Teleosts are classical models for the study of oogenesis and shift in steroidogenesis

Abstract

Information on the molecular mechanisms of oocyte growth and maturation provides novel inputs to our understanding of teleostian oogenesis. Oogenesis is a complex process primarily coordinated by two steroid hormones, estradiol-17β (E2) and 17α,20β-dihydroxy-4-pregnen-3-one (17α,20β-DP), under the influence of pituitary gonadotropins, luteinizing hormone (LH) and follicle stimulating hormone (FSH). Oocytes originated from primordial germ cells get arrested in the diplotene stage of prophase I of first meiotic division until they mature. Vitellogenesis is a key event where E2 stimulates the production of vitellogenin in liver, transport into the developing oocytes where it is selectively taken up into the oocyte by a receptor-mediated process involving specific cell-surface receptors. The events of oogenesis were tightly modulated by several genes which are involved at the level of gonadal steroidogenesis under the influence of FSH. Ovarian aromatase (cyp19a1a) is considered to be one of the rate limiting enzymes for oogenesis. It is well established that production of E2 and 17α,20β-DP by ovarian follicles occurs via the coordination of two cell layers, the theca and granulosa. A shift in the steroidogenesis pathway from E2 to 17α,20β-DP occurs in the ovarian follicle layer immediately prior to oocyte maturation. During maturation, gonadotropins primarilyregulate the shift, E2 to 17α,20β-DP in the cholesterol biosynthesis pathway by enhancing the transcription of 20β-hydroxysteroid dehydrogenase (20β-HSD) in granulosa cells. LH enhances the production of 17α,20β-DP by decreasing E2 levels through the reduction of cyp19a1a and/or P450c17-I in ovarian follicle. Conversely, the levels of 20β-HSD and/or P450c17-II mRNA were elevated upon the stimulation of LH. Transcript level changes were also seen at the level of enzyme activity to bring about shift in steroidogenesis. Thus, 17α,20β-DP is regarded as maturation inducing hormone in several teleosts. Transcription factors such as Ad4BP/SF-1, Foxl2 and CREB are potentially contributes for the expression of these steroidogenic enzyme genes during final oocyte maturation. © 2013 Nova Science Publishers, Inc.