Abstract
During the long migration from river habitats to the spawning ground, the Japanese eel undergoes sexual maturation. This spawning migration occurs concurrently with morphological changes, such as increases in eye size; however, the mechanisms by which sex steroids and their receptors influence these changes in peripheral tissues remain unclear. The aim of this study was to investigate changes in the eyes of female Japanese eels during sexual maturation, and our research focused on estrogen receptor (ER)α and ERβ transcripts. During ovarian development, the gonadosomatic index increased and yolk-laden oocytes developed rapidly. These changes occurred in conjunction with a steady increase in plasma levels of estradiol-17β (E2). Concomitant increases in transcript levels of ERα and ERβ in eye, brain, pituitary, and ovary were also observed. Fluorescence in-situ hybridization analyses revealed that ERα and ERβ transcripts were present in the choriocapillary layer and photoreceptor layer of the eyes, and the analysis also revealed that their signals in these layers became stronger in mature females compared to those observed in immature females, suggesting that under the influence of gonadotropins, morphological changes in the eyes are regulated by E2 through the activation of its receptors. In conclusion, E2 plays a crucial role in physiological adaptations that occur in peripheral tissues during the spawning migration.
Highlights
The reproductive activities of female fish are directly and indirectly regulated by sex steroids that are synthesized in the ovarian follicles and secreted into the blood circulation under influence of gonadotropins such as follicle stimulating hormone (FSH) and luteinizing hormone (LH).Cells 2019, 8, 310; doi:10.3390/cells8040310 www.mdpi.com/journal/cellsAmong the sex steroids, estradiol-17β (E2 ) is converted from testosterone in the ovarian granulosa cells through the catalytic activity of aromatase [1,2]
The results of our present study demonstrated that Gonadosomatic index (GSI) and oocyte diameter (OD) increased in the salmon pituitary extract (SPE)-injected females
This result was in agreement with the previous studies in which significant increases in those parameters were recorded during sexual maturation from yellow to silver eels [19,22]
Summary
The reproductive activities of female fish are directly and indirectly regulated by sex steroids that are synthesized in the ovarian follicles and secreted into the blood circulation under influence of gonadotropins such as follicle stimulating hormone (FSH) and luteinizing hormone (LH). Estradiol-17β (E2 ) is converted from testosterone in the ovarian granulosa cells through the catalytic activity of aromatase [1,2]. Elevated transcription of ERs in female fish has been reported in the brain (including the pituitary), liver, and ovary, and changes in the transcript levels of ERs in the liver are closely related to regulation of vitellogenin synthesis in most teleosts [10,12].
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