Evidence for collagen fibrils inside the cells of granular cell myoblastomas.

An electron microscope study was performed on two cases of subcutaneous granular cell myoblastomas. Phosphotungstic acid staining reveals the intracellular formation of collagen fibrils both inside the granules and scattered outside them, but always in relation with ribosomes or ergastoplasmic sacs. Pericellular deposition of basal-membrane-like material is a constant feature; however it appears to be more a precollagen material, extruded by cells, than a true basal membrane. This observation strongly supports the view that granular cells in Abrikossoff tumors are fibroblasts of muscle (and nerve?) sheaths abnormally producing precollagen material.