Differentiation
 
 Differentiation is the expression of specialized functions by a cell (1). Differentiation may be reversible. For example, bacteria express sets of enzymes to facilitate the use of the nutrients that they sense in their surroundings. When the bacteria sense a change in their surroundings, such as a change in the nutrients in their culture medium, they inactivate the set of enzymes appropriate for the first medium and express a different set of enzymes to facilitate use of the new nutrients. As their surroundings change, the bacteria can change back and forth between different sets of enzymes indefinitely.
 In most cases, however, differentiation is not reversible. Once a cell begins expressing specialized functions, its future choices become limited. For example, human cells that begin expressing heart-specific proteins no longer have the potential to form skin. As cells become more specialized, they find their potential fates more restricted. The restriction of fates is called determination, and it was often shown to occur long before differentiation of any specialized functions. As our molecular tools have become more sophisticated, the distinction between determination and differentiation has become more blurred. Looking for the first detectable signs of differentiation, more sensitive techniques allow us to find molecular differences at earlier stages. When pushed to the limits, molecular techniques have increasingly shown that the basis of determination is the expression of a specialized gene product or function, that is, merely the first step in differentiation.
 
References
1. B. I. Balinsky (1975) An Introduction to Embryology, 4th ed., W. B. Saunders, Philadelphia, p. 468.