Evidence for S stage quiescence in regeneration blastema cells.

Working on the temporal structure of the S stage of regeneration blastema cells we have discovered that some blastema cells, containing a 3C quantity of DNA, do not incorporate radioactive thymidine, or that they proceed through the synthetic phase with a speed undetectable by a radioactive pulse. We report the identification of several short-term periods of quiescence for regeneration blastema cells in vivo, operating during the S stage of the cell cycle. Blastema cell smears from regenerating urodele forelimbs were used in this study. Combined microdensitometric scanning of Feulgen-stained nuclei and microradioautographic labelling of the same nuclei have been exploited to assess DNA replication. A total of 3,600 blastema cell nuclei were scanned, mapped on photographic film and subsequently processed for radioautography. It was found that 49.7% of the cells with a DNA content ranging between 2C and 4C were unlabelled after 1 h exposure to tritiated thymidine.