<P> In developmental biology, cellular differentiation is the process where a cell changes from one cell type to another . Most commonly the cell changes to a more specialized type . Differentiation occurs numerous times during the development of a multicellular organism as it changes from a simple zygote to a complex system of tissues and cell types . Differentiation continues in adulthood as adult stem cells divide and create fully differentiated daughter cells during tissue repair and during normal cell turnover . Some differentiation occurs in response to antigen exposure . Differentiation dramatically changes a cell's size, shape, membrane potential, metabolic activity, and responsiveness to signals . These changes are largely due to highly controlled modifications in gene expression and are the study of epigenetics . With a few exceptions, cellular differentiation almost never involves a change in the DNA sequence itself . Thus, different cells can have very different physical characteristics despite having the same genome . </P> <P> A specialized type of differentiation, known as' terminal differentiation', is of importance in some tissues, for example vertebrate nervous system, striated muscle, epidermis and gut . During terminal differentiation a precursor cell formerly capable of cell division, permanently leaves the cell cycle, dismantles the cell cycle machinery and often expresses a range of genes characteristic of the cell's final function (e.g. myosin and actin for a muscle cell). Differentiation may continue to occur after terminal differentiation if the capacity and functions of the cell undergo further changes . </P>

When do cells first become different kinds of cell—muscle cells nerve cells reproductive cells etc