Reproduction in Domestic Animals

Dominant Follicle Selection in Cows, Mares and Women 49been determined in the three species. Subsequently, twoimportant intrafollicular functions regulated during DFselection (FSH and LH-responsiveness, insulin-likegrowth factor (IGF) binding protein production) areproposed as examples of conserved DF selection mechanisms.Finally, new candidate genes for attainment ofdominance are presented from genomic bovine studies.Dominant Follicle Selection: Follicle WaveMorphologyAt specific times during the luteal phase of the oestrouscycle in cows, but also at intervals of 7–10 days in mostother reproductive states (and this includes the prepubertal,post-partum and early pregnant anoestrousstate), rapid growth of a cohort of small (from largerthan 1 mm in diameter) antral follicles to 3–5 mm andbeyond is observed using daily ovarian ultrasound(Roche et al. 1998; Ireland et al. 2000). This observationhas been termed ‘emergence of a follicular wave’ and isfollowed by continued growth of a constantly reducingnumber of cohort follicles over the next 3 days, whereonly the minority will reach diameters of 6 mm, two orthree follicles will reach 7–8 mm, and generally only onefollicle is selected to be the DF and grows beyond 8 mmin diameter (Fig. 1). For a large number of in vivostudies, the time of DF selection in the bovine has beenroutinely defined using ultrasound criteria such as thetime of divergence between the DFs continued rapidgrowth vs the reduced growth shown by its closestcompetitor, the largest subordinate follicle (SF1) (onsetof deviation; Ginther et al. 1997), or a minimum size of8.5 mm and minimum diameter difference of 1 to>2 mm to the SF1 (Ginther et al. 1997; Mihm et al.1997, 2008). All other cohort (subordinate) folliclescease growth at different times after wave emergence,decrease in size, and eventually disappear. In general,two or three follicle waves emerge during the lutealFig. 1. The relationship between the transient rise in FSH, follicle wavegrowth, which culminates in selection of the DF, and the follicularsecretions oestradiol and inhibin in cows, mares and women. Note thatin women this is most likely the ovulatory follicle wave which emergesat luteolysis and grows during the early follicular phase, in mares theovulatory follicle wave emerges in the second half of the luteal phaseand the DF is selected at luteolysis, and in cows the most studied folliclewave is the first wave of the cycle, which is anovulatory and emerges atovulation, with DF selection occurring in the early luteal phase.Cohort, dominant and subordinate follicles in mares are generally twicethe size of follicles in the other two species. Note the longer FSH declinein mares compared with cows, and the particularly long FSH decline inwomen. DF, dominant follicle, SF, subordinate folliclesphase of the 21-day oestrous cycle in cattle, eachselecting a DF, and with the final DF selected atluteolysis or shortly after and thus becoming theovulatory DF in the relatively short follicular phase(Savio et al. 1988; Ginther et al. 1989). Very rarely theemergence of a wave has been detected which may growfor one day but does not lead to selection of a new DF(Mihm et al. 1999). There are several interesting featuresof DF selection in the bovine, apparent when studyingthis process closely by ultrasound: (1) the future DFgenerally seems to emerge first, 6–7 h before the largestSFI, and the size advantage is maintained throughoutthe common parallel growth phase until deviation(Ginther et al. 1997); (2) the number of cohort folliclesemerging over a 48–72 h period is highly variablebetween animals and can also vary within the cycle,ranging from only 2 to more than 50 small antralfollicles per wave (Burns et al. 2005); (3) follicle numbersper wave in animals representing the two extremes, thatis in animals with very low (25) numbers of follicles per wave are consistentwithin the cycle and from cycle to cycle (Burns et al.2005); (4) follicle numbers per wave do not usuallyinfluence the process of selecting one single DF, as nohigher incidence of co-dominant follicles has beenreported in animals showing very high follicle numbers(Burns et al. 2005), or in animals treated exogenouslywith bovine growth hormone which increases thenumber of small antral follicles (cohort follicles) peranimal (Gong et al. 1991). The exception are cows fromherds selected for twinning such as those described inEchternkamp 2000; (5) the diameter of the DF at thetime of selection (approximately 8.5 mm) is very consistentacross all (oestrous and anoestrous) reproductivestates, and across different breeds and types of cattle,and in cyclic animals is independent of when during theluteal phase (beginning, mid, end) the DF emerges(Roche et al. 1998; Ireland et al. 2000; Ginther et al.2001a); and (6) co-dominance in animals not geneticallyselected for twinning is generally rare (approximately5%) but can occur either temporarily with late selectionof a single follicle, or throughout the complete dominanceperiod, leading to twin ovulations if the last waveof the cycle is affected (Echternkamp 2000; Kulick et al.2001) Overall, these features indicate that the DFselection mechanism in the bovine is very robust andin unselected cows can only be inhibited by pharmacologicalinterference which elevates FSH or abolishesGnRH pulsatility (Prendiville et al. 1995; Gong et al.1996; Mihm et al. 1997).In contrast to cows, in mares and women not allfollicle waves detected during an interovulatory intervalwill result in the selection of a DF (these are termed‘minor waves’ where cohort follicles never reach the sizeof a DF; details of major and minor wave follicledynamics in both species are reported in Ginther 1993;Baerwald et al. 2003; Ginther et al. 2004a). The ovulatorywave in mares, which is the only follicle waveguaranteed to result in selection of a DF, emerges in thesecond half of the luteal phase leading to selection of theDF coincident with luteal regression. In women, however,the ovulatory wave only emerges during the firsthalf of the follicular phase, and again is the only waveÓ 2008 The Authors. Journal compilation Ó 2008 Blackwell Verlag