Pattern of recruitment of oocytes to second growth phase in normal toads, and in hypophysectomized toads, Bufo bufo bufo (L.), treated with gonadotropin (HCG)

Abstract

The size frequency distribution of first growth phase (FGP) oocytes has been determined in the ovaries of spawned toads before and after recruitment of oocytes to the second growth phase (SGP). The experiment included unoperated toads and toads with extirpated pars distalis of the hypophysis, treated 6 days a week for 4 weeks and for 6–7 weeks with injections of 50 IU of human chorionic gonadotropin (HCG). The number of oocytes recruited to the SGP did not differ between the unoperated controls and the two groups of hypophysectomized HCG treated groups. Moreover, the rate of growth of the SGP oocytes was the same in the three groups. Recruitment of oocytes to the SGP depleted the populations of FGP oocytes of about 17% of the stock. The shapes of the size frequency distribution curves after recruitment of oocytes indicated that the recruited oocytes originated from FGP oocytes varying from 0.8 down to 0.3 mm in diameter, with about two-thirds of the oocytes originating from the range of 0.4 to 0.6 mm in diameter. It was, furthermore, indicated that the sensitivity toward gonadotropin increases with the size reached by the oocytes during the FGP. It was finally indicated that during the 2 months that followed spawning the loss of FGP oocytes to the SGP was the only, or main, factor that contributed to the change in size frequency distribution of the FGP oocytes; that is, recruitment to SGP is not immediately followed by oogenesis and first growth to restore number and size frequency distribution within the population of FGP oocytes.