10.4Cell Differentiation and Stem Cells
During the developmental process, cells derived from one fertilized egg become units with various functions such as muscle cells, neurons and epithelial cells. This phenomenon is called cell differentiation.
Embryonic cells in the early developmental stage have the potential to become various cell types. These cells, which have not yet undergone differentiation, are referred to as undifferentiated cells. However, as development proceeds, most cells are differentiated into units with particular functions. Each differentiated cell type shows the different gene expression pattern and level. (Fig. 10-7). In other words, in differentiated cells, the expression of certain genes necessary to fulfill their cellular functions is enhanced, whereas the expression of unnecessary genes is suppressed.
In differentiated cells, the suppression of unnecessary genes is performed through chemical modification of the genes themselves (or of proteins that bind to them) or through the binding of special proteins to them. Such suppression may be temporary or semi-permanent (see Chapter 4). If the suppression is unlocked - thus allowing gene expression - the cells regain the potential to be transformed into other cells.
Cells that retain the ability to differentiate into many cell types exist in the various tissues of the human body. These are known as stem cells, and are believed to be involved in the repair of damaged tissues. Attempts to collect, grow and differentiate these cells in order to artificially create tissues and organs have recently been made. The aim is to create tissues and organs using stem cells obtained from patients and implant them back into the body. The method of implanting the patient’s own tissues avoids immunological rejection and raises fewer ethical issues than other methods (such as techniques that use human embryonic cells).
Fig. 10-7. Cell Differentiation
A model showing changes in the gene expression pattern and level during cell differentiation. The gene expression patterns occurring in three types of differentiated cell are schematically outlined. Differentiated cells have the same gene set as the fertilized egg, but the gene expression pattern and level differ.