Differentiation of mammalian somatic cells: DNA and hemoglobin synthesis in fetal mouse yolk sac erythroid cells.

The relationship between the synthesis and replication of DNA and the capacity to synthesize specialized protein on a stable messenger RNA has been examined in yolk sac erythroid cells differentiating in vivo in fetal mice. Evidence is presented that by day 11 of gestation, at least 98 per cent of yolk sac erythroid cells are synthesizing hemoglobin. These cells are shown to incorporate (3)H-thymidine and to replicate, with more than twofold increase in cell number between days 10 and 11. Division of these cells continues through day 13. Hemoglobin formation in these cells is resistant to actinomycin D from day 10. During this period, ribosomal content per cell decreases by more than a factor of 2. These data indicate that in yolk sac erythroid cells, hemoglobin formation proceeds independent of continued RNA formation in cells capable of DNA synthesis and replication.