Multicystic peritoneal mesothelioma. A fine structure study with special reference to the spectrum of phenotypic differentiation exhibited by mesothelial cells.

Multicystic peritoneal mesothelioma evidenced a complete spectrum of morphodifferentiation extending from classic epithelial-like surface mesothelial cells to myoid-fibroblastoid mesenchymal-like subsurface mesothelial cells. The contemporaneous presence of both epithelial- and mesenchymal-type submicroscopic features in individual cells strongly supported the concept of a holistic histogenetic/differentiative nature of surface and subsurface mesothelial cells on the one hand, and of a single origin for all serosal neoplasms, apart from their actual cytodifferentiation features, on the other. In addition, a profusion of smooth-membrane-bound intracellular collagen was found in myoid-fibroblastoid cells; this finding was interpreted to be, at least in part, due to tangential sectioning of convoluted cell surfaces enfolding extracellular collagen fibrils or to cell processes wrapping around them. A certain finalism might be seen in this event, especially considering the shorter-than-normal period length we found in collagen fibrils; the myoid-fibroblastoid cells might be hypothesized to play a contractile role in shortening collagen fibrils in a process of stroma contraction similar to wound repair or hypertrophic scarring.