Morphological Characteristics of the Donkey (Equus asinus) Uterus during Estrus: Light, Scanning and Transmission Electron Microscopic Study

The aim of the present study was to describe the morphological characteristics of the donkey uterus during estrus. Specimens from 8 donkey uteri were fixed in glutaraldehyde solution and examined using light (LM), scanning (SEM) and transmission (TEM) electron microscopy. Light microscopy revealed that the donkey endometrium is arranged into folds of variable heights separated by deep grooves. The surface epithelium consisted of tall columnar (18.60 ± 0.65 µm) cells that were either ciliated and microvillus in nature. A wide variety of staining intensities was evident within the cells and the nuclei presented as round to elongate-oval in shape. Subepithelial capillaries were sparse with a mean diameter of 7.36 ± 0.41 µm. Numerous, highly branched uterine glands were observed within the endometrium, of which the diameter and luminal width varied widely (mean: 67.60 ± 3.7 µm). The glandular epithelium ranged from tall to low columnar and, hence, varied greatly in height (mean: 19.03 ± 0.85 µm). Scanning electron microscopy revealed that the endometrial folds were irregularly orientated in the uterine body, but in the uterine horns they were organised in a longitudinal parallel fashion. The uterine gland openings were mainly located along the deep grooves with only a few of them situated on the folds. The apical surfaces of cells were polygonal in shape and were either ciliated or microvillous in nature. The proportion of each of these cell types varied from one region to another, but the microvillous cells were the most prolific. Apical blebs, with smooth or wrinkled surfaces, were evident on some of these secretory cells. Transmission electron microscopy revealed that the cytoplasm of the surface and glandular epithelial cells contained few cell organelles. The surface epithelium contained mitochondria, membranous vesicles, endoplasmic reticulum, small dense bodies and numerous glycogen deposits, particularly in the microvillous cells. The glandular epithelium was resting on a well-developed basal lamina and was differentiated into ciliated and microvillus cells. Both cells types contained a few mitochondria, small dense bodies and some vacuoles. In conclusion, the morphological structure of the donkey and mare endometrium appears similar and this may facilitate the process of interbreeding between the two species. Occurrence of secretory activity during estrus in the donkey uterus corresponds closely to that observed in other species during estrogen dominance, which proceeds ovulation. Accumulation of glycogen in luminal epithelial cells during estrus may provide a source of energy for a developing embryo once it enters the uterine lumen post ovulation.