# Histomorphometric Analysis of the Endometrium of Jennies (Equus asinus) and Mares (Equus caballus) in Estrus: Anatomical Differences and Possible Reproductive Implications

## TL;DR

This study compares the uterine anatomy of donkeys and horses to explain why donkeys have lower success in assisted reproductive techniques.

## Contribution

The first quantitative histomorphometric analysis of the endometrium in reproductively sound jennies compared to mares.

## Key findings

- Jennies have taller luminal epithelium and larger glands compared to mares.
- Glandular tissue percentage in the stratum compactum is higher in jennies.
- These anatomical differences may explain the species-specific uterine response during assisted reproduction.

## Abstract

The donkey population has significantly decreased over the last few decades, and some breeds are now considered endangered. Assisted reproductive techniques (ARTs) play a key role in saving donkeys from extinction. However, the application of current ARTs often yields poorer fertility results in donkeys than in the species they are usually compared to, i.e., horses. It has been suggested that the key to this disparity might lie in the species-specific uterine response, but the anatomical and physiological differences between the uteri of these species remain unknown. In this study, we measured and compared the uterine cells and glands of reproductively sound donkeys (jennies) and horses (mares). Jennies displayed larger and more widely distributed glands than mares. This study provides the first quantitative characterization of the jenny endometrium, highlighting its differences with mares. These species-specific structural variations may partly underlie the distinct uterine response seen during ARTs.

Assisted reproductive techniques are often extrapolated from horses to donkeys, despite poorer fertility outcomes in jennies. This issue has been attributed to unknown uterine species-specific differences. This study compared, through histomorphometry, the endometrium of jennies and mares. Endometrial biopsies (N = 12) were taken from reproductively sound jennies (n = 6) and mares (n = 6) in estrus. Histomorphometric analysis evaluated luminal (LE, µm) and glandular epithelium height (GE, µm), glandular lumen diameter (LD, µm), glandular area (GA, µm2), the number of glands (#G), and glandular tissue percentage (GT, %), measured in the stratum compactum (SC) and spongiosum (SS). A total of 30 measurements of glandular size parameters and 10 fields of glandular density parameters per sample were recorded. 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. These findings suggest that the histomorphometric features observed in reproductively sound jennies reflect anatomical differences that might partly explain previously observed species differences in post-breeding uterine response. In conclusion, histomorphometry revealed significant endometrial differences between species, with jennies displaying taller luminal epithelium, greater glandular size, and higher glandular tissue percentage in the SC than mares.