Anti-Müllerian hormone (AMH) is a granulosa-cell derived glycoprotein which plays an important regulatory role during folliculogenesis. Antral follicle count (AFC) and AMH have been used in several species to assess ovarian reserve, but relatively little is known about these parameters in mares. The objectives of this study were to determine (i) plasma AMH concentrations and AFC in mares of different ages, as well as their repeatability within and across oestrous cycles; (ii) the relationship between plasma AMH concentrations and AFC with regard to age and follicle size; and (iii) endocrine and molecular changes in growing and preovulatory follicles in relation to AFC and peripheral AMH concentrations. In Exp. 1, young (3–8 years), middle-aged (9–18 years), and old (>18 years) mares were examined by transrectal ultrasonography over 2 to 3 oestrous cycles and changes in the number and size of all detectable antral follicles were recorded. Concentrations of AMH were determined by ELISA. In Exp. 2, molecular and endocrine differences in growing and dominant equine follicles were examined in relationship to variation in AFC, peripheral AMH concentrations and mare age. Follicular fluid oestradiol and AMH concentrations were determined by immunoassay, and expression of AMH, AMHR2, ESR1, ESR2, FSHR, IGF1, INHA, INHBA, and CYP19A1 was determined by quantitative RT-PCR. Antral follicle counts were significantly lower in old mares than in young and middle-aged mares, and AMH concentrations were significantly lower in old mares than in middle-aged mares. A positive relationship was detected between AFC and AMH, and this relationship varied by mare age with a strong correlation in older mares (ρ = 0.86; P < 0.0001), a moderate correlation in middle-aged mares (ρ = 0.60; P = 0.01), and no correlation (ρ = 0.40, P < 0.4) in young mares. Circulating AMH concentrations were significantly related to the number of antral follicles between 6 and 20 mm in diameter, and the repeatability of AFC and AMH concentrations was high within and between oestrous cycles. Within growing follicles, expression of AMH, AMHR2, ESR2, and INHA in granulosa cells was significantly associated with AFC and plasma AMH concentrations, whereas expression of ESR1 and FSHR were significantly correlated with AMH concentrations alone. In follicular fluid, AMH concentrations declined and oestradiol concentrations increased in dominant compared to growing follicles. Our findings indicate that the relationship between AMH and AFC varies across age groups, and that variations in AFC and plasma AMH concentrations are associated with molecular differences in the equine follicle, which, in turn, may have an impact on follicular function.
Funded by the Albert G. Clay Endowment at the University of Kentucky.
Follicle Structure Influences the Availability of Oxygen to the Oocyte in Antral Follicles
