Telomere fluorescence measurements in granulocytes and T lymphocyte subsets point to a high turnover of hematopoietic stem cells and memory T cells in early childhood

To study telomere length dynamics in hematopoietic cells with age, we analy zed the average length of telomere repeat sequences in diverse populations of nucleated blood cells. More than 500 individuals ranging in age from 0 t o 90 yr, including 36 pairs of monozygous and dizygotic twins, were analyze d using quantitative fluorescence in situ hybridization and now cytometry. Granulocytes and naive T cells showed a parallel biphasic decline in telome re length with age that most likely reflected accumulated cell divisions in the common precursors of both cell types: hematopoietic stem cells. Telome re loss was very rapid in the first year, and continued for more than eight decades at a 30-fold lower rate. Memory T cells also showed an initial rap id decline in telomere length with age. However, in contrast to naive T cel ls, this decline continued for several years, and in older individuals lymp hocytes typically had shorter telomeres than did granulocytes. Our findings point to a dramatic decline in stem cell turnover in early childhood and s upport the notion that cell divisions in hematopoietic stem cells and T cel ls result in loss of telomeric DNA.