Electron Microscopy of Emperipolesis

In a previous communication (1), one of us described the propensity for emperipolesis exhibited by the cells of the mouse ascites lymphocytic tumor, Lymphoma No. 1 (L, 1). Emperipolesis, a term coined by Humble, Jayne, and Pulvertaft (2) to describe the "inside round about wandering" of lymphocytes within cells, has been observed by several investigators to occur under a variety of conditions (cf. reference 1). The question that presents itself when cells are observed to be existing within other cells is-what are the relationships between the engulfing cell and the engulfed cells? Are the engulfed cells in intimate contact with the cytoplasm of the engulfing cell or are membranes present which separate one cell from the other? These questions are answered in part by the micrographs accompanying this note. The macro-phage shown in the phase photomicrograph (Fig. 1) contains five living L~ 1 tumor cells and one dead cell. The very granular cytoplasm of the macrophage can readily be distinguished from the clear cytoplasm of the tumor cells contained in it, but the nature of the boundary between the two cytoplasms cannot be determined by phase micros-copy. A thin section of a similar group of cells, fixed in chrome-osmium and photographed with the electron microscope, shows that for the most part the malignant lymphocytes are separated from the cytoplasm of the macrophage by two membranes, one belonging to the lymphocytes and one belonging to the macrophage (Figs. 2 and 4). Thus, the macrophage cytoplasm is not only enclosed in an outer membrane but in an inner membrane as well. While the plasma membrane of each of the * lymphocytes is intact, there are areas where the inner membrane of the macrophage is not so distinct and in these areas macrophage cytoplasm appears to be in contact with the lymphocyte membrane (Figs. 3 and 5). The continuity of the outer membrane shows that at the level of the section, at least, the tumor cells are indeed entirely within the macrophage. Although the nature of the plasma membrane of cells is only poorly understood, there is no reason to think that the inner membrane of the macro-phage is different from the outer one. It is more interesting to speculate on the reason for its existence. The macrophage shown here has reacted to the presence of living lymphocytes within it by forming another membrane, so that the relationship between the two cell types …