The microscopic histopathology of hematoxylin and eosin stained nodular fasciitis tumors (see above and three below figures) consists of spindle-shaped myofibroblastic cells (i.e. cells with features of smooth muscle cells and fibroblasts.). These cells are in a myxoid (i.e. more blue or purple compared to normal connective tissue because of excessive uptake of the hematoxylin stain) or a collagenous (high content of collagen fibers) tissue background. The neoplastic myofibroblasts are arranged in whorls and/or short bundles. These cells may show high rates of replicating as judged by their mitotic index but these mitoses are normal in appearance. The tumor tissues often contain red blood cells, lymphocytes and giant osteoclast-like giant cells and may contain sites of bone-like tissue. NF is sometimes classified into three subtypes based on its predominant histopathological pattern: myxoid or reactive (type I), cellular (type II), and fibrous (type III). These patterns appear related to the duration of the lesion with the myxoid variant tending to have the shortest duration and the cellular and fibrous variants tending to have progressively longer durations.Immunohistochemical analyses indicate that the cells in NF usually express smooth muscle actin, muscle specific actin, and vimentin proteins but generally do not express CD34, S-100 protein, desmin, trypsin, factor VIII, F4/80 (also termed macrophage-specific antigen), or HLA-DR1 proteins. Uncommonly, the cells in NF tumors also express the CD68 (a histiocyte-specific marker) protein.
