Flux of Cells
Flux of Cells
Cell division is important for growth, for replacement of cells lost to natural attrition and wear and tear, and for wound healing. Cell division is especially rapid during early development of the organism. At birth the human infant has about 2 trillion cells from repeated division of a single fertilized egg. This immense number could be attained by just 42 cell divisions, with each generation dividing once every six to seven days. With only five more cell divisions, the cell number would increase to approximately 60 trillion, the number of cells in a mature man weighing 75 kg. But of course no organism develops in this machinelike manner. Cell division is rapid during early embryonic development, then slows with age. Furthermore, different cell populations divide at widely different rates. In some the average period between divisions is measured in hours, whereas in others it is measured in days, months, or even years. Cells in the central nervous system stop dividing altogether after the early months of fetal development and persist without further division for the life of the individual. Muscle cells also stop dividing during the third month of fetal development, and future growth depends on enlargement of fibers already present.
In other tissues that are subject to wear and tear, lost cells must be constantly replaced. It has been estimated that in humans about 1% to 2% of all body cells—a total of 100 billion—are shed daily. Mechanical rubbing wears away the outer cells of the skin, and food in the alimentary canal rubs off lining cells. The restricted life cycle of blood corpuscles involves enormous numbers of replacements, and during active sex life of males many millions of sperm are produced each day. Such losses of cells are made up by mitosis.
Normal development, however, does entail cell death in which the cells are not replaced. They may become senescent, accumulating damage from destructive oxidizing agents and eventually dying. Other cells undergo a programmed cell death, or apoptosis (Gr. apo-, from, away from; + ptosis, a falling) (a-puh-TOE-sis), which is in many cases necessary for the continued health and development of the organism. For example, during embryonic development of vertebrates, excess immune cells that would attack the body’s own tissues “commit suicide” in this manner, and nerve cells die to create cerebral convolutions. Apoptosis consists of a well-coordinated and predictable series of events: The cells round up and form bulges from the cytoplasm, the nuclear membrane and other organelles break down, and the DNA is broken up by enzymes.
Apoptosis currently is receiving a great deal of attention from researchers. One of the most valuable laboratory models is a tiny free-living nematode, Caenorhabditis elegans.The effects of apoptosis are not always beneficial to the organism. For example, an important disease mechanism in AIDS (acquired immune deficiency syndrome) seems to be an inappropriate triggering of programmed cell death among important cells of the immune system.
Cell division is important for growth, for replacement of cells lost to natural attrition and wear and tear, and for wound healing. Cell division is especially rapid during early development of the organism. At birth the human infant has about 2 trillion cells from repeated division of a single fertilized egg. This immense number could be attained by just 42 cell divisions, with each generation dividing once every six to seven days. With only five more cell divisions, the cell number would increase to approximately 60 trillion, the number of cells in a mature man weighing 75 kg. But of course no organism develops in this machinelike manner. Cell division is rapid during early embryonic development, then slows with age. Furthermore, different cell populations divide at widely different rates. In some the average period between divisions is measured in hours, whereas in others it is measured in days, months, or even years. Cells in the central nervous system stop dividing altogether after the early months of fetal development and persist without further division for the life of the individual. Muscle cells also stop dividing during the third month of fetal development, and future growth depends on enlargement of fibers already present.
In other tissues that are subject to wear and tear, lost cells must be constantly replaced. It has been estimated that in humans about 1% to 2% of all body cells—a total of 100 billion—are shed daily. Mechanical rubbing wears away the outer cells of the skin, and food in the alimentary canal rubs off lining cells. The restricted life cycle of blood corpuscles involves enormous numbers of replacements, and during active sex life of males many millions of sperm are produced each day. Such losses of cells are made up by mitosis.
Normal development, however, does entail cell death in which the cells are not replaced. They may become senescent, accumulating damage from destructive oxidizing agents and eventually dying. Other cells undergo a programmed cell death, or apoptosis (Gr. apo-, from, away from; + ptosis, a falling) (a-puh-TOE-sis), which is in many cases necessary for the continued health and development of the organism. For example, during embryonic development of vertebrates, excess immune cells that would attack the body’s own tissues “commit suicide” in this manner, and nerve cells die to create cerebral convolutions. Apoptosis consists of a well-coordinated and predictable series of events: The cells round up and form bulges from the cytoplasm, the nuclear membrane and other organelles break down, and the DNA is broken up by enzymes.
Apoptosis currently is receiving a great deal of attention from researchers. One of the most valuable laboratory models is a tiny free-living nematode, Caenorhabditis elegans.The effects of apoptosis are not always beneficial to the organism. For example, an important disease mechanism in AIDS (acquired immune deficiency syndrome) seems to be an inappropriate triggering of programmed cell death among important cells of the immune system.
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