Efeito do período de exposição ao estrógeno prévia à progesterona sobre a histomorfometria do endométrio de éguas cíclicas e acíclicas

Abstract

The use of acyclic mares as recipients represents an alternative to increase the availability of females in embryo transfer (ET) programs. Estrogen- and progesterone-based protocols are used for endometrial preparation, and the duration of estrogen exposure may influence uterine receptivity. This study evaluated the effects of different periods of exposure to estradiol cypionate (ECP) prior to long-acting progesterone (LA P4) on the endometrial histomorphometry of mares. Sixteen anestrous mares were subjected to treatments with ECP for seven days (Long Estrus - LE, n=5), for two days (Short Estrus - SE n=5), or received only LA P4 (No Estrus - NE, n=6), and were subsequently evaluated during the cyclic phase and classified according to estrus duration (Cyclic Short Estrus - CSE n=6, Cyclic Long Estrus - CLE n=6). Uterine biopsies were collected four days after progesterone administration in acyclic mares, or after ovulation in cyclic mares, processed using the classic paraffinembedding method and stained with Hematoxylin and Eosin (HE), and subsequently analyzed for luminal epithelial height (LEH), glandular epithelial height (GEH), endometrial thickness (ET), and glandular density (GD). For statistical analysis, the data were organized in different ways. Initially, comparisons were performed among all groups together LE (n = 5), SE (n = 5), CLE (n = 5), CSE (n = 5), and NE (n = 6). Subsequently, they were grouped according to the presence or duration of estrus, regardless of cyclicity LE (Cyclic + Acyclic - C+AC, n = 10), SE (Cyclic + Acyclic - C+AC (n = 10), and NE (n = 6), and then reorganized and compared among cyclic mares (CLE + CSE, n = 10), acyclic mares (LE + SE + NE, n=16), and acyclic mares treated with estradiol (LE + SE, n = 10), regardless of estrus duration. When the groups were organized according to the presence and duration of estrus, it was observed that, for LEH, the LE C+AC and SE C+AC groups showed higher values than the NE group. Regarding GEH, the LE C+AC group also showed higher values than the NE group. In the comparison among the five groups, LEH was greater in the CSE and CLE groups compared to the NE group. GEH showed higher values in the CLE and LE groups when compared to the CSE group. Regarding GD, the CSE group presented higher values compared to the NE and CLE groups. No significant differences were observed in endometrial thickness (ET) among the evaluated groups. Overall, greater exposure to estrogen promoted an increase in luminal and glandular epithelial height compared to mares without estrus, with no effect on endometrial thickness. These findings highlight the modulatory role of estrogen on the endometrial epithelium, contributing to the understanding of uterine preparation in recipient mares in embryo transfer programs.