02414naa a2200337 a 450000100080000000500110000800800410001910000180006024501010007826000090017952015060018865000160169465000180171065000110172865000330173965000350177265000240180765000140183165000170184565000140186265000170187665000100189365000190190365000140192265000170193665000100195365000240196365300140198770000160200177300590201715335212024-01-03       1981    bl uuuu     u00u1 u    #d1 aFOSTER, D. L.  aEndocrine mechanisms governing transition into adulthood in famale sheep.h[electronic resource]  c1981  aIt is proposed that the first follicular phase in the lamb is initiated when responsiveness to oestradiol inhibition of LH secretion decreases sufficiently to permit the expression of an inherent hourly LH pulse rhythm. The hourly LH pulse rhythm is believed to drive oestradiol production to levels that induce the first LH surge. This hypothesis is based upon several considerations. First, pulsatile LH secretion invariably occurs at low frequencies in immature lambs whereas hourly pulses are manifest in postpubertal lambs and mature ewes during the follicular phase of the oestrous cycle. Second, intravenous administration of LH at hourly intervals to immature lambs results in an increase in follicular size, induction of an LH surge, ovulation and corpus luteum formation. Third, hourly LH pulses will occur in the immature female if the ovaries are removed; the hourly frequency in the immature ovariectomized lamb can be reduced by exogenous oestradiol. Fourth, in the chronically oestradiol-treated ovariectomized lamb, responsiveness to oestradiol inhibition of LH secretion becomes markedly decreased during the pubertal period, and the hourly LH pulse rhythm is expressed. It is further proposed that a minimum body weight and short daylengths are necessary for the reduction in oestradiol negative feedback. With regard to photoperiod, the delay in onset of cyclicity in lambs born in the wrong season (autumn) may be due to retarded maturation or post-maturational seasonal anoestrus.  aBody weight  aEndocrinology  aEstrus  aFollicle-stimulating hormone  aGonadotropin-releasing hormone  aLuteinizing hormone  aOvulation  aPharmacology  aPregnancy  aReproduction  aSheep  aEndocrinologia  aEstradiol  aOestrus Ovis  aOvino  aReprodução Animal  aPuberdade1 aRYAN, K. D.  tJournal Reproduction Fertilitygv. 30, p. 75-90, 1981.