Newts (Triturus cristatus) made anemic with acetylphenylhydrazine (APH) fail to regenerate erythrocytes (RBC's) immediately and exhibit a latent period of 1.5-2 wk during which animals lack RBC's and are aplastic. With the establishment of erythroid regeneration at 10-14 days, relatively homogeneous populations of successive erythropoietic stages occur in the blood. This feature makes possible biochemical analyses of events in early, intermediate, and late developmental stages, respectively, each of which can be obtained in vivo with minimal contamination by other stages. Previous studies have described a primitive cell population referred to as "erythroid precursor cells" (EPC's) which precedes the appearance of definitive erythroid elements. The present studies show that EPC's and early erythroid cells are engaged mainly in ribosomal production, including synthesis of rRNA and ribosomal proteins. Moreover, EPC's and early erythroid cells also synthesize tRNA and a presumed Hb-mRNA which has been identified by its sedimentation rate at 9-12 s and its content of-polyadenylic acid. In intermediate stages, there occurs a fourfold decrease in the level of RNA synthesis and, while rRNA continues to be formed, there is a disproportionate accumulation of the two major cytoplasmic rRNA species in favor of the large ribosomal subunit RNA. In late developmental stages, the level of RNA synthesis is markedly diminished with little or no evidence of formation of defined RNA classes. Correlated radioautographic and biochemical studies with radioactive 8-aminolevulinic acid and leucine indicate that EPC's and other early erythroid elements synthesize not only hemoglobin but also ferritin and ribosomal proteins. It is concluded that: (a) erythroid RNA synthesis is most pronounced in the early developmental stages, being manifested predominantly by rRNA production but including tRNA and HbmRNA; (b) intermediate developmental stages show both "ribosomal wastage" and decreased growth rate, marking a pivotal point between the transcriptional activities of early stages and translational activities of late stages; (c) EPC's represent a cell population already committed to RBC formation and are excluded from a role as the pluripotential stem cell.The process of erythrocyte (RBC)development derived from an undifferentiated hemopoietic occurs as a series of successive stages which are stem cell. Although functional and cytogenetic 206

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