Cellular Senescence
The human body is thought to be composed of anywhere between 50-100 trillion cells. Some of these cells remain undifferentiated and possess the ability to become any type of tissue. These are the stem cells. Most others develop into specific tissue types such as kidney, liver, etc. The cells of some of these tissues (skeletal and heart muscle, nerve, fat) do not undergo cell division during our lives while others (like skin, gut, hair) do. At about the time of puberty the individual cells begin to age and this process is referred to as cellular senescence. Over the past few decades there has been much research on this topic and scientists are developing a clearer understanding of what causes our cells to age.
Oxygen is utilized by all living organisms to produce energy. This is done in small structures within the cell called mitochondria. As with everything that has a plus side, there is also a downside. These oxidative reactions produce by products called free radicals which in turn can cause damage to DNA. We are now all familiar with the fact that our chromosomes are composed of DNA and are found in the nucleus of the cell. Mitochondria also have DNA but their DNA is more sensitive to the damaging free radicals. This is because of the structure of the DNA itself and the fact that it is present near the production site of the free radicals.
Our body produces chemical substances called antioxidants that help neutralize these free radicals. With time this process becomes less effective. The end result is thought to be one of the major causes of aging. The rapidity with which an animal can repair this damage correlates to the life expectancy of that particular species.
The most interesting aspect of cellular senescence relates to changes in chromosomal telomeres. Telomeres are the end caps of individual chromosomes. You may recall that chromosomes (46 in humans) occur in pairs and are ribbon like structures. These end caps function like the plastic tip of a shoe lace. They prevent the DNA of the chromosome from unraveling when the DNA replicates during cell division. If this protective device was not present the results would be catastrophic for the organism. It is estimated that cells can undergo about 60 cell divisions during their lifetime. The interesting point is that each time the cell divides the telomere loses some of its substance (which is made of DNA and protein). Once it reaches a critical size an adjacent gene is activated and further cell divisions are prevented. Certain cells, however, produce an enzyme called telomerase which permits the rebuilding of these lost molecular structures and thus the cell can continue to divide. Cells that possess this enzyme can divide indefinitely and are in a sense ‘immortal’. Sperm and certain cells in the immune system are examples. Cancer cells possess telomerase and this is, of course, detrimental.
Next month we will examine how cellular aging manifests itself at the tissue level and in ways that are visible to the naked eye.
Oxygen is utilized by all living organisms to produce energy. This is done in small structures within the cell called mitochondria. As with everything that has a plus side, there is also a downside. These oxidative reactions produce by products called free radicals which in turn can cause damage to DNA. We are now all familiar with the fact that our chromosomes are composed of DNA and are found in the nucleus of the cell. Mitochondria also have DNA but their DNA is more sensitive to the damaging free radicals. This is because of the structure of the DNA itself and the fact that it is present near the production site of the free radicals.
Our body produces chemical substances called antioxidants that help neutralize these free radicals. With time this process becomes less effective. The end result is thought to be one of the major causes of aging. The rapidity with which an animal can repair this damage correlates to the life expectancy of that particular species.
The most interesting aspect of cellular senescence relates to changes in chromosomal telomeres. Telomeres are the end caps of individual chromosomes. You may recall that chromosomes (46 in humans) occur in pairs and are ribbon like structures. These end caps function like the plastic tip of a shoe lace. They prevent the DNA of the chromosome from unraveling when the DNA replicates during cell division. If this protective device was not present the results would be catastrophic for the organism. It is estimated that cells can undergo about 60 cell divisions during their lifetime. The interesting point is that each time the cell divides the telomere loses some of its substance (which is made of DNA and protein). Once it reaches a critical size an adjacent gene is activated and further cell divisions are prevented. Certain cells, however, produce an enzyme called telomerase which permits the rebuilding of these lost molecular structures and thus the cell can continue to divide. Cells that possess this enzyme can divide indefinitely and are in a sense ‘immortal’. Sperm and certain cells in the immune system are examples. Cancer cells possess telomerase and this is, of course, detrimental.
Next month we will examine how cellular aging manifests itself at the tissue level and in ways that are visible to the naked eye.
posted by Dr. Lou Moretti at
2:16 PM
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