CELL DIFFERENTIATION

After conception, the fertilized ovum undergoes a series of divisions, ultimately forming approximately 200 dif­ferent cell types. The formation of different types of cells and the disposition of these cells into tissue types is called cell differentiation, a process controlled by a system that switches genes on and off. Embryonic cells must become different to develop into all of the various organ systems, and they must remain different after the signal that initi­ated cell diversification has disappeared. The process of cell differentiation is controlled by cell memory, which is maintained through regulatory proteins contained in the individual members of a particular cell type. Cell differ­entiation also involves the sequential activation of multi­ple genes and their protein products. This means that after differentiation has occurred, the tissue type does not revert to an earlier stage of differentiation. The process of cell differentiation normally moves forward, producing cells that are more specialized than their predecessors. Usually, highly differentiated cell types, such as skeletal muscle and nervous tissue, lose their ability to undergo cell division.

Although most cells differentiate into specialized cell types, many tissues contain a few stem cells that appar­ently are only partially differentiated. These stem cells are still capable of cell division and serve as a reserve source for specialized cells throughout the life of the or­ganism. They are the major source of cells that make re­generation possible in some tissues. Stem cells have varying abilities to differentiate. Some tissues, such as skeletal muscle tissue, lack sufficient numbers of undif-ferentiated cells and have limited regenerative capacity. Stem cells of the hematopoietic (blood) system have the greatest potential for differentiation. These cells can po­tentially reconstitute the entire blood-producing and im­mune systems. They are the major ingredient in bone marrow transplants. Other stem cells, such as those that replenish the mucosal surface of the gastrointestinal tract, are less general but can still differentiate. Cancer cells are thought to originate from undifferentiated stem cells (see Chapter 8).