The gastric hypercellular microleiomyoma as a precursor lesion for clinical gastrointestinal stromal tumors.

Differentiation features and proliferation activity of 67 gastric microleiomyomas (microLMs) and 53 clinical gastrointestinal stromal tumors (GISTs) of the stomach were compared. The 67 microLMs were divided into two categories on the basis of cellularity: 53 hypocellular and 14 hypercellular types, and the 39 GISTs were divided into 13 low-grade and 40 high-grade lesions. Immunohistochemically, 49 hypocellular microLMs (92%) were positive for alpha-smooth muscle actin and desmin, whereas only 16 (30%) were stained for vimentin. Conversely, all 14 hypercellular microLMs were positive for vimentin, and only one (7%) was positive for alpha-smooth muscle actin and desmin. Five low-grade (38%) and 14 high-grade GISTs (35%) were positive for alpha-smooth muscle actin, and 12 low-grade (92%) and all 40 high-grade GISTs were stained for vimentin. CD34 was positive in 10 hypocellular microLMs (19%), all 14 hypercellular microLMs, 10 low-grade GISTs (77%), and 38 high-grade GISTs (95%). Hypercellular microLM thus showed similarities to clinical GIST and also exhibited significantly higher proliferation activity than hypocellular microLM on analysis of the Ki-67 labeling index and argyrophilic nucleolar organizer regions staining. The findings indicate that the hypercellular microLM may be a direct precursor for clinical GIST, both showing a primitive mesenchymal cell nature.