Various ultrastructural changes occur during follicular growth in the rat oocyte nucleolus. The nucleolus, which has a reticulated fibrillogranular structure at the primordial and primary follicle stages, becomes entirely compact and is made up of a conspicuous and homogeneous mass at the antral follicle stage. In order to define the nature and the functions of this homogeneous mass, cytochemical methods allowing detection of nucleic acids, proteins and lipids were performed at the light microscopic and ultrastructural levels. The results obtained suggest that this nucleolar mass is probably composed of acid proteins which are not silver stained. This proteinaceous mass could be a special kind of nucleolar secretion providing material for meiotic resumption in the oocyte. Cytochemical researches now in progress should supply new information concerning the exact nature and the role of the nucleolar compact mass, which is the essential nucleolar component at the antral follicle stage and which really plays a role in the nucleolus in the first stages of embryogenesis.

The ultrastructural evolution of the nucleolus was followed during follicular growth by means of a silver staining procedure. The oocyte nucleolus in the primordial and primary follicles consists of strands of dense fibrillar silver-stained component and aggregates of granules which are devoid of silver grains. Small fibrillar centres are also recognized and appear to have less silver stainability. At the secondary follicle stage, a new nucleolar component appears in the reticulated oocyte nucleolus. This component is devoid of silver grains. During follicle growth, at the antral follicle stage, this new component seems to fuse and the nucleolus becomes constituted of a compact homogeneous mass which exhibits a vacuole at the end of the oocyte maturation. The results obtained suggest that this nucleolar mass is essentially made of proteins and particularly of acidic proteins.

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