Ultrastructural study of minute uterine leiomyomas.

Minute uterine leiomyomas, less than 3 mm in diameter, were studied by electron microscopy. In five of 15 cases, morphologically different types of smooth muscle cell were identified. In the central region of leiomyomas, most myoma cells were characterized by filaments sporadically located in the cytoplasm and well-developed organelles. These cells were interpreted as immature smooth muscle cells. In the outer layer of nodules, the cells were a more mature form of smooth muscle cell and resembled normal myometrial cells. The differences suggest that differentiation of the smooth muscle cells occurs early in the growth of uterine leiomyoma.