Immunoelectron microscope localization of bromodeoxyuridine incorporated into DNA of Ehrlich tumor cell nucleoli

The distribution of DNA within the nucleolus of Ehrlich tumor cells has been investigated by means of a recent immunocytochemical approach involving an electron microscopic detection of incorporated 5-bromodeoxyuridine (BUdR) into DNA by an anti-BUdR monoclonal antibody. An immunogold method has been performed on ultrathin sections of cells embedded in Lowicryl K4M. In the nucleolus, gold particles are essentially found over the perinucleolar chromatin adn over its intranucleolar invaginations which are connected with the fibrillar centers. In addition, a few gold particles are also observed in the fibrillar centers, preferentially toward their peripheral regions. In contrast, the dense fibrillar component is completely devoid of labeling. The results are discussed in the context of other recent findings concerning the functional organization of the nucleolus.