Histomorphometric Analysis of the Endometrium of Jennies and Mares: Anatomical Differences and Possible Reproductive Implications

Assisted reproductive technologies (ARTs) are often extrapolated from horses to don-keys, despite poorer fertility outcomes in jennies, highlighting the need to identify spe-cies-specific traits to optimize ARTs in donkeys. This study compared, through histo-morphometry, the endometrium of jennies and mares. Endometrial biopsies were taken from reproductively sound jennies (n = 6) and mares (n = 6) in estrus. Histomorpho-metric analysis evaluated luminal (LE, µm) and glandular epithelium height (GE, µm), glandular lumen diameter (LD, µm), glandular area (GA, µm²), number of glands (#G), and glandular tissue percentage (GT, %), measured in the stratum compactum (SC) and spongiosum (SS). Results were statistically compared between species (jennies vs. mares), parity status (maiden vs. foaling), and stratum (SC vs. SS). Jennies exhibited higher (P < 0.05) values than mares for LE, LD-SC, GA-SC, and GT-SC. Greater glan-dular size and density in mares have been linked to susceptibility to endometritis, in-trauterine fluid accumulation, aging, and poorer reproductive outcomes in mares. These findings suggest that the histomorphometric features observed in reproductively sound jennies reflect anatomical differences that might contribute to a distinctive en-dometrial inflammatory response compared to mares. In conclusion, histomorphome-try revealed significant endometrial differences between mares and jennies, the latter displaying larger luminal epithelium, greater glandular size, and density than mares.