[Fibrillar centers and ring-shaped nucleoli in various tissue cells of 11-day-old chick embryos]

The fine structure of nucleoli in different tissues of 11-day old chick embryos was studied. Nucleoli of highly specialized cells, such as lymphocytes, granulocytes and endothelial cells of liver and kidney capillaries contain well developed fibrillar centres and are characterized by peripheral location of the RNP material surrounding the central electron transparent core (ring-shaped nucleoli). However, lens fibers from annular and central zones of this organ are devoid or ring-shaped nucleoli. In nucleoli of mitotically more active cells (hepatocytes, nephrocytes and lens epithelium), which have clear nucleolonemal composition, light fibrillar components are described having homogenous fibrillar texture and consisting of fibers about 100 A in diameter. By main structural criteria, they are very similar to fibrillar centres. It is concluded that these two elements of interphase nucleolus may be equivalent structures.