Morphologic comparisons among equine endometrium categories I, II, and III, using light and transmission electron microscopy.

Objective: To evaluate whether the pathologic changes observed by light microscopy in endometrium of categories II and III were reflected by cellular changes and to describe differences in the endometrial cell ultrastructure during estrus and diestrus.

Animals: 18 healthy mares.

Procedure: Endometrial tissues biopsied during the physiologic breeding season were categorized, using light microscopy, and were studied, using transmission electron microscopy (TEM).

Results: Using TEM, glycogen granules were associated with giant mitochondria for all endometrial types during diestrus. Development of rough endoplasmic reticulum (RER) and Golgi apparatus suggested protein synthesis in the endometrial glands during diestrus. TEM did not reveal major ultrastructural differences, between endometrium of categories I and II. This was unlike differences identified by light microscopy. The most extensive pathologic changes were seen in category-III tissue (TEM and light microscopy). Category-III endometria had a large number of light cells with more degenerative structures and fewer organelles, and lacked cilia in the lumen of the glands. This tissue had extensive fibrotic tissue in the lamina propria and many inflammatory cells in most tissue layers.

Conclusions: The severe ultrastructural changes may be one of the many factors decreasing the fertility of mares with category-III, compared with category-1 and -2, endometrium.