Motility of L 5222 rat leukemia cells in the flattened state. Evidence against emperipolesis.

Emperipolesis is the term for the assumed penetration of living cells into other living cells. As reported earlier, L 5222 rat leukemia cells, migrating in vitro, change from a spherical to a spread configuration when they meet flat cells, and continue to move in this shape within the contours of the target cells. Whether or not this close cellular association corresponded to emperipolesis could not be determined with phase and interference contrast cinemicrography alone. In combination with transmission electron microscopy, it could be demonstrated that the compartment, in which the spread leukemia cells move, is not the cytoplasm of the target cells, but the narrow space created by the target cells and the underlying glass surface. Thus, emperipolesis could be ruled out for L 5222 leukemia cells. On this basis the reported observations on emperipolesis are reviewed, and a critical attitude regarding the occurrence of emperiopolesis in general is advocated.