Hormonal events surrounding spontaneous onset of puberty in female rats

In an effort to define more completely the hormonal events surrounding onset of puberty in female rats, concentration of sex steroids and gonadotropins in serum, ovaries and pituitaries were quantified. Simultaneously the sites and intensity of synthesis of the hormones were observed immunohistochemically. It was found that an increase in tissue concentration of follicle stimulating hormone (FSH), luteinizing hormone (LH), progeterone (Po) and estradiol (E2) preceded vaginal opening (VO). However, the serum levels of these hormones were not significantly increased until the day of VO except for serum E2 levels which were fairly high during the early prepubertal period. Coincident with an increase in pituitary gonadotropin levels was an increase in the staining intensity for pituitary gonadotropes. However, the increase in ovarian steroid levels was not always coincident with the staining intensity for steroid-producing cells. The site or localization of steroid synthesis in the ovary was the thecal layer, but not the granulosa, of various growing follicles. The present results clearly indicated that the preovulatory changes in gonadotropins and sex steroids in blood and the tissues are similar to those noted in cycling adult rats, suggesting that prepubertal gonadotropin surge is induced via a common mechanism. They also indicated that ovarian steroids, especially estrogens, are synthesized mainly in the thecal layer of growing follicles.