NORMAL PROCESS OF GROWTH AND DIFFERENTIATION (CELLS)
Each organ consists of a number of different types of cells arranged in a characteristic pattern. For example, the stomach has an inner lining, a muscle layer and an outer protective coat. The lining contains acid-producing and mucus-producing cells. The middle layer contains muscle cells, blood vessels with the different types of blood cells within them, lymph vessels, lymph cells, and supporting cells which hold all the others together. The outer layer contains supporting cells and nerve cells.
Some cells, such as the acid-producing cells, occur only in the stomach. Some, such as the muscle cells, are also found in the intestines, bladder and other organs. Others, such as blood and lymph vessels, are found throughout the body. Each type of cell has a unique job and a unique appearance. A muscle cell cannot do the job of a nerve cell. An acid-producing cell cannot do the job of a blood cell, and so on.
The process of developing special functions is called differentiation or maturation. A cell with few or no special functions is called undifferentiated, immature or primitive. The only function of undifferentiated cells is to multiply when necessary to produce cells which are capable of developing into specialised cells. Once cells specialise they often lose the ability to multiply. For example, mature red blood cells are so specialised for carrying oxygen around the body that they cannot reproduce themselves. As old cells wear out or are lost by bleeding, new red blood cells come from undifferentiated cells in the bone marrow, not from the mature red cells in the blood.
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