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.

Cell division is encouraged during the night to allow the best possible replication with a minimum of interfering radiation.