title: Radiocarbon dating of the human eye lens crystallines reveal proteins without carbon turnover throughout life.
abstract: BACKGROUND
  Lens crystallines are special proteins in the eye lens. Because the epithelial basement membrane (lens capsule) completely encloses the lens, desquamation of aging cells is impossible, and due to the complete absence of blood vessels or transport of metabolites in this area, there is no subsequent remodelling of these fibers, nor removal of degraded lens fibers. Human tissue ultimately derives its (14)C content from the atmospheric carbon dioxide. The (14)C content of the lens proteins thus reflects the atmospheric content of (14)C when the lens crystallines were formed. Precise radiocarbon dating is made possible by comparing the (14)C content of the lens crystallines to the so-called bomb pulse, i.e. a plot of the atmospheric (14)C content since the Second World War, when there was a significant increase due to nuclear-bomb testing. Since the change in concentration is significant even on a yearly basis this allows very accurate dating.
  
  
  RESULTS
  Our results allow us to conclude that the crystalline formation in the lens nucleus almost entirely takes place around the time of birth, with a very small, and decreasing, continuous formation throughout life. The close relationship may be further expressed as a mathematical model, which takes into account the timing of the crystalline formation.
  
  
  CONCLUSIONS
  Such a life-long permanence of human tissue has hitherto only been described for dental enamel. In confront to dental enamel it must be held in mind that the eye lens is a soft structure, subjected to almost continuous deformation, due to lens accommodation, yet its most important constituent, the lens crystalline, is never subject to turnover or remodelling once formed. The determination of the (14)C content of various tissues may be used to assess turnover rates and degree of substitution (for example for brain cell DNA). Potential targets may be nervous tissues in terms of senile or pre-senile degradation, as well as other highly specialised structures of the eyes. The precision with which the year of birth may be calculated points to forensic uses of this technique.
authors.affiliation: Department of Forensic Medicine, University of Copenhagen, Copenhagen, Denmark. n.lynnerup@antrolab.ku.dk
authors.forename: Niels
authors.initials: N
authors.lastname: Lynnerup
authors.forename: Henrik
authors.initials: H
authors.lastname: Kjeldsen
authors.forename: Steffen
authors.initials: S
authors.lastname: Heegaard
authors.forename: Christina
authors.initials: C
authors.lastname: Jacobsen
authors.forename: Jan
authors.initials: J
authors.lastname: Heinemeier
mesh_terms: D002244:Carbon / Q000378:metabolism*; D002250:Carbon Radioisotopes; D005136:Eye Proteins / Q000378:metabolism*; D006801:Humans; D007908:Lens, Crystalline / Q000378:metabolism*
chemical_list: D002250:Carbon Radioisotopes; D005136:Eye Proteins; D002244:Carbon