Ringed sideroblasts were studied by means of transmission electron microscopy in patients suffering from refractory anaemia with ringed sideroblasts (RARS) of myelodysplastic syndrome (MDS) to provide more information on the structural organization of nucleoli in these abnormal erythroblasts. For control of the electron microscopic observations nucleoli in erythroblasts were also visualized by two widely used cytochemical procedures for the demonstration of RNA and AgNOR proteins. In contrast to previously described ultrastructure of nucleoli in "normal" erythroblasts, nucleoli of ringed erythroblasts in RARS of MDS were frequently characterized by a reduced incidence or lack of dense ribonucleic acid (RNA) containing granular components. Since the dense RNA containing granular components represent preribosomes, such sideroblasts in RARS of MDS exhibit a further nucleolar abnormality, which reflects a severe alteration of the nucleolar ribosome assembly in these abnormal cells. On the other hand, the alteration of the preribosome assembly was not noted in early developmental stages of ringed sideroblasts such as proerythroblasts. In addition, nucleoli in advanced or terminal stages of few ringed sideroblasts also did not exhibit such nucleolar abnormality and thus confirm a great structural and functional variability of these cells. The defect of RNA containing structures in nucleoli of advanced and terminal stages of erythroblasts are in a hormony with the light microscopic cytochemistry, which demonstrated a significantly smaller incidence of micronucleoli in specimens stained for RNA than in those stained for AgNOR (silver stained nucleolus organizer region) proteins.

Institute of Haematology and Blood Transfusion Haematology Clinic Institute of Medical Postgraduate Studies Prague Czech Republic