ABSTRACT
Life for most eukaryotes, and certainly all mammals, begins as a single totipotent stem cell, the zygote. This cell contains the same complement of genes-no more and no less-as does every adult cell that will make up the organism once development is complete. Nonetheless, this cell has the unique characteristic of being able to implement every possible program of gene expression and is thus totipotent. How is this possible? It is now known that the selective activation and repression of genes distinguishes cells with different developmental potentials. Unraveling this complex series of genetic changes accompanying the progressive restriction of developmental potential during ontogeny is the realm of modern developmental biology. In contrast to the zygote, which has unlimited developmental potential, an intestinal epithelial cell or a granulocyte, for example, is a highly developed cell type that is said to be differentiated. These cells are fixed with respect to their developmental potential and thus no longer possess the ability to contribute to other tissue types. Indeed, intestinal epithelial cells and granulocytes are incapable of undergoing further division and are said to be terminally differentiated. These mature cells have therefore undergone a process whereby they each have acquired a unique and complex repertoire of functions. These functions are usually associated with the cellular morphologic features and/or enzymatic profiles required to implement a specific developmental or functional program. We will come back to the tissue dynamics of these two cell types in a later section of this chapter.
