Noncanonical structural-functional organization of nucleoli in maturing oocytes of sea urchin Paracentrotus lividus

The dynamics of structural and functional organization of the nucleolus in the oocytes of P. lividus is described. At the late stages of oogenesis the nucleolus is composed of two main components, namely the peripheral zone (PZ) and the central zone (CZ) which are spatially separated. This two-component structure of the nucleolus is formed, at early stages of oogenesis, by stepwise segregation of the fibro-granular component and by its migration to the nucleolar periphery. Absence of morphologically distinct fibrillar centers and dense fibrillar component in nucleoli of both somatic cells and oocytes makes it possible to classify the nucleoli of P. lividus as 'noncanonical' type. Based on detailed morphological and cytochemical analysis the following molecular interpretation of nucleolar ultrastructure in oocytes of P. lividus is proposed: 1) the PZ, containing RNP-positive granules 15 nm in size, but lacking Ag-NOR proteins and BrU incorporation, can be considered a structural equivalent of the granular component of 'typical' nucleoli; 2) the CZ, which is the site of incorporation of RNA precursors, contains intranuclear DNA, RNP-fibers and accumulates Ag-NOR proteins, corresponds to both FC and DFC of 'typical' nucleoli; 3) nucleolar growth during oogenesis, leading to the 1000-fold increase of nucleolar volume, seems to be correlated with the stockpiling of nonfunctioning mature preribosomal particles which will be utilized during embryogenesis.