Studies on splenic erythropoiesis in the mouse. I. Ribosomal ribonucleic acid metabolism

The erythropoietic mouse spleen was used as a model system to study RNA (ribonucleic acid) metabolism during erythroid cell development. The murine spleen becomes a major site of red cell production when hematopoiesis is accelerated. Induction of a hemolytic anemia with phenylhydrazine resulted in a 5 fold increase in spleen weight and an increase in hemoglobin-containing nucleated cells to 70 per cent of the splenic cell population. During the period of splenic erythropoiesis, cytoplasmic RNA concentration increased 3 fold, reaching a maximum 1 day prior to peak splenic erythropoiesis and 2 days prior to maximal splenic DNA (deoxyribonucleic acid) concentration. The bulk of the accumulated RNA was ribosomal RNA (rRNA). In vitro measurements of cytoplasmic rRNA production revealed a rapid early increase in 28S and 18S rRNA synthesis of 7 to 9 fold occurring 3 days prior to peak splenic erythropoiesis and diminishing within 24 hours. The increased synthesis of rRNA was accompanied by an increase in nucleolar RNA content as demonstrated by histochemical staining. Nucleolar RNA content also diminished in parallel with the decrease in rRNA synthesis. Synthesis of 28S and 18S rRNA by splenic tissue in vitro was sensitive to low concentrations of actinomycin D as well as to puromycin, cycloheximide, and chloramphenicol, but not to bromodeoxyuridine. These results indicate that activation of erythroblast nucleoli and the concomitant increase in rRNA synthesis are early events in erythroid cell development.