ABSTRACT
Different types of cells arise early in embryonic development, and epigenetic control of gene expression is used to establish specific gene expression patterns that distinguish individual types of cells. The early embryos of most multicellular eukaryotes begin as groups of cells that are termed totipotent because each cell seems to have an equal ability to become any of the cell types generated as the embryo develops. This chapter introduces the basic concepts behind the cellular differentiation that permits development of the embryo. With the exception of erythrocytes and post-meiotic gametes, most of the cells of the developing organism contain an identical copy of the original genome that was generated by the combination of the two parental genomes shortly after fertilization. The basis of cell differentiation is that specific patterns of gene expression apply to different types of cells, but the fact that they all probably possess the same genome implies that epigenetic regulation must control the cell-type-specific expression pattern.
