The nucleolus is the most obvious structure in the interphase nucleus of eukaryotic cells. It is located in the nucleus of the cell. Contributes to reproduction and assembles ribosomes. The once assembled ribosomes exit the nucleus and enter the cytoplasm. The study of nucleolus structure, dynamics and function was not only paid close attention to by early cytologists, but also has been highly valued by researchers in various related fields since the important functions of nucleolus were discovered in the 1960s. Nucleoli can be easily seen in stained cells under a light microscope, in living cells under a phase contrast microscope, or in the nuclei of isolated cells. They are usually single or multiple homogeneous spherical bodies. A medium round or oval granular structure with no outer membrane. The nucleolus is composed of three major parts: particle component, fiber center and dense fiber component. Nucleolar components include rRNA, rDNA and ribonucleoprotein. The nucleolus is the site for rRNA gene storage, rRNA synthesis and processing, and the assembly of ribosomal subunits. The size and shape of the nucleolus vary with the species of organism, cell type and cell metabolic state. Cells with strong protein synthesis and active growth, such as secretory cells and oocytes, have large nucleoli, which can account for 25% of the total nuclear volume. Cells without protein synthesis ability, such as muscle cells and dormant plant cells, have small nucleoli. . The number of nucleoli in various organisms is generally certain. For example, the African clawed frog has two nucleoli. Human cells have only one nucleolus. During the cell cycle, the nucleolus is a highly dynamic structure that exhibits periodic disappearance and reconstruction during mitosis. When the cell divides, the nucleolus disappears. After the division is completed, the two daughter cells produce new nucleoli. Nucleolus organizer region: the rRNA sequence region, which is related to the formation of interphase nucleoli and constitutes one or several specific chromosome segments of the nucleolus. This segment of DNA is transcribed into rRNA. The human nucleolus organization area is located at one end of 10 (5 pairs) chromosomes. There are originally 10 new nucleoli, but they are all very small and quickly merge into one large nucleolus.