Proceedings of the 5th International Symposium on EQUINE ...

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Havemeyer Foundation Monograph Series No. 33530a)Parallel phaseBeginning <strong>of</strong> deviationDominantfollicleDiameter (mm)2520151013 mm22.5 mmLargestfollicle2nd largestfollicleSubordinatefollicle50 1 2 3 4 5 6 7 8 9 10 11b)FSHLH3530OestradiolDominantfollicleDiameter (mm)252015105InhibinTwo largest folliclesDeviationSubordinatefollicle0 1 2 3 4 5 6 7 8 9 10 11Days after wave emergenceFig 1: Schematic 2-follicle model <strong>of</strong> <strong>the</strong> size advantage <strong>of</strong> <strong>the</strong> largest follicle (a) and <strong>the</strong> hormonal aspects (b) <strong>of</strong> follicledeviation in mares. The largest follicle emerges, on average, about one day before <strong>the</strong> second-largest follicle, and <strong>the</strong>2 follicles grow in parallel (a). When <strong>the</strong> follicles reach 13 mm, <strong>the</strong>y begin to secrete a biologically active form <strong>of</strong>inhibin. As a result, <strong>the</strong> FSH surge that stimulated emergence <strong>of</strong> <strong>the</strong> follicles, begins to decline (b). Both follicles secreteincreasing inhibin causing <strong>the</strong> continuing FSH decline, and <strong>the</strong> declining FSH is utilised by both follicles (2-wayfunctional coupling). The parallel phase ends and follicle deviation begins when <strong>the</strong> largest follicle reaches 22.5 mm(a). Deviation is characterised by continued growth rate <strong>of</strong> <strong>the</strong> largest or developing dominant follicle and decreasinggrowth rate <strong>of</strong> <strong>the</strong> smaller or developing subordinate follicle. Deviation is established in less than one day or before <strong>the</strong>second-largest follicle can reach a similar diameter (represented by <strong>the</strong> width <strong>of</strong> <strong>the</strong> vertical bar in panel a). Duringthis time, <strong>the</strong> FSH/follicle coupling changes from multiple-follicle to single-follicle. The more-developed larger folliclecontinues <strong>the</strong> FSH/follicle coupling because it is able to utilise <strong>the</strong> reduced concentrations <strong>of</strong> FSH. However, <strong>the</strong> lessdevelopedsmaller follicle is unable to maintain follicle coupling because <strong>the</strong> declining concentrations <strong>of</strong> FSH havereached a level below its requirements. Just before <strong>the</strong> beginning <strong>of</strong> deviation, oestradiol is secreted by <strong>the</strong> largestfollicle under <strong>the</strong> influence <strong>of</strong> increased concentrations <strong>of</strong> LH (b). The increasing oestradiol acts synergistically withplateaued inhibin to continue <strong>the</strong> reduction in FSH concentrations after deviation. The elevated LH continues tostimulate <strong>the</strong> production <strong>of</strong> oestradiol by <strong>the</strong> developing dominant follicle and exerts a trophic effect on <strong>the</strong> dominantfollicle within 2 days after <strong>the</strong> beginning <strong>of</strong> deviation.17
Equine Embryo TransferREFERENCESBergfelt, D.R., Gastal, E.L. and Gin<strong>the</strong>r, O.J. (2000)Systemic oestradiol and inhibin concentrations inresponse to experimentally reduced LHconcentrations during follicle deviation in mares.Biol. Reprod. Submitted.Donadeu, F.X. and Gin<strong>the</strong>r, O.J. (2000) Effects <strong>of</strong> folliclenumber and diameter on <strong>the</strong> secretion <strong>of</strong> inhibin and<strong>the</strong> suppression <strong>of</strong> follicle stimulating hormone inmares. Biol. Reprod. Submitted.Gin<strong>the</strong>r, O.J. (2000a) Selection <strong>of</strong> <strong>the</strong> dominant follicle incattle and horses. Anim. Reprod. Sci. 60-61, 61-79.Gin<strong>the</strong>r, O.J. (2000b) The FSH-follicle couplinghypo<strong>the</strong>sis for follicle selection. Biol Reprod. Suppl.62, 1, 92.Gin<strong>the</strong>r, O.J., Bergfelt, D.R., Kulick, L.J. and Kot, K.(2000a) Selection <strong>of</strong> <strong>the</strong> dominant follicle in cattle:role <strong>of</strong> two-way functional coupling betweenfollicle-stimulating hormone and <strong>the</strong> follicles. Biol.Reprod. 62, 920–927.Gin<strong>the</strong>r, O.J., Bergfelt, D.R., Kulick, L.J. and Kot ,K.(2000b) Selection <strong>of</strong> <strong>the</strong> dominant follicle in cattle:role <strong>of</strong> oestradiol. Biol. Reprod. 63, 383–389.Gin<strong>the</strong>r, O.J., Bergfelt, D.R., Beg, M.A. and Kot, K.(2000c) Follicle selection in cattle: role <strong>of</strong>luteinising hormone. Biol. Reprod. in press.Miller, K.F., Wesson, J.A. and Gin<strong>the</strong>r, O.J. (1979)Changes in concentrations <strong>of</strong> circulatinggonadotropins following administration <strong>of</strong> equinefollicular fluid to ovariectomized mares. Biol.Reprod. 21, 867-872.18
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