, the gilthead seabream (Sparus aurata): expression and cellular localization

Expression of insulin-like growth factor (IGF)-I, IGF-II, and IGF type I receptor (IGF-1R) genes was studied in gonads at different developmental stages of the protandrous hermaphroditic species the gilthead seabream (Sparus aurata) by reverse transcription-polymerase chain reaction and Northern blot analysis. Both IGF-I and IGF-II mRNA levels were highest in bisexual gonads and decreased during gonadal development. Regardless of the stage of gametogenesis, IGF-II mRNA levels exceeded those of IGF-I. Transcripts for IGF-1R RNA were detected in gonads at all stages studied. A major transcript of 11 kb was found in gonads and in gill arch and brain, but it was not found in liver and muscle. Distribution of the two types of IGF-1R and IGF-I in gonads was studied by immunohistochemistry. Immunoreactive IGF-I was found in the granulosa and theca cells of follicles at different vitellogenic stages and in oocytes at the chromatin-nucleolus and perinucleolus stage. In the testis, immunoreactive IGF-I was found in somatic cells of the cyst wall, interstitial cells, and spermatogonia A. In addition, IGF-1R was detected in the membrane of previtellogenic oocytes and in the theca and granulosa cells of vitellogenic and late vitellogenic follicles. In the testis, a positive reaction was identified in spermatogonia A and spermatids for the germ cells and in somatic cells of the cyst walls and interstitial cells. Local expression and production of IGFs and their receptors in fish gonads support a role for the IGF system in fish gonadal physiology.