Maturation, Aging, and Death

Abstract

Our body is not a fixed structure. For the first 18 years or so, from the moment an egg is fertilized by a sperm and starts to grow in the womb, through birth and into adulthood, new tissues and new organs are being laid down. But even after that, there is turnover of many tissues in that some cells are degraded and replaced by new ones. A cut on the skin is repaired, a sunburnt skin returns to normal. Inside our body repair goes on as well: The cells lining the alimentary tract are continually being sloughed off and replaced; the same is true of the cells lining all parts of the respiratory system. An infection or an inflammation leads to the recruitment of white cells to the site of injury; when the cells have completed their task, they die off and are eventually replaced by new ones. Within cells, molecules are continually being replaced. A glucose molecule has a life of minutes. A protein molecule has a life of hours, days, or months, depending on its function: Enzymes turn over fast, structural proteins like actin, myosin, or collagen, slowly. RNA is turning over fast, as new proteins are made. Only DNA remains stable, without any turnover. What changes are superimposed on this continual wear and tear of our molecules, as we mature in childhood, and as we age in adulthood? These are the questions that are addressed in this chapter.