by Eric Yager

Created on: June 23, 2008

Stem cells have received a great deal of public attention over the past few years due both to their interesting biological characteristics and their therapeutic potential. What makes stem cells unique over other cells found in the human body is that they are capable of renewing themselves for long periods and, importantly, they can be induced to become specialized cell types such as muscle cells or insulin-producing pancreatic cells. To better realize the therapeutic potential of stem cells, much of the current scientific research has been aimed at understanding how cells differentiate.

Cell differentiation is the process by which unspecialized cells give rise to specialized cell types and involves changes in a cell's size, shape, gene expression, biochemistry, and function. Differentiation occurs during embryonic development and in adults during tissue repair and normal cell turnover. Scientists have just begun to understand the internal and external signals that regulate cell differentiation. Specialized cell types are defined by their particular pattern of gene expression and differentiation involves the turning on and turning off of particular genes. In addition, external signals including physical contact with neighboring cells and exposure to chemicals secreted by other cells within the microenvironment also regulate cell differentiation.

Through the study of embryogenesis, the process by which an embryo is formed and develops, two types of cell differentiation have been identified. The first type of differentiation is called conditional specification. Initially, each cell has the capacity to become many different cell types. However, interactions of a cell with surrounding cells and the environment can restrict the fate of that cell. This mode of differentiation is called conditional specification because how a cell becomes specialized is dependent upon the conditions in which the cell finds itself. For example, if a cell is removed from an early embryo that uses conditional specification, the remaining embryonic cells will alter their differentiation pathway as to compensate for the loss of that cell type and function. Removed from the environment of the embryo, the isolated cell is now free to give rise to a wide variety of cell types, even those not normally part of the embryo. Conditional specification has been observed in all vertebrates and few invertebrates.

The second type of cell differentiation is called autonomous specification. Autonomous specification

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