Regulation of ovarian cycle in the toad Bufo bufo bufo (L.): Effects of autografting pars distalis of the hypophysis, of extirpating gonadotropic hypothalamic region, and of partial ovariectomy

Abstract

Autografting of the pars distalis of the hypophysis to an eye muscle or to the median eminence in spawned toads affected ovarian development: weight of ovaries and size and number of vitellogenic oocytes stagnated, or even regressed slightly. The small difference observed in gonadotropic activity of the ectopically and orthopically grafted pars distalis suggest that gonadotropin is largely autonomously secreted during the initial phase of ovarian development. An autonomous gonadotropic function of the pars distalis was, moreover, indicated by the finding that, in experiments performed in autumn, previtellogenic oocytes started vitellogenic gonadotropin-dependent growth in toads bearing an ectopically autografted pars distalis even after extirpation of the region of hypothalamus that contains the gonadotropic centre. The effect of extirpating one ovary and two-thirds of the second ovary on growth of oocytes in the remaining ovarian fragment was studied in spawned toads. The operation greatly increased recruitment of previtellogenic oocytes to the vitellogenic growth phase. However, the rate of growth of the oocytes remained unaltered. It was found, moreover, that partial ovariectomy in toads with the pars distalis of the hypophysis autografted (to an eye muscle or to the median eminence) resulted in an increase in ovarian weight and a recruitment of oocytes to the secondary growth phase that was similar to that observed in the normal unoperated controls. As autografting of pars distalis reduced ovarian growth in toads with intact ovaries, partial ovariectomy seems to induce compensatory growth of the remaining ovarian fragment also in toads that bear an ectopically autografted pars distalis. Mechanisms of compensatory growth are discussed. It is suggested that specific ovarian factor(s) secreted to the blood may participate in regulating ovarian function, especially recruitment of oocytes to vitellogenic growth phase, for instance, by regulating (decreasing) the sensitivity of the ovary toward gonadotropin.