Each neuron begins life as a progenitor cell. These cells form in the central part of the developing brain and then move outward along the supportive glial cells until they reach their pre-programmed location in the brain. As a progenitor cell reaches its destination its status changes and it now is "committed" to become one of numerous types of neurons. The neuron begins to grow axons and dendrites. These fibers will eventually form a synapse or connection with those of other neurons. Once the neuron has taken on its specialized function it is considered "differentiated."
In developmental biology, cellular differentiation is the process by which a cell changes from one cell type to another. Usually this is because a less specialized type becomes a more specialized type, such as during cell growth. 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.
A cell that can differentiate into all cell types of the adult organism is known as pluripotent. Such cells are called embryonic stem cells in animals and meristematic cells in higher plants. 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. 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.
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