Reproduction in Domestic Animals

Follicles and Mares 2251982). Similar to pre-antral stages, little is known on thedevelopment of antral follicles before they reachapproximately 2 mm, the smallest diameter that canusually be detected by transrectal ultrasonography. Ithas been reported that the growth of an equine folliclefrom 0.1 to 1 mm takes approximately two oestrouscycles (Driancourt 1979) and that atresia duringthis early phase of antral development is rare (Ginther1992).As in other farm species and humans, the developmentof antral follicles in the horse is characterized bythe periodic growth of cohorts of follicles or follicularwaves (Sirois et al. 1989; Bergfelt and Ginther 1993).Characterization of follicular waves has involved theultrasonic day-to-day identification of individual follicles(Bergfelt and Ginther 1993; Gastal et al. 1997) andthe use of a statistical method that avoids the need tomaintain the identities of individual follicles duringserial ultrasound examinations (Ginther and Bergfelt1992; Donadeu and Ginther 2002b). Follicular wavesin the horse can be identified in relation to follicles2 mm in diameter and larger; however, it is not knownwhether earlier antral and pre-antral stages are alsocharacterized by wave-like patterns of growth,a question that has also not been clarified in otherspecies (Mizunuma et al. 1999; McGee and Hsueh2000). Follicular waves and their regulation with anemphasis on normal oestrous cycles will be described inthis and the next section. Follicular wave patternscharacteristic of other reproductive states will bedescribed in a separate section.Characteristics of a follicular wave. Follicular waveemergence has been normally defined for experimentalpurposes as occurring when the largest follicle reaches 6or 13 mm, depending on the study (Ginther et al.2003a). Identification of wave emergence often requiresday-to-day ultrasonic evaluation of the ovary usuallyafter aspiration of all follicles from previous waves.A follicular wave initially involves the simultaneousgrowth of a variable number of follicles at a commonrate of between 2 and 3 mm ⁄ day. A recent studyinvolving ablation of all follicles during the middle ofan oestrous cycle in pony mares reported a mean ofapproximately 12 follicles emerging during the commongrowth phase of the ablation-induced wave (Gastalet al. 2004). Approximately two follicles emerged eachday during the first 4 days after wave emergence with aprogressive decrease in the numbers of follicles emergingthereafter. The exact number of follicles emerging withinwaves is affected by factors such as season (Donadeuand Ginther 2003).The phase of common follicle growth is followed bythe selection of a single follicle (occasionally twofollicles) which is manifested as a deviation in diameterbetween the two largest follicles of the wave beginningwhen the largest follicle reaches approximately 22 mm(Ginther et al. 2003a). Deviation begins a mean of7 days before ovulation and is characterized by thecontinuous growth (at an unchanged rate) of the largest(selected) follicle as a dominant follicle and thesimultaneous cease in growth and subsequent regressionof smaller, subordinate follicles. Once it grows toapproximately 35–45 mm, the dominant follicle normallyeither ovulates or ceases to grow and begins toregress, depending on whether an ovulatory LH surgeoccurs. The establishment of dominance is mediated bya differential increase in trophic support to the largestfollicle of a wave by mechanisms that will be explainedin more detail in the next section. This leads to profoundchanges in follicular cell function that are necessary forthe eventual full maturation of the follicle to anovulatory-competent state and that are reflected indramatic changes in global gene expression, changesthat have begun to be characterized in other species,most notably cattle (Sisco et al. 2003; Fayad et al. 2004;Mihm et al. 2008).Dominance does not seem to be a pre-determinedtrait among the follicles growing in a wave becausefollicle ablation studies have demonstrated that allfollicles have similar capacity to become dominant,a capacity that in subordinate follicles is lost withinapproximately 48 h after the beginning of deviation(Gastal et al. 2004). The same studies showed that inapproximately 61% of waves the first follicle toemerge at 6 mm maintains its size advantage oversmaller follicles through the common growth phaseand becomes dominant. The likelihood of the largestfollicle becoming dominant increases as it approachesthe expected diameter at the beginning of deviation(Gastal et al. 2004). In a few instances, yet, the largestfollicle ceases or slows down its growth during thecommon growth phase and is replaced by the secondlargest follicle (or sometimes even a smaller follicle)which then becomes the dominant follicle.Types of follicular waves. Follicular waves have beenclassified as major waves (referred to in the literatureand throughout this review simply as follicular waves)or minor waves, depending on whether they involve thedevelopment of a readily identifiable dominant follicleor they produce only smaller follicles, respectively(Ginther 1993; Donadeu and Ginther 2002b). Thenumber of follicular waves during an oestrous cyclevaries between species. In the horse, as in humans, onlyone or two major follicular waves develop during eachcycle (Ginther et al. 2004c). A major wave (namedprimary wave) always emerges during the middle of theequine oestrous cycle and produces the ovulatoryfollicle. Approximately 25% of interovulatory intervalsinvolve an additional major wave (secondary wave) thatdevelops during the first half of the cycle (Sirois et al.1989; Bergfelt and Ginther 1993). The incidence ofsecondary waves is significantly higher in some breedssuch as Thoroughbreds, and some of these waves mayproduce ovulations. The ability to ovulate in thepresence of high progesterone levels during dioestrusseems to be unique to the horse (Ginther 1992). Minorwaves (largest follicle usually