Comparative ultrastructure of maturing toad (Bufo ictericus) and rabbit (Oryctolagus cuniculus) erythroid cells with regard to hemoglobin biosynthesis.

1. Toad and rabbit maturing erythroid cells were comparatively analysed with regard to their ultrastructural modifications involved in hemoglobin (Hb) biosynthesis. 2. The mitochondrial inner membrane differentiates to a lamellated body that, successively, gives rise to prehemosomal vesicles, prohemosomes, and to hemoglobinized organelles called hemosomes. 3. The prehemosomal vesicle involves ferruginous inclusions, taken as iron sources for heme biosynthesis, as well as the polypeptide globin chains, assembling themselves in the course of volume reduction. 4. From the prohemosomal stage onwards, where possibly heme biosynthesis occurs, hemosomes are formed; these organelles are presumably sites where the final Hb biosynthesis could take place. 5. All development stages leading to hemosome formation are similar in toad and rabbit erythroid cells, except that, in the toad, the structural prohemosome characteristics persist in hemosomes. 6. Through toad erythroid cell fractionation and electrophoresis of the organelle lysate supernatant, a wide and a weak cytoplasmic Hb bands were obtained; the latter coincides with the intraorganellar Hb band.