Topography and enterocyte morphology of the small bowel mucosal surface in equine granulomatous enteritis.

The jejunal mucosa of 4 cases of equine granulomatous enteritis and 2 control horses was investigated by light microscopy and by scanning and transmission electron microscopy. Attention was focused upon changes in mucosal topography and enterocyte morphology in the inflamed mucosa. Structural changes ranged in severity from only a slight thickening and shortening of villi to the appearance of a virtually flat mucosa, upon which crypts opened directly or through shallow cavities encircled by collars of epithelial cells. Between these extremes, the mucosa showed a variety of patterns, all characterized by distinctly abnormal villus projections. These were often united by epithelial bridges and were commonly markedly short, broad and irregular. Enterocytes of mildly changed mucosae showed a normal histology and fine structure, whereas more severely changed specimens displayed a flattened surface epithelium with ultrastructural abnormalities, the most consistent being a pronounced shortening of microvilli. In particular, greatly flattened cells showed evidence of cellular injury, such as prominence of cytolysosomes and degenerative changes of the rough endoplasmic reticulum, while other cells were chiefly characterized by an abundance of non-membrane-bound ribosomes and other features signifying an immature state. Cell-membrane-tight junctions of the surface epithelium appeared to be intact. No intracellular micro-organisms were detected. It is suggested that several factors are involved in the creation of the abnormal mucosal topography in this disease, including excessive enterocyte loss, crypt cell destruction, inflammatory distension of villi and villus fusions.