TELOMERE METRICS. “Harley et al. observed that the mean telomere length decreased by 2 to 3 kilobase pairs (kbp) during the serial passage of several strains of normal human diploid fibroblasts. The decrease was found to be progressive and averaged 50 base pairs for each population doubling. The telomere shortening seen in aging normal human fibroblasts also occurs in vivo in skin epidermal cells, peripheral blood leukocytes and colon mucosa epithelia.”
http://protein.bio.msu.ru/biokhimiya/contents/v62/full/62111380.html