A long-lived thymidine pool in epithelial stem cells

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Abstract
The labeling index (LI) of the individual basal cell positions of the anterior column of mouse tongue filiform papillae was assessed with time after an injection of [3H]TdR at 12.00 h (the minimum point in the circadian LI rhythm). An initial doubling of the LI in the stem cell zone due to cell division was followed, by a 2nd rise of 14-16% 16 h after injection and this occurred even in the presence of vincristine. Although the uptake of [3H]TdR and the initial LI doubling were largely prevented by a preceding injection of hydroxyurea, the 14-16% LI rise was still observed. The possible explanations are discussed, the favored one being that an average of 1 of the 6 or 7 cells (the stem cell) in each stem cell zone can store [3H]TdR in a long-lived precursor pool for at least 16 h before being utilized for DNA synthesis. This complements previously published work which suggested that on cell in each stem cell zone may selectively segregate DNA at mitosis.