[Ultrastructure and functions of ring-shaped nucleoli, fibrillar centers and nucleolar vacuoles]

Evidence on the ultrastructure of ring-shaped nucleoli in several cell types is reviewed. Detailed attention is paid to these structures in PEK culture cells after the action of actinomycin D, to infiltrating carcinomas of human breast after different degrees of malignization, and to hepatocytes of 11 day old chick embryos. On the basis of our own and literary data, the ring-shaped nucleoli are classified into two groups: 1) those with centrally located fibrillar centres, and 2) those containing large central vacuole. Connections of fibrillar centres with the nucleolus organizer regions in the interphase nucleolus and mitotic chromosomes are considered in detail. Literary and our own data on the genesis and functional activity of ring-shaped nucleoli with the central vacuole, as well as nucleoli with different degrees of vacuolization (vacuolized nucleoli) are specially analyzed. Cellular function, properties and significance of the central vacuole in low differentiated cells are discussed.