Intracellular localization of 55Fe after serum-mediated uptake by human erythropoietic cells.

Marrow cells of 3 patients with normoblastic erythropoiesis and 1 with sideroblastic erythropoiesis were incubated with serum-bound 55Fe Cl3 and studied by the technique of electron microscope autoradiography. In the 3 normoblastic marrows, both the mean grain counts and the percentages of grains situated over the cytoplasm (excluding the mitochondria) were higher in the early erythroblasts than in the intermediate and late erythroblasts. The percentages of autoradiographic grains situated over the mitochondria of early intermediate and late erythroblasts were 11.9-27.4, 56.5-61.1 and 65.3-65.9, respectively. Although the mean grain counts over these three cell classes in the sideroblastic marrow were similar to those in the normoblastic marrows, the proportions of grains over the mitochondria were higher, particularly in the early erythroblasts. In all 4 cases (1) some grains were situated at the junction between heterochromatin and euchromatin and (2) a few of the 55Fe-labelled immature-looking cells did not display the ultrastructural features usually attributed to pronormoblasts.