Ultrastructural aspects of the final stage of hemoglobin biosynthesis.

1. Electron microscopic observations on rabbit embryo, adult rabbit, guinea pig, and human immature erythroid cells showed characteristic hemoglobinized organelles distinguishable from mitochondria by their highly dense matrix, two or three longitudinally arranged double lamellae, and smaller diameters. 2. The presence of hemoglobin (Hb) within these organelles was also demonstrated by electrophoresis of the concentrated supernatant from the isolated, washed and osmotically lysed organelle fraction. The term hemosome has been suggested for these organelles because of their Hb content. We propose that they are the sites of heme integration into the four polypeptide globin chains. 3. The frequency of hemosomes is higher in the peripheral blood erythroid cells of embryos than in the liver erythroid cells, coinciding with the higher Hb synthesis rate in peripheral blood than in the liver. 4. Peripheral blood reticulocytes of rabbits with anemia induced by bleeding presented a lower hemosome frequency than normal reticulocytes. The decrease paralleled the decay of Hb biosynthesis activity. Moreover, Hb biosynthesis induced in HeLa cell and epithelial cell tissue cultures was always associated with the formation of hemosomes. 5. Hemosomegenesis was studied in epithelial tissue culture cells experimentally induced to synthesize Hb, allowing the identification of several stages of hemosome formation in erythroid cells. The morphological data suggest that mitochondria are successively modified to lamellated bodies, prehemosomal vesicles, prohemosomes and hemosomes. These organelles have also been detected in erythroid cells of representative specimens from other vertebrate classes.