Mesotheliomas: a light- and electron-microscopical study concerning histogenetic relationships between the epithelial and the mesenchymal variants.

Mesotheliomas have variable histological patterns which may suggest a predominance of epithelial cells, a predominance of mesenchymal cells, or a mixture of epithelial and mesenchymal cells. An adenomatoid tumor of the testis, a fibrous mesothelioma, and an epithelial mesothelioma were studied by light and electron microscopy to search for evidence of a histogenetic link between the various cell types. Each neoplasm was composed of poorly differentiated spindle-shaped cells and variably differentiated cells having epithelial, mesenchymal, and combined epithelial and mesenchymal features. The differentiated neoplastic cells exhibited a spectrum of cytodifferentiation with typical mesothelial cells at one end, typical fibroblasts at the other end, and transitional forms in between. These observations are presented in detail as morphological evidence for a direct histogenetic relationship between the epithelial-appearing cells and the mesenchymal-appearing cells of human mesothelial neoplasms.