Spleen - Inflammation

Inflammation is an uncommon lesion in the spleen of rodents, and when present, infectious agents are the most likely cause (Figure 1 and Figure 2, arrowheads). In NTP studies, the five standard categories of inflammation are acute, suppurative, chronic, chronic active, and granulomatous. In acute inflammation, the neutrophil is the predominant infiltrating cell, though fewer macrophages and lymphocytes may also be present. Edema or hyperemia may be evident. The neutrophil is also the predominant cell type in suppurative inflammation, but the neutrophils are aggregated, and many of them are degenerate (suppurative exudate) (Figure 1 and Figure 2, arrows). Cell debris, from both the resident cell populations and infiltrating leukocytes; proteinaceous fluid containing fibrin, fewer macrophages, occasional lymphocytes, or plasma cells; and, possibly, an infectious agent (Figure 1 and Figure 2, arrowheads) may also be present within the exudate. Grossly, these lesions would be characterized by the presence of pus. In the tissue surrounding the exudate, there may be fibroblasts, fibrous connective tissue, and mixed inflammatory cells, depending on the chronicity of the lesion. Lymphocytes predominate in chronic inflammation. Lymphocytes also predominate in chronic active inflammation, but there are also a significant number of neutrophils. Chronic and chronic active inflammation may contain macrophages. Granulomatous inflammation is another form of chronic inflammation, but this diagnosis requires the presence of a significant number of aggregated, large, activated macrophages, epithelioid macrophages, and/ or multinucleated giant cells.