Local PEComatosis of the Appendix: New Insights Into the Histogenesis of Nodular Granular Muscle Degeneration and Granular Cells/Granular Cell Lesions of the Appendix.

Nodular granular muscle degeneration (NGMD) of the appendix is a rare histologic curiosity characterized by distinctive nests of polygonal epithelioid cells with abundant pale-pink eosinophilic granular cytoplasm, mostly distributed in the inner layer of the muscularis propria or submucosa of the appendix. Although the nature of the cells of interest in NGMD of the appendix has not been completely elucidated, it is believed that they denote degenerative smooth muscle cells of the appendiceal muscularis propria, a histologic finding described as granular cells/granular cell lesions of the appendix in the 1960s. In this article, we described a new case of NGMD of the appendix and documented for the first time that this peculiar lesion actually represents a form of perivascular epithelioid cell proliferation based on its dual immunopositivity for myogenic and melanocytic markers. We also analyzed the old medical literature on granular cells/granular cell lesions of the appendix to shed some light on this ill-defined morphologic finding and its relationship to NGMD of the appendix. Since NGMD of the appendix is a lesion of perivascular epithelioid cells, the term NGMD is a misnomer, and hence, the designation "local PEComatosis of the appendix" is proposed for this unusual phenomenon.