When using scanning electron-microscopy as a principal method, autoradiographs differ from those of TEM. Grains of reduced silver are revealed as bright, shining dots above the nuclei of DNA synthesis phase, i.e. of S phase #1, #2, #3, #4. This methodology allows counting the percentage of cells in S phase (proliferative pool or proportion of dividing cells) inside a cellular population. The image depicts a fragment of aortic endothelial lining near a damaged area of endothelial stratum. Cells that are found here do not usually migrate to the damaged zone. However, they divide intensively to produce a sufficient cell number for complete repair of the damage. The proliferative pool of endothelial cells comprises generally not more than 0.1-0.3% of cells. If the proliferation becomes active, the proportion of dividing cells may increase 10-100-fold in damage repair areas of endothelial strata. You may see in the image that about a half of visible cells are operating their S phase or have already passed it. The nuclei of other cells #1, #2, #3 do not accumulate ³Н-thymidine. In this case, a brief label has been used; this means that a radioactive precursor is administered for a short period of time (several minutes) followed by the tissue fixation. The technique does not reflect the total size of the proliferative pool but allows tracking cells whose nuclei have already incorporated ³Н-thymidine during the interval of exposure.