Ultrastructural and electron-immunocytochemical characterization of cells in epiretinal membranes

Electron-immunocytochemical staining for three intermediate filament (IF) proteins, keratin (K), glial fibrillary acidic protein (GFAP), and vimentin (V), and for the macrophage marker, EBM/11 (E), was performed on epiretinal membranes obtaining during vitrectomies performed for proliferative vitreoretinopathy (PVR), postdetachment macular puckers (PDMPs), idiopathic macular puckers (IMPs), or macular puckers associated with other disease processes. The ultrastructural and immunocytochemical characteristics of the cells were compared. Unstained cells outmembered stained cells for each of the markers in almost all membranes. Six cell types, based on ultrastructure, were found in the majority of epiretinal membranes: 1) polarized cells with microvilli on the free border and foot processes anchoring them to extracellular matrix that consistently stained negative for all of the immunocytochemical markers; 2) spindle-shaped fibroblastlike cells that were generally negative for all markers, but rarely positive for V; 3) large undifferentiated cells with large, lightly stained nuclei and little cytoplasm that frequently expressed one of the intermediate filament (IF) proteins; 4) poorly differentiated cells that contained numerous mitochondria and frequently expressed one of the IF proteins; 5) undifferentiated, pigment-laden cells that rarely stained for any of the above IF proteins, but occasionally showed K or V positivity in a portion of the cell, suggesting that they may be losing or acquiring these proteins, and that rarely expressed GFAP; and 6) small, round, mononuclear cells with short processes that were sometimes, but not always, positive for E and that were consistently negative for K, V, and GFAP. In addition to these morphologic types, transitional cells demonstrating features of two or more of the above cell types were seen, suggesting that phenotypic changes between the various cell types can occur. The amount of extracellular matrix in epiretinal membranes showed a correlation with disease process (PVR > PDMP > IMP), and a negative correlation with the percentage of cells expressing a highly differentiated polarized morphology and with the percentage of cells staining for IF proteins. These data suggest that both cell morphology and IF protein expression may be dependent in part on microenvironment and that neither alone can be used to identify unequivocally the derivation of particular cells found in epiretinal membranes. The integration of ultrastructural and immunocytochemical data may provide a more accurate determination of the cell of origin and of phenotypic changes that have occurred. In some cases, however, both ultrastructural and IF protein composition taken together are insufficient for the precise identification of all cells. The data presented here as well as other information available in the literature suggests a prominent role for RPE and retinal glia in epiretinal membrane formation as well as a possible role for macrophages.