Does the uterine inflammatory response differ between mares and donkey jennies inseminated with frozen donkey semen?

Abstract

Frozen donkey semen produces poor fertility rates in jennies (0-11%) while showing normal fertility in mares (33-53%). This may be related to interspecific differences in the interaction of the uterus with frozen semen. This study aimed to evaluate and compare the inflammatory process in the endometrium of donkey jennies and mares inseminated with cryopreserved donkey semen. A total of 36 estrous cycles from 12 Pega donkey jennies and 36 estrous cycles from 12 mares (Breton and Mangalarga), aged between 3 and 14 years old were used. Once ovulation was detected, the animals were inseminated deeply in the horn ipsilateral to ovulation with thawed donkey semen containing 1 billion sperm. The following parameters were evaluated in both species at ovulation/before insemination (time 0h) and after insemination (times 6h, 12h, 24h, 48h, 72h, 96h and 120h): uterine vascularization, endometrial cytology, serum progesterone levels and pregnancy rate. Normally distributed data were analyzed using a mixed-model ANOVA and a Wilcoxon test for non-normally distributed data, using R software. Donkey jennies showed a higher percentage of colored pixels in the uterine body region at T6 compared to mares. Also, the percentage of inflammatory cells was higher in the donkey jenniescompared to the equine cytology at T12 (52.75 versus 37.44) and T48 (16.00 versus 3.98). The inflammatory peak was reached at T6 in both species (55.77 versus 57.31) but remained elevated until T12 only in donkeys. Although a complete resolution of the inflammation occurred within 48 hours after AI in both species, the percentage of inflammatory cells remained higher in jennies than in mares. An eosinophilic uterine infiltrate was present in donkeys, while it was not observed in mares. In addition, donkey jennies showed a significant increase in monocytes at T12, compared to the other times. Also, jennies showed a higher mean P4 level (13 ng/ml), compared to the mares (6 ng/ml), both species had progesterone concentrations compatible with the maintenance of pregnancy. The pregnancy rate 14 days after AI was higher in mares than in donkeys, reaching a rate of 54.55% (6/11) versus 0% (0/12), respectively. In conclusion, the inflammatory uterine response in the presence of frozen donkey semen differs between mares and jennies. Jennies have a later inflammatory resolution, marked by the presence of intense polymorphonuclear and eosinophilic infiltrates. These differences may contribute to the lower fertility rates in this species.