Human Body : Growth

The human body starts as a single cell, which multiplies into seventy-eight different organs at birth. It continues to grow throughout a long period of childhood and adolescence, after which external signs of growth cease. Only hair and nails continue to grow externally into old age. The drooping of ears and nose, which may make them appear longer, is caused by the effect of gravity on cartilage, not by growth.
Internally there continues to be unseen growth, that keeps the human body alive. The lens of the eye is the only part of the body, which is set for life. (Unless artificially replaced in a cataract operation) Otherwise, there is constant regeneration, performed without the drama seen in Dr Who!
This is due to the presence of stem cells, which are able to divide and multiply, to replenish the body with blood cells, brain cells, heart muscle and bone cells. Other cells do not have this capacity. The outer layer of the skin is replaced every month. The lining of the lungs is replaced every two weeks, the stomach lining every two to nine days and the liver is replaced annually. It takes rather longer to grow a new skeleton! The bones are broken down and replaced every twelve years.
It is thought that stem cells have a maximum life of one hundred and twenty years, until they cease to be able to perform the function of renewal, so that the human body can no longer sustain life. Despite medical advances, human life expectancy currently falls well short of one hundred and twenty years. However, the activity of the stem cells provide the human body with at least the possibility of longevity. 
Research is being done into their use in organ transplants, where a heart using the human bodies own cells, would be accepted by the patient,  without the need for anti – rejection medication. Potentially the possibility of further extension of the human life span exists.  

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