Ultrastructure and immunohistochemical localization of estradiol in three thecomas

Three ovarian thecomas were studied by ultrastructural and immunohistochemical techniques. In each tumor, a small number of tumor cells stained for estradiol. Vacuolated as well as plump non-vacuolated tumor cells were positive, but spindle-shaped cells were negative. Ultrastructural examination showed two principle cell types. Type I cells were immature mesenchymal cells that differed from typical steroid-secreting cells because only a minority had conspicuous smooth endoplasmic reticulum, although mitochondria often had tubular cristae. Type II cells were distinguished by abundant intermediate (10-nm) microfilaments and round mitochondria with incomplete cristae and empty centers. Focal smooth muscle differentiation was present in each tumor. Adherens-type intercellular junctions, including desmosomes in one case, and degenerate cells with markedly vesiculated cytoplasmic membrane systems were also present. Lipid was not abundant, but it was more conspicuous in degenerate cells and in type II cells. Thecoma cells thus closely resemble both ovarian stromal cells and theca interna cells, which are known to be capable of steroidogenesis. The localization of estradiol in a minority of tumor cells in each thecoma suggests that thecoma cells, too, are capable of steroid synthesis and supports the popular concept that hyperestrogenism in patients with thecomas may be the result of estradiol secretion by these tumors.