<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> There are multiple levels of cell potency, the cell's ability to differentiate into other cell types . A greater potency indicates a larger number of cells that can be derived . A cell that can differentiate into all cell types, including the placental tissue, is known as totipotent . In mammals, only the zygote and subsequent blastomeres are totipotent, while in plants many differentiated cells can become totipotent with simple laboratory techniques . A cell that can differentiate into all cell types of the adult organism is known as pluripotent . Such cells are called meristematic cells in higher plants and embryonic stem cells in animals, though some groups report the presence of adult pluripotent cells . Virally induced expression of four transcription factors Oct4, Sox2, c - Myc, and Kfl4 (Yamanaka factors) is sufficient to create pluripotent (iPS) cells from adult fibroblasts . A multipotent cell is one that can differentiate into multiple different, but closely related cell types . Oligopotent cells are more restricted than multipotent, but can still differentiate into a few closely related cell types . Finally, unipotent cells can differentiate into only one cell type, but are capable of self - renewal . In cytopathology, the level of cellular differentiation is used as a measure of cancer progression . "Grade" is a marker of how differentiated a cell in a tumor is . </P>

The need for differentiation in a multicellular organism