Reproduction in Domestic Animals

228 FX Donadeu and HG PedersenBased on reported hormone–follicle associations, deficientFSH levels do not seem to be responsible for thereduction in follicular growth in some mares betweendays 11 and 40 of pregnancy (Ginther and Bergfelt1992), an effect that may instead be accounted for byreduced levels of LH (due to persistent progesteronenegative feedback), similar to the effects of the seasonalreduction in circulating LH on follicle growth during thefall transitional period (Ginther et al. 2003b).The mechanisms responsible for the dramatic reductionin follicular growth during mid and late pregnancyin mares have not been clarified but likely involves atemporally associated decrease in circulating FSH(reviewed in Ginther 1992). This is different from theanovulatory season during which changes in FSH levelsdo not seem to play a major role in the decrease infollicle growth during deep anoestrus (Donadeu andWatson 2007). Reduced follicle numbers during midpregnancyare also likely attributable to ovulation orluteinization of follicles into accessory corpora luteaunder the influence of chorionic gonadotropin (Ginther1992). Further complexity into the regulation of folliclegrowth during pregnancy is provided by the observationthat the effects of season on hormones and folliclescontinue to occur during pregnancy (Ginther 1992).The natural pressure to produce a foal each year in aspecies with an 11-month-long pregnancy is reflected inan early post-partum ovulation in the mare (foal heat,typically within 2 weeks of parturition). A steadyincrease in the diameter of the largest follicle andin the numbers of follicles after parturition resultingin ovulation 14 days later was recently shown inArabian mares (Gunduz et al. 2008). The post-partumincrease in follicular growth is induced by an increase ingonadotropin secretion at the time of parturition (Hineset al. 1987; Ginther et al. 1994). Studies have shown thatthe follicular response to parturition may vary inindividual horses or different types of horses, and maynot readily occur in primiparous mares (Nagy et al.1998), in the presence of a nursing foal (Ginther 1992) orduring the winter, when the negative effects of seasonmay prevail over the positive effects of parturition(Ginther et al. 1994).Follicle development before puberty. Follicle developmentin spring-born pre-puberal pony fillies has recentlybeen studied in detail (Nogueira and Ginther 2004),adding to limited information on follicle profiles fromearlier studies (reviewed in Ginther 1992). Follicularactivity during 2–10 months of life was characterized bya progressive increase in mean follicle diameter (fromapproximately 6 to 10 mm) and mean follicle numbers(from 3 to 17 follicles) between 2 and 5 months of age,a short plateau in activity coinciding with the wintermonths and a re-initiation of follicle growth after7 months of age leading to the onset of the firstovulatory season in spring (Nogueira and Ginther2004). Changes in follicle activity during the first yearof life were positively correlated with changes incirculating gonadotropins, consistent with a regulatoryrole of season on gonadotropin and follicular activitybeginning early during life in mares. Follicular growthbefore puberty was characterized by the development of(minor) follicular waves. Remarkably, these waves werenot temporally associated with statistically significantcirculating FSH surges, an observation that warrantsfurther investigation.Effects of aging on follicular activity. As highlighted in arecent review, many age-related changes in follicularactivity in the horse resemble those occurring in humans(Carnevale 2008). Follicular activity during oestrouscycles begins to decrease in mares 20 years of age orolder eventually leading to a cease in ovarian activity(Carnevale et al. 1993, 1994). Interovulatory intervalsfirst become longer in these mares due to longerfollicular phases associated with a primary follicularwave that emerges later and contains less follicles. Inaddition, the ovulatory LH surge is less pronounced inthe older mares, and there is a higher incidence ofultrasonically atypical ovulations characterized by acentral hypoechogenic area at the ovulatory site. Thereduction in follicular activity and frequency of ovulationin mares ‡20 years old is associated with an overallelevation in concentrations of FSH and LH during thefollicular phase, a phenomenon that also occurs duringthe peri-menopausal period in women. This is eventuallyassociated with persistent ovarian inactivity with follicles