Myofibroblasts and myoepithelial cells in human breast carcinoma

Summary

Ultrastructural studies of 11 human breast carcinomata revealed that most stromal cells could be arranged in a cell spectrum from fibroblasts, with abundant rough endoplasmic reticulum, to myofibroblasts. In 4 out of 11 cases, myoepithelial cells were observed in the parenchyma at the periphery of some carcinomatous duct-like structures or carcinoma cell nests. The distinction between myofibroblasts and myoepithelial cells was usually easy from their respective locations. Their ultrastructural features were summarized as follows. Myofibroblasts: (1) abundance of rough ER and other cytoplasmic organelles; (2) bundles of microfilaments, 50–70 Å in diameter and associated dense bodies. Myoepithelial cells: (1) bundles of microfilaments 50–70 Å in diameter and associated dense bodies (a common feature); (2) dense bundles of tonofilaments, 80–100 Å in diameter; (3) typical desmosomes which connected them with adjacent myoepithelial or carcinoma cells. Myofibroblasts were occasionally located closely contiguous with carcinoma cells, giving an appearance resembling myoepithelial cells. Even in these instances a distinction between myofibroblasts and myoepithelial cells was possible, since myoepithelial cells had dense bundles of tonofilaments and typical desmosomes, which were not observed in myofibroblasts. No cell types intermediate between myofibroblasts and myoepithelial cells were detected. We could not decide whether myoepithelial cells were neoplastic or not despite the facts that they showed obscured polarity and had partially or completely lost their basal lamina. We conclude that fibroblasts, myofibroblasts and probably some, if not all, smooth muscle cells belong to the same cell system. Myofibroblasts in our material are derived from fibroblasts, while myoepithelial cells are epithelial in origin.