Abstract
The anti-Müllerian hormone (AMH) plays an inhibitory role during folliculogenesis by regulating the number of follicles entering the growing pool. Antral follicle counts (AFC) are highly correlated with serum AMH concentrations and both appear to be related to the ovarian reserve in several species. Few data on AMH and AFC in mares exist, especially with regard to fertility. Therefore, the objective of the current study was to investigate the interrelationship between antral follicle count, serum AMH concentrations and fertility outcome in mares. One hundred and twenty-seven mares were enrolled in the study and grouped according to their reproductive status. Around time of estrus, serum AMH concentrations and AFC before and after ovulation were determined. Mares were artificially inseminated and pregnancy diagnosis was performed 14 to 18 days later. A high inter-individual variability in AFC and AMH concentration and a positive relationship between AMH and AFC for follicles ≤ 30 mm in diameter were observed, with a stronger correlation in mares older than 18 years. A high correlation between AFC measurements before and after ovulation was identified. The AFC after ovulation was higher than AFC before ovulation. AMH concentrations were neither related to the mares' reproductive status nor to age, number of cycles needed for pregnancy and pregnancy outcome. Excepted for a higher AFC in the middle-aged mares (9-18 years) compared to the younger mares (< 9 years), no associations between AFC and age, reproductive status as well as fertility of mares were found. This study confirms the relationship between AFC and AMH and a high degree of reproducibility of AFC measurements. However, based on our findings, neither AFC nor AMH are useful predictors of fertility in mares.
Highlights
Antral follicle counts (AFC) are highly correlated with serum anti-Müllerian hormone (AMH) concentrations and both appear to be related to the ovarian reserve in several species
The objective of the current study was to investigate the interrelationship between antral follicle count, serum AMH concentrations and fertility outcome in mares
AMH concentrations were neither related to the mares’ reproductive status nor to age, number of cycles needed for pregnancy and pregnancy outcome
Summary
The number of follicles present in female ovaries is established during the fetal period and, due to apoptotic processes resulting in atresia, it already starts to decrease before birth.[1,2] The size of the pool of non-growing follicles (NGF), which includes primordial, intermediate and small primary follicles, varies greatly between individuals and species and is age-dependent.[2,3,4,5,6,7,8,9] The total number of resting follicles at birth averages 520’000 in humans and 150’000 in bovines.[3,8] Estimates for the number of oocytes present in the ovaries of the mare at birth are not available. AMH concentrations and AFC decline with age in all the aforementioned species, but there is a great variability of both AMH concentration and AFC even between individuals of the same age and the onset of reproductive senescence associated with reduced ovarian reserve varies considerably.[8,12,15,18,21,30, 43,44,45,46,47] In some studies, no significant correlations between AMH and/or AFC and pregnancy rate have been reported in mares and women, indicating that besides the number of primordial follicles present in the ovaries and the AFC and AMH correlated to it, other factors such as oocyte quality are likely to play a role.[33,34,35] On the other hand, the study by Ball et al (2019)[12] demonstrated a relationship between AMH and fertility, which was reduced in mares with low serum AMH
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