title: Human hair genealogies and stem cell latency.
abstract: BACKGROUND
  Stem cells divide to reproduce themselves and produce differentiated progeny. A fundamental problem in human biology has been the inability to measure how often stem cells divide. Although it is impossible to observe every division directly, one method for counting divisions is to count replication errors; the greater the number of divisions, the greater the numbers of errors. Stem cells with more divisions should produce progeny with more replication errors.
  
  
  METHODS
  To test this approach, epigenetic errors (methylation) in CpG-rich molecular clocks were measured from human hairs. Hairs exhibit growth and replacement cycles and "new" hairs physically reappear even on "old" heads. Errors may accumulate in long-lived stem cells, or in their differentiated progeny that are eventually shed.
  
  
  RESULTS
  Average hair errors increased until two years of age, and then were constant despite decades of replacement, consistent with new hairs arising from infrequently dividing bulge stem cells. Errors were significantly more frequent in longer hairs, consistent with long-lived but eventually shed mitotic follicle cells.
  
  
  CONCLUSIONS
  Constant average hair methylation regardless of age contrasts with the age-related methylation observed in human intestine, suggesting that error accumulation and therefore stem cell latency differs among tissues. Epigenetic molecular clocks imply similar mitotic ages for hairs on young and old human heads, consistent with a restart with each new hair, and with genealogies surreptitiously written within somatic cell genomes.
authors.affiliation: Department of Pathology, University of Southern California Keck School of Medicine, Los Angeles, CA 90033, USA. pck2973@unitel.co.kr
authors.forename: Jung Yeon
authors.initials: JY
authors.lastname: Kim
authors.forename: Simon
authors.initials: S
authors.lastname: Tavaré
authors.forename: Darryl
authors.initials: D
authors.lastname: Shibata
mesh_terms: D001683:Biological Clocks; D002455:Cell Division / Q000235:genetics; D019175:DNA Methylation; D006197:Hair / Q000166:cytology* / Q000254:growth & development; D018859:Hair Follicle / Q000166:cytology; D006801:Humans; D013234:Stem Cells / Q000166:cytology*