DNA replication patterns of normal human leukocyte cultures. Time sequence of DNA synthesis in relation to the H3-thymidine incorporation over the nucleolus.

IN RECENT YEARS many papers dealing with DNA synthesis in short-term cultures of human peripheral blood”6 have appeared. There has been increasing evidence that the small lymphocytes under stimulation of phytohemaglutinin1’ 9 and other substances10’2 are able to proliferate and produce mito515, but very few researchers have tried to investigate the time sequence of the DNA synthesis in the interphase nuclei of these proliferating lymphocytes. Prescott and Bender1 ha ’e described two patterns of H3-thymidine incorporation in the interphase nuclei of the leukocyte cultures-i.e., a diffuse and a localized pattern of labeling. Further, Lima-de-Faria and Reitalu4 have demonstrated the incorporation of 113..thymidine on the heterochromatin situated around the nucleolus of the leukocytes cultivated in vitro. Stimulated by these studies and by our previous observation’3 that the labeling of the interphase nuclei which synthesize DNA is not done at random and that in some cells H3-thymidine is incorporated in the nucleolus, we have undertaken an autoradiographic study of normal human peripheral blood cultures to determine ( 1 ) which patterns of H3-thymidine incorporation can be demonstrated in the nuclei of PHA-stimulated lymphocytes, and (2) the time sequence of the observed patterns of labeling.