The Hayflick Limit

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                                 December 29, 1990

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                                The Hayflick Limit

       In the early  60's, Dr. Leonard Hayflick carried out research at the
       Wistar Institute in Philadelphia which  led  to the discovery of the
       "Hayflick Limit".

       Hayflick found that lung tissue appeared to die out  after the cells
       had divided a  certain  number of times (roughly 50).  Cell cultures
       were also frozen after dividing 25  times.   On  revival,  the cells
       would continue until the 50 division limit was reached, then die.

       As the cells approached the end of their division limit,  the  cells
       would take on   the  appearance  of  old  tissue.   This  appearance
       included age pigments (lipofuscin)  which  is  also  found  in  aged
       hearts and brain cells.

       The discovery of the Hayflick Limit led to theories  speculating  on
       the existence of   a   cellular  "clock".   These  clocks  could  be
       internally regulated for  each  individual   cell   (accounting  for
       variations below and above the 50 limit) or controlled  systemically
       by the hypothalamus of the brain.

       Another possibility is  that control is exerted by the DNA since DNA
       is known to contain our genetic blueprints.  Dr. James Fries and
       Dr. Lawrence Crapo write,

            "Probably, aging just happens,  as  the  result  of cumulative,
             random, and  inevitable  errors  in translation  of  DNA  into
             protein.  The  errors  may even be a crucial part of a process
             that allows  variation  among   individuals  and  thus  allows
             natural selection."

       Mistakes in cell  division tend to accumulate from  the  actions  of
       viruses, free radicals,   radiation  and  chemicals  to  affect  the
       healthy replication of DNA.  As the  system ages and the DNA becomes
       more damaged, the DNA repair mechanism can no longer  perform to its
       optimum level.

       Organisms and animals  which  have better DNA repair mechanisms tend
       to live longer than those who don't.

       One technique used  by  lower  life   forms   is  to  minimize  cell
       replication during daylight hours to prevent radiation  damage  from
       ionizing rays such as UV.
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       Cell division is  encouraged  during  the  night  to  allow the best
       possible replication with a minimum of interfering radiation.

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