The Nucleolus

Abstract

The nucleolus is the only organized body present in the nucleus and it is conspicuous when suitably stained. The nucleolus is the site for the synthesis and biogenesis of ribosomal ribonucleic acid (rRNA) in eukaryotes. The chromatin segment containing DNA that codes for ribosomal RNA is found within the nucleolus. However, the nucleolus is not only the chromosomal location containing cistrons coding for rRNA but it is also the site for processing its primary gene products and as such contains the derivatives of this product, their associated proteins, and most of the enzymatic machinery required for the processing of ribosomes. Studies on the nucleoli in specialized cells help in understanding the importance of nucleoli in cell physiology. Among the specialized cells, liver cells have been studied most extensively for these organelles because the nucleoli in these cells are prominent and can be studied under different experimental and pathological conditions. Blood cells are convenient material for the studies of cytological changes during cell maturation and differentiation. These studies show that the differentiation and maturation of these cells are accompanied by gradual changes in nucleolar morphology. The most important changes are the decrease in size and number of nucleoli and also in the RNA content of nucleoli during differentiation.