Clear Cell Leiomyoma (Leiomyoblastoma) of the Uterus: Ultrastructural Observations

The infrastructure of a benign uterine epithelioid leiomyoma with prominent clear cells (“leiomyoblastoma”) was studied to define the subcellular changes that yield the distinctive light microscopic pattern of this rare lesion. The main constituent of the cytoplasm was multiple cytolysosomes composed of vesicular, membranous material. Aggregates of glycogen also contributed to the clear appearance of the cytoplasm seen by light microscopy. In addition, the cells contained scattered aggregates of typical smooth muscle myofilaments with densities that certified that the lesion was a variant of a leiomyoma. The fine structural features of this lesion are distinctly different than those described in gastrointestinal leiomyoblasto-mas and indicate that the uterine clear cell leiomyoma is a unique lesion that should be separated from nonuterine leiomyoblastomas. Furthermore, the ultrastructural features of the uterine clear cell tumor suggest the lesion represents a leiomyoma that has undergone sublethal cell injury. The term “leiomyoblastoma” implies a primitive tumor and is inappropriate; “epithelioid leiomyoma, clear-cell type” is preferred.