The density and distribution of electric charge on the surface of rabbit bone marrow cells was visualized by electron microscopy after the cell surfaces had been stained with charged colloidal iron particles. Expulsed erythroid nuclei are less negatively charged than any other cell in the bone marrow. They carry from about one-half to one-third of the charge density on the remaining future reticulocyte. The reduction in the surface charge density is already apparent when the nucleus is partially expulsed. Practically no positive charge was found on its surface or on the surface of any other bone marrow cell. The possibility that the reduced negative charge on the surface of expelled erythroid nuclei is one of the means by which the macrophage distinguishes it from other bone marrow cells is discussed.

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