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

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Havemeyer Foundation Monograph Series No. 3FOLLICULAR WAVES AND SELECTION OFFOLLICLES IN MARESO. J. Gin<strong>the</strong>r, D. R. Bergfelt and F. X. DonadeuAnimal Health and Biomedical Sciences, 1656 Linden Drive, University <strong>of</strong> Wisconsin, Madison,Wisconsin 53706, USAFollicle selection in mares is <strong>the</strong> mechanismwhereby only one <strong>of</strong> many available follicles <strong>of</strong> awave becomes <strong>the</strong> ovulatory follicle. Part <strong>of</strong> thisreport is a synopsis <strong>of</strong> a review on selection thatwas written in July 1999 and <strong>the</strong> references up tothat month are cited (Gin<strong>the</strong>r 2000a). The o<strong>the</strong>rpart discusses studies completed from August1999 to August 2000.Follicular waves in mares can be classified asmajor waves (characterised by dominant andsubordinate follicles) and minor waves (largestfollicle does not attain <strong>the</strong> diameter <strong>of</strong> a dominantfollicle). Both types <strong>of</strong> waves develop inassociation with a surge in FSH concentrations. Inmajor waves, <strong>the</strong> future dominant follicle isdetected earlier, on average, than <strong>the</strong> o<strong>the</strong>rfollicles, and <strong>the</strong> follicles grow at a similar rate forseveral days (parallel growing phase). Thedifference in mean diameter between <strong>the</strong> 2 largestfollicles at first detection and during <strong>the</strong> parallelphase is 2–3 mm, which is equivalent to a growingperiod <strong>of</strong> approximately one day. When <strong>the</strong> largestfollicle reaches a mean <strong>of</strong> about 22.5 mm, <strong>the</strong>parallel phase ends and follicle deviation begins.Deviation is recognised by a continuing growthrate <strong>of</strong> <strong>the</strong> largest or developing dominant follicleand a decreasing growth rate <strong>of</strong> <strong>the</strong> subordinatefollicles. The mean diameter differences between<strong>the</strong> 2 largest follicles at <strong>the</strong> beginning <strong>of</strong> deviationsuggests that <strong>the</strong> largest follicle becomesestablished as <strong>the</strong> dominant follicle within one dayor before <strong>the</strong> next largest follicle reaches a similardiameter (Fig 1).The FSH surge that stimulated emergence <strong>of</strong><strong>the</strong> wave begins to decline in concentrations when<strong>the</strong> largest follicle reaches a diameter <strong>of</strong> about 13mm. The FSH decline continues during <strong>the</strong>remaining parallel growing phase and for severaldays after follicle deviation. The necessity for lowconcentrations <strong>of</strong> FSH for deviation is consistentwith <strong>the</strong> formation <strong>of</strong> multiple dominant folliclesfollowing administration <strong>of</strong> FSH or a substance(anti-inhibin) that raises <strong>the</strong> endogenousconcentrations <strong>of</strong> FSH.According to studies in cattle, all <strong>of</strong> <strong>the</strong>follicles <strong>of</strong> <strong>the</strong> wave continue to utilise <strong>the</strong>declining FSH concentrations during <strong>the</strong> parallelgrowing phase. Results <strong>of</strong> a recent study in mares(Donadeu and Gin<strong>the</strong>r 2000) demonstrated thatmore than one follicle <strong>of</strong> <strong>the</strong> parallel phasecontributes to <strong>the</strong> FSH decline. The effect <strong>of</strong> FSHon <strong>the</strong> follicles and conversely <strong>the</strong> effect <strong>of</strong> <strong>the</strong>follicles on FSH during <strong>the</strong> parallel phase havebeen described as a 2-way functional coupling,involving multiple follicles. The FSH/multiplefolliclecoupling becomes an FSH/single-folliclecoupling at <strong>the</strong> end <strong>of</strong> <strong>the</strong> parallel phase or <strong>the</strong>beginning <strong>of</strong> diameter deviation. Two-wayFSH/follicle coupling has been postulated to be<strong>the</strong> basis <strong>of</strong> follicle selection (Gin<strong>the</strong>r 2000a,b).Ironically, <strong>the</strong> pool <strong>of</strong> growing follicles directs acontinuing decline in available FSH, despite <strong>the</strong>follicles continuing requirement for FSH. By <strong>the</strong>time <strong>the</strong> FSH concentrations reach a precariouslevel at <strong>the</strong> end <strong>of</strong> <strong>the</strong> parallel phase, only <strong>the</strong> mostdeveloped or largest follicle is able to utilise <strong>the</strong>low levels and to direct a continuing decline. Theability <strong>of</strong> <strong>the</strong> developing dominant follicle toutilise <strong>the</strong> low FSH concentrations, at leastinitially, has been demonstrated in cattle; a similarstudy has not been done in mares.Results <strong>of</strong> a recent study in mares indicate thatinhibin is <strong>the</strong> substance that is secreted by <strong>the</strong>multiple follicles and causes <strong>the</strong> FSH declineduring <strong>the</strong> parallel growing phase (Donadeu andGin<strong>the</strong>r 2000). All follicles 6 mm or larger were15
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
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