Telomeres in the haemopoietic system.

The limited life span of most blood cells requires the continuous production of cells, which in adults exceeds 10(12) cells/day. This impressive production of cells (approximately 4 x 10(16) cells over a lifetime) is achieved by the proliferation and differentiation of committed progenitor cells, which themselves are derived from a population of pluripotent stem cells with self-renewal potential. Paradoxically, the large majority of stem cells in adult bone marrow are quiescent cells. One possibility is that stem cells, like other somatic cells, have only a limited replicative potential (< 100 divisions). This hypothesis is supported by two key observations and the consideration that, in theory, 55 divisions can yield 4 x 10(16) cells. First, it was shown that 'candidate' stem cells purified from fetal and adult tissue showed dramatic functional differences in turn-over time and the ability to produce cells with stem cell properties, Second, these functional differences were found to correlate with a measurable loss of telomere repeats despite the presence of low but readily detectable levels of telomerase in all purified cell fractions. In order to address questions about the role of telomeres in normal and malignant haemopoiesis, we developed a quantitative fluorescence in situ hybridization technique. Here we review the characteristics of this novel tool to assess the number of telomere repeats at the end of individual chromosomes and provide an overview of recent observations.