Intramembranous particles in erythrocyte, reticulocyte and erythroblastic leukemic cells of the rat: a model system for erythrocyte maturation.

The density and size distribution of intramembranous particles (IMP) were determined for cells of the erythroid series. The number and size of IMP were measured on both fracture faces of erythroblastic leukemia cells, phenylhydrazine-induced reticulocytes and mature erythrocytes. We found that the number of IMP adhering to the protoplasmic fracture face of the plasma membrane increased with increasing maturation, while the number of particles adhering to the external fracture face did not correlate with maturational stage. In general, the mean size of particles adhering to both fracture faces decreased with increasing maturation after the erythroblastic stage. We interpret these results to mean that the IMP seen are derived from more than one macromolecular species and that they are distributed asymmetrically in the plasma membrane.