<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> 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>
when do cells first become different kinds of cell -- muscle cells nerve cells reproductive cells etc
when do cells first become different kinds of cell -- muscle cells nerve cells reproductive cells etc
numerous times
numerous times