The Fine Structure of Uterine Sarcomas

Abstract

This paper describes the ultrastructure of uterine sarcomas, which are considered under two major categories — endometrial sarcomas and leiomyosarcomas. Endometrial sarcomas show a wide range of cytological and histologic patterns. On the basis of morphology it seems appropriate to distinguish between three subgroups: - pure homologous endometrial sarcomas - pure heterologous endometrial sarcomas - mixed mesodermal tumors (MMT) with or withour heterologous elements The mixed mesodermal tumors (MMT) contain different types of sarcoma and carcinoma mixed together. The carcinomatous components may be adenocarcinoma of endometrial or tubal type, undifferentiated carcinaoma, squamous or clear cell carcinoma. The differentiation of these elements is organ- or system-specific in the sense that it reflects the epithelial potentialities of the Müllerian duct. To a certain degree the sarcomatous elements of MMT represent more or less distorted reflections of normal - hormone dependent - tissue reactions of the endometrial stroma. Thus on the basis of our morphologic findings we distinguish 4 homologous cell types: 1) undifferentiated tumor cell (tumor stem cell) 2) “fibroblastic” tumor cell 3) neoplastic granular cell 4) leiomyoblastic tumor cell Most authors agree that the undifferentiated tumor cells in MMT are derived from the endometrial stromal cell. Thus this neoplastic cell and its normal counterpart are characterized by a large nucleus and a small rim of cytoplasm with only few organelles. The “fibroblastic” cell closely resembles cells of the mid-secretory cycle. Cytoplasmic organelles and specializations of the cell surface indicate a synthesis of collagen. A second homologous cell differentiation is the neoplastic granular cell, which on the basis of histochemical and electron microscopic findings is a complete reflection of the normal granular cells of the endometrium. The latter are most specific to the functioning endometrium, and accordingly are found in large numbers in the late secretory phase of the endometrial cycle. Another homologous cell element is the leiomyoblast which, when fully differentiated, contains tracts of longitudinally oriented myofibrils. The best known heterologous elements in MMT are rhabdomyosarcoma, chondrosarcoma, and far less often osteo- and liposarcoma. It is now generally assumed that differentiation to these tissues represents neoplastic mesenchymal metaplasia of the tumor cells of MMT. Thus the ultrastructural findings show the development of highly differentiated rhabdomyoblasts or neoplastic cartilage from immature sarcoma cells. From our findings it can be stated that the neoplastic stromal cell has a many fold mesenchymal potency, which is especially realized in MMT. Contrary to the mesenchymal elements, the epithelial components strictly retain their Müllerian character. Pure heterologous sarcomas are probably very rare. They are characterized by just one line of differentiation. Pure homologous sarcomas are not yet fully defined, although we presume that on account of its cellular features it may be regarded a “stem cell sarcoma” of the endometrium with occasional differentiation into “fibroblastic”, smooth muscle tissue, neoplastic granular cells, or decidual cells. Electron microscopic studies on leiomyosarcomas disclose a wide range of cytologic differentiation. According to cytoplasmic appearance three main cells can be distinguished: un(de)differentiated cells, myoblastic cells, and fibroblast-like cells. Derivation of these cells from the smooth muscle cell of the uterine wall is suggested. The cytoplasm of more differentiated tumor cells usually contains focal myofilament bundles. The low cytoplasmic organization of undifferentiated cells is considered a sign of dedifferentiation, while the fibroblastic tumor cell may show the dual potentiality of smooth muscle cells.