In this study, we investigate the expression of muscle markers, including the specific skeletal muscle markers myogenin and myoD1, in neuroendocrine carcinomas (NECs). The study included 23 NECs from various sites (14 small cell NECs and 9 large cell NECs). These were stained with desmin, myogenin and myoD1. Positive staining with at least one muscle marker was observed in 14 cases (61%). 8 (35%), 8 (35%) and 11 (48%) of the cases were positive with desmin, myogenin and myoD1 respectively. In most, but not all, cases positive staining was focal generally involving < 10% of tumour cells. Expression of muscle markers is not uncommon in NECs. This represents an important diagnostic pitfall of which pathologists should be aware. In reporting this phenomenon, we speculate on the pathogenesis of this \"aberrant\" expression of muscle markers.

BACKGROUND: Historically, Grocott\'s methenamine silver (GMS) stain has been used in cytopathology to highlight Pneumocystis jiroveci and other fungal organisms. Several nonfungal organisms, however, can show distinct GMS staining patterns that are important to recognize.
METHODS: We prospectively and retrospectively identified nonfungal pathogenic organisms on GMS-stained liquid-based and cytospin preparations of respiratory cytologic specimens. The organisms included parasitic worms, viruses, and assorted bacteria. Nine cases were identified, including two cases each of Strongyloides stercoralis, Cytomegalovirus, Mycobacterium tuberculosis, Nocardia species, as well as one case of anthrax-like Bacillus cereus.
RESULTS: The nonfungal organisms had silver deposition in varying locations including the internal organs and/or cuticle of Strongyloides stercoralis larvae, the intranuclear inclusions of Cytomegalovirus infected cells, the surfaces of partially acid-fast Nocardia species and acid-fast Mycobacterium tuberculosis, and the cell walls and central endospores of Bacillus cereus. In 3 of the 9 cases, organisms were not clinically suspected. It was the aberrant GMS staining that pointed to the diagnosis and led to the performance of the definitive stain, culture, or other test.
CONCLUSIONS: GMS is a chromic acid, sodium bisulfate stain that precipitates silver ions in fungal polysaccharide walls, producing the characteristic black stain on light microscopy. It is helpful to recognize aberrant GMS staining to avoid misdiagnosis of fungal elements. GMS stains several nonfungal human pathogens and may be a particularly useful diagnostic aid when the infectious condition is not clinically suspected or the number of organisms is sparse and otherwise difficult to visualize by routine staining methods.