The nucleolus is a spherical structure (Figure 3-13) that is rich in rRNA and protein. It is usually basophilic when stained with hematoxylin and eosin. As seen with the electron microscope, the nucleolus consists of three distinct components: (1) From one to several pale-staining regions are composed of nucleolar organizer DNA sequences of bases that code for rRNA (Figure 3-14). In the human genome, five pairs of chromosomes contain nucleolar organizers. (2) Closely associated with the nucleolar organizers are densely packed 5- to 10-nm ribonucleoprotein fibers that comprise the pars fibrosa, which consists of primary transcripts of rRNA genes. (3) The pars granulosa consists of 15- to 20-nm granules (maturing ribosomes; see Figure 3-14). Proteins, synthesized in the cytoplasm, become associated with rRNAs in the nucleolus; ribosome subunits then migrate into the cytoplasm. Heterochromatin is often attached to the nucleolus (nucleolus-associated chromatin), but the functional significance of the association is not known. The rRNAs are synthesized and modified inside the nucleus. In the nucleolus they receive proteins and are organized into small and large ribosomal subunits, which migrate to the cytoplasm through the nuclear pores.
Photomicrograph of two primary oocytes, each with its pale cytoplasm and round, dark-stained nucleus. In each nucleus the nucleolus, very darkly stained, is clearly seen. The sectioned chromosomes are also seen, because they are condensed. These cells stopped at the first meiotic division. Meiosis will proceed just before ovulation (extrusion of the oocyte from the ovary; see Chapter 22: The Female Reproductive System).
Electron micrograph of a nucleolus. The nucleolar organizer DNA (NO), pars fibrosa (PF), pars granulosa (PG), nucleolus-associated chromatin (NAC), nuclear envelope (NE), and cytoplasm (C) are shown.
Large nucleoli are encountered in embryonic cells during their proliferation, in cells that are actively synthesizing proteins, and in rapidly growing malignant tumors. The nucleolus disperses during the prophase of cell division but reappears in the telophase stage of mitosis.
The nuclear matrix is the component that fills the space between the chromatin and the nucleoli in the nucleus. It is composed mainly of proteins (some of which have enzymatic activity), metabolites, and ions. When its nucleic acids and other soluble components are removed, a continuous fibrillar structure remains, forming the nucleoskeleton. The fibrous lamina of the nuclear envelope is part of the nuclear matrix. The nucleoskeleton probably contributes to the formation of a protein base to which DNA loops are bound.
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