Endocrine profile and OPU-ICSI outcomes in mares: a comparative study

Despite recent improvements in equine ovum pick-up (OPU) combined with intracytoplasmic sperm injection (ICSI), there is still significant inter-individual variability. In this study, serum concentrations of the oocyte-secreted factors growth differentiation factor 9 (GDF9) and bone morphogenetic protein 15 (BMP15), as well as the GDF9/BMP15 complex, anti-Müllerian hormone (AMH), progesterone (P4), and free and conjugated estradiol-17β (E2), were determined in 59 mares (3–24 years) at the time of OPU. Mares were classified retrospectively, based on the number of embryos obtained, into low or high outcome groups, using two or three embryos as the threshold values. Results of OPU-ICSI were not affected by the mare’s age, body condition score, cycle stage, number of aspirated follicles, number of recovered oocytes, or the recovery rate (P > 0.05 in all cases). Mares with <2 or ≥2 embryos had similar hormone levels (P > 0.05 in all cases), but there was a >90% likelihood of obtaining ≥2 embryos per session when P4 and E2 concentrations were considered simultaneously. Mares with ≥3 embryos/session had lower P4 and higher AMH (P < 0.05 in both cases). GDF9 and BMP15 were detectable in 14 and 12% of mares, respectively, while the GDF9/BMP15 complex was detected in 93% of mares, with no correlation to other factors. In conclusion, these preliminary findings suggest a negative correlation between high P4 levels and OPU-ICSI outcome in horses. Lay summary We analyzed the relationship between the hormone profiles of mares at the time of egg collection and the outcome of intracytoplasmic sperm injection (ICSI), as ICSI success often varies significantly between individuals. There were no differences in the hormone profiles of mares producing fewer than two or two or more embryos. However, mares with three or more embryos/session showed significant differences in their hormone profiles compared to those producing fewer than three embryos/session. These changes were mainly related to steroid hormones produced by the ovary, such as progesterone and estradiol, and to the concentration of AMH. Our results suggest a possible influence of the hormone profile of the mare on the number of embryos obtained.