Lymphatic vessels in the uterine endometrium of virgin rats

Uteri from four diestrous rats fixed by perfusion were examined histologically for the presence of lymphatic vessels in the endometrium, using 1 μm epoxy sections stained with toluidine blue. For each animal, multiple random sections were prepared from 3 tissue samples. In all sections, lymphatics were readily apparent in the myometrium, being identified by their irregular shape and contents of stained lymph proteins, which contrasted with the clear, empty blood vessels. While similar lymphatics were not seen in the endometrium of most tissue samples, one sample from each of three females revealed a small lymphatic vessel near the bases of the uterine glands, clearly in the endometrium. One of these was followed in serial alternating light and electron microscopic sections. The vessel maintained its relative position to the endometrial glands for a considerable distance, and also joined with two separate branches that appeared along its course. Ultrastructural observations, including a thin endothelium without a continuous basal lamina, the presence of anchoring filaments, and only mononuclear cells and cellular debris in the lumen, confirmed the vessel's identification as a lymphatic. The vessel's size was normally large enough to be followed by light microscopy; however, at some places along its course, the lumen narrowed to such a degree that it was only identifiable ultramicroscopically. These observations refute claims that there are no lymphatic vessels in rodent uterine endometrium, but indicate that they are small and sparsely distributed.