Development of nucleolar apparatus in the chick primitive erythroid cells.

The primitive erythroid line cells of chick embryos were studied during embryonic days 2-14 by means of a cytochemical method to investigate the appearance and frequency of the main nucleolar types. The populations of erythroblasts and erythrocytes were classified according to the presence of functionally dominant nucleoli in their nuclei. In the course of primitive erythroid cell differentiation and maturation, compact nucleoli and nucleoli with nucleolonemas (both supposed to be RNA biosynthetically active) were gradually replaced by ring-shaped nucleoli and finally by micronucleoli reflecting the reversible and irreversible inhibition of RNA synthesis, respectively. The occurrence of the main nucleolar types and their values in primitive erythroid cells of the developing chick depend not only on the maturation stage of the blood cells, but also on the developmental stage of the chick embryo. In comparison with the definitive erythroid line of the post-hatching chick and hen, the cells of the chick embryonic primitive erythroid line possess relatively high values of "active" nucleolar types. These are still present in advanced maturation stages, and occur also as definitive erythroid lines of lower vertebrates.