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

Equine Embryo Transferablated 10 days after ovulation. Circulating FSHconcentrations increased and inhibin decreasedwithin one or 2 days after ablation. Ei<strong>the</strong>r <strong>the</strong>largest, 3 largest, or all follicles <strong>of</strong> <strong>the</strong> subsequentnew wave were retained and <strong>the</strong> remainder ablatedbefore <strong>the</strong>y reached >10 mm. Data werenormalised to <strong>the</strong> day <strong>the</strong> largest follicle <strong>of</strong> <strong>the</strong>post-ablation wave reached 13 mm (expectedbeginning <strong>of</strong> <strong>the</strong> FSH decline). Concentrations <strong>of</strong>FSH decreased and inhibin increased during <strong>the</strong>remaining parallel growing phase. In <strong>the</strong> onefolliclegroup, <strong>the</strong> increase in inhibinconcentrations after <strong>the</strong> beginning <strong>of</strong> <strong>the</strong> FSHdecline was delayed and was temporallyassociated with a slower decrease in FSH than in<strong>the</strong> groups with three or all follicles retained. Theinterrelationships among FSH, follicles, andinhibin were also demonstrated in a group inwhich all follicles <strong>of</strong> <strong>the</strong> new wave were ablated at13mm from <strong>the</strong> beginning <strong>of</strong> <strong>the</strong> FSH decline until<strong>the</strong> end <strong>of</strong> <strong>the</strong> parallel phase. This effect wasattributable to <strong>the</strong> secretion <strong>of</strong> inhibin, based on<strong>the</strong> temporal relationships between increasinginhibin concentrations and decreasing FSHconcentrations and on <strong>the</strong> positive relationshipbetween <strong>the</strong> number <strong>of</strong> retained follicles and <strong>the</strong>extent <strong>of</strong> <strong>the</strong> increase in inhibin. Near <strong>the</strong> expectedend <strong>of</strong> <strong>the</strong> parallel growing phase or beginning <strong>of</strong>deviation, concentrations <strong>of</strong> FSH were no longerdifferent among groups with various numbers <strong>of</strong>retained follicles, and inhibin appeared to reachmaximum concentrations. At <strong>the</strong> beginning <strong>of</strong>deviation, <strong>the</strong> FSH concentrations apparently arealready too low in most waves for survival <strong>of</strong> <strong>the</strong>less-developed smaller follicles.In a recent cattle study (Gin<strong>the</strong>r et al. 2000b),an increase in FSH concentrations occurredimmediately following ablation <strong>of</strong> <strong>the</strong> largestfollicle at <strong>the</strong> beginning <strong>of</strong> deviation and aminimal single dose <strong>of</strong> oestradiol at <strong>the</strong> time <strong>of</strong>ablation delayed <strong>the</strong> FSH increase. This findingindicated that oestradiol was one <strong>of</strong> <strong>the</strong> FSHdepressingfactors that was lost when <strong>the</strong> largestfollicle was ablated. Thus, oestradiol contributes to<strong>the</strong> continued depression in FSH concentrationsafter <strong>the</strong> expected beginning <strong>of</strong> deviation. Asimilar study has not been done in mares.However, increasing oestradiol concentrations aresecreted from <strong>the</strong> largest follicle in mares justbefore <strong>the</strong> beginning <strong>of</strong> deviation. The continuingdecrease in concentrations <strong>of</strong> FSH at <strong>the</strong> beginning<strong>of</strong> deviation likely reflects <strong>the</strong> synergistic effect(Miller et al. 1979) <strong>of</strong> high concentrations <strong>of</strong>inhibin and <strong>the</strong> increasing concentrations <strong>of</strong>oestradiol. However, <strong>the</strong> relative contributions <strong>of</strong>inhibin and oestradiol in <strong>the</strong> suppression <strong>of</strong> FSH atvarious times during deviation have not beendetermined adequately. The role <strong>of</strong> <strong>the</strong> continuingdecline in FSH after <strong>the</strong> beginning <strong>of</strong> deviation isnot known. It may serve as an assurance that FSHis adequately depressed for deviation to occur inall waves or may prevent <strong>the</strong> emergence <strong>of</strong> ano<strong>the</strong>rwave.Increasing or transiently elevated LHconcentrations encompass deviation in both maresand cattle (Gin<strong>the</strong>r et al. 2000c). Concentrations <strong>of</strong>LH have been experimentally reduced by doses <strong>of</strong>progesterone that did not alter <strong>the</strong> concentrations<strong>of</strong> FSH. In initial studies in mares, <strong>the</strong> reduced LHwas associated with a smaller maximum diameter<strong>of</strong> <strong>the</strong> dominant follicle. However, this effectoccurred well after <strong>the</strong> beginning <strong>of</strong> follicledeviation; <strong>the</strong> growth pr<strong>of</strong>ile <strong>of</strong> <strong>the</strong> second-largestfollicle was not altered. In a more recent study inmares (Bergfelt et al. 2000), experimentalreduction in concentrations <strong>of</strong> LH beginningbefore deviation was associated with reducedcirculating concentrations <strong>of</strong> oestradiol. However,<strong>the</strong> LH and oestradiol reduction did not alter FSHconcentrations until 2 days after <strong>the</strong> expectedbeginning <strong>of</strong> deviation. At that time, an increase inFSH concentrations occurred as well as areduction in diameter <strong>of</strong> <strong>the</strong> dominant follicle.These results indicated that <strong>the</strong> increasedconcentrations <strong>of</strong> LH that encompass deviation areutilised by <strong>the</strong> developing dominant follicle for <strong>the</strong>secretion <strong>of</strong> oestradiol. The oestradiol, as notedabove, contributes to continued FSH suppressionand may also be important for facilitating growth<strong>of</strong> <strong>the</strong> dominant follicle and a change fromprimarily FSH to LH dependency.The research model that our laboratory iscurrently using for <strong>the</strong> mechanism <strong>of</strong> follicleselection in mares is shown (Fig 1). It isrecognised that good research models are modifiedand poor models fade away.16