Ultrastructural Evidence of in Vitro Interaction among Burkitt Lymphoma Cells: Possible Relevance to the “Phagocytic” Activity of Starry Sky Histiocytes in Vivo

Summary Cells with morphologic characteristics of histiocytes or macrophages have been observed in cultures of human lymphoma cells derived from Burkitt tumors. Aggregation of the smaller lymphoid cells around the larger hisiocytic cells resulting in “rosette formation” was seen. Morphologically, intact small lymphoma cells and cell remnants were identified within the cytoplasm of the larger cells; however, none of the cells in the cultures were able to phagocytize small particulates such as latex spherules, bacteria, or carbon particles. These findings suggest that the whole cell inclusions in the histiocytes may be the result of active entry of the smaller cells into the larger cells in a process related to emperipolesis rather than true phagocytosis or engulfment. In sections and imprints of Burkitt tumors, morphologically intact cells as well as cellular debris were found within the cytoplasm of the “clear histiocytes.” The findings in vitro may be relevant in interpreting the significance of the “starry sky” histiocytes: It cannot be assumed that the starry sky histiocytes are actively phagocytic, and the possibility of “emperipolesis” in vivo must be considered.