Cellular localization of ¹²⁵I-rTpo within the spleen of NF-E2-null and control mice analyzed by electron microscopic autoradiography. Silver grains (arrows) associated with (A) megakaryocyte fragments/platelet-like structures (P) found within or in close contact with macrophages (M) in NF-E2^−/− mice and (B) with a mature platelet (P) from a control mouse. Bar, 2 μm.