Computerized measurement of the DNA content, areas, and autoradiographic grains of the same nuclei: Demonstration that lightly (³H)thymidine-labeled bone marrow cells are predominantly in G0/G1 and G2

A computerized 'flying spot' microdensitometer and scanning stage have been combined to measure the cellular DNA content, nuclear areas, and autoradiographic grain areas of the same cells. The slide positions of the Feulgen-stained, (³H)thymidine-labeled cells are mapped with the computerized stage, and nuclear DNA content and areas are then determined by integral absorbance measurements at 588 nm. Following autoradiograph preparation, the cells are relocated and the areas of the autoradiographic grains over each nucleus are measured at a light wavelength (625 nm) and an optical density setting (>0.10) that do not detect the Feulgen stain. The microcomputer calculates the portion of each nucleus covered with autoradiographic grains (grain area proportion, GAP), and it links the GAP value to the DNA content of each nucleus in the computer file for subsequent sorting and analysis. By using this system in a study of mouse bone marrow cells labeled in vivo with (³H)thymidine, we found that all S-phase cells were clearly labeled after 8 or more days of autoradiographic exposure. Prolonged exposures (up to 64 days) led to detection of lightly labeled cells (0.1 < GAP <0.8) with G1/G0 and G2 DNA content.