sentences:  (2002), using pulse/chase experiments with tritiated thymidine, found long-term label-retaining cells in the small intestinal crypts of neonatal mice.These researchers hypothesized that long-term incorporation of tritiated thymidine occurred because neonatal mice have undeveloped small intestines, and that pulsing tritiated thymidine soon after the birth of the mice allowed the 'immortal' DNA of adult stem cells to be labeled during their formation.These long-term cells were demonstrated to be actively cycling, as demonstrated by incorporation and release of BrdU.