The dynamic ultrastructural and topographical changes occurring in the nucleus of synchronized TG cells during the cell cycle were investigated, with special reference to the region in which the nucleolus is linked to the nuclear envelope. The number, location, and ultrastructural aspect of the nuclear components involved in this spatial relationship were compared in 20 reconstructed nuclei for several time points from early to late interphase. The results showed that the ultrastructure of the nucleolus—envelope region is remarkably stable during interphase, that it consists of the outermost layer of condensed chromatin at least at the specific point where the nucleolus is tightly associated with the nuclear envelope, and that the duration of this association depends on the persistence of this outermost layer of chromatin. These observations provide support for the hypothesis that the nucleoli where the rDNA genes are localized, are attached to this specific layer of peripheral condensed chromatin and through it are kept associated to the nuclear envelope. A clear sequence of changes was seen to affect each of the components making up the nucleolus—envelope region. These changes included nucleolar fusion, expansion of the nucleolus toward the center of the nuclear volume, lengthening of the nucleolar canals, and changes in the type of location of the nucleolus—envelope region along the nuclear envelope.