In most vertebrates, sex steroids play a critical role in gonadal development, maturation of germ cells, and development of secondary sexual characteristics. Sex steroids are synthesized in steroid-producing cells (SPCs) in the testis known as Leydig cells, as well as in thecal and granulosa cells in the ovary. In SPCs, cholesterol is sequentially catalyzed by a set of steroidogenic factors and enzymes in order to produce sex steroids. Therefore, integrated expression of the genes involved in steroidogenesis is critical for the proper production of sex steroids. In the present study, regulatory mechanisms of steroidogenic factors and enzymes were examined. We focused on hsd3b, star and ad4bp/sf-1 as well as the description of temporal and spatial expression of these genes during gonadal development in medaka (Oryzias latipes). During testicular development, hsd3b, star and ad4bp/sf-1 were co-expressed in the interstitial somatic cells subsequent to the formation of the seminiferous tubule precursor, suggesting that ad4bp/sf-1 regulated the transcription of both hsd3b and star. During ovarian development, the expression pattern of hsd3b coincided with that of cyp11a1, but not with that of aromatase. Although ad4bp/sf-1 was mainly expressed in presumptive follicular cells, it was also detected in hsd3b positive interstitial cells in the developing ovary. Contrary to our expectations, the onset of star expression occurred during a later stage of ovarian development than the expression of other steroidogenic enzymes. Thus, the regulation mechanism of star transcription appears to differ from that of the other steroidogenic enzymes in the developing ovary, but not in the developing testis.
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