Effect of exogenous estradiol-17β on vitellogenic oocyte growth in ovaries of the toad Bufo bufo bufo (L.) during the postspawning period

Abstract

The effects of injecting different doses of estradiol-17β for 3 or 7 weeks on ovarian function have been studied in toads during the postspawning period in an attempt to evaluate the possible role played by estrogen in the mechanisms that control the number of previtellogenic oocytes recruited to the second growth phase and their growth rate. It was found that intramuscular injections of peanut oil, which was used to dissolve estradiol-17β, retarded growth of the oocytes, but did not significantly reduce the recruitment of oocytes to the second growth phase. Injection of the physiologically active dose of 1 μg of estradiol-17β three times a week had no clear effects on ovarian development. Only large doses of estradiol-17β, 5 μg or more, reduced the number and size of second growth phase oocytes and caused atresia among the vitellogenic oocytes. The results are discussed and it is concluded that secretion of estradiol-17β, or other estrogens, from the ovary does not seem to play an essential role in the mechanisms that control gonadotropin secretion during the period of vitellogenic growth in the toad ovary.