DEVELOPMENT OF NUCLEOLAR APPARATUS IN THE CHICK PRIMITIVE ERYTHROID-CELLS

The primitive erythroid line cells of chick embryos were studied durin g embryonic days 2-14 by means of a cytochemical method to investigate the appearance and frequency of the main nucleolar types. The populat ions of erythroblasts and erythrocytes were classified according to th e presence of functionally dominant nucleoli in their nuclei. In the c ourse of primitive erythroid cell differentiation and maturation, comp act nucleoli and nucleoli with nucleolonemas (both supposed to be RNA biosynthetically active) were gradually replaced by ring-shaped nucleo li and finally by micronucleoli reflecting the reversible and irrevers ible inhibition of RNA synthesis, respectively. The occurence of the m ain nucleolar types and their values in primitive erythroid cells of t he developing chick depend not only on the maturation stage of the blo od cells, but also on the developmental stage of the chick embryo. In comparison with the definitive erythroid line of the post-hatching chi ck and hen, the cells of the chick embryonic primitive erythroid line possess relatively high values of ''active'' nucleolar types. These ar e still present in advanced maturation stages, and occur also as defin itive erythroid lines of lower vertebrates.