Oligodendrocyte progenitor cells (OPCs), also known as oligodendrocyte precursor cells, NG2-glia, O2A cells, or polydendrocytes, are a subtype of glia in the central nervous system named for their essential role as precursors to oligodendrocytes. They are typically identified by coexpression of PDGFRA and NG2. OPCs play a critical role in developmental and adult myelinogenesis by giving rise to oligodendrocytes, which then ensheath axons and provide electrical insulation in the form of a myelin sheath, enabling faster action potential propagation and high fidelity transmission without a need for an increase in axonal diameter. The loss or lack of OPCs, and consequent lack of differentiated oligodendrocytes, is associated with a loss of myelination and subsequent impairment of neurological functions. In addition, OPCs express receptors for various neurotransmitters and undergo membrane depolarization when they receive synaptic inputs from neurons. Structure OPCs are glial cells that are typically identified by coexpression of NG2 (a chondroitin sulfate proteoglycan encoded by CSPG4 in humans) and platelet-derived growth factor receptor alpha (encoded by PDGFRA). They are smaller than neurons, of comparable size to other glia, and can either have a bipolar or complex multipolar morphology with processes reaching up to ~50 μm. OPCs comprise approximately 3-4% of cells in the grey matter and 8-9% in white matter, making them the fourth largest group of glia after astrocytes, microglia and oligodendrocytes. OPCs are present throughout the brain, including the hippocampus and in all layers of the neocortex. They distribute themselves and achieve a relatively even distribution through active self-repulsion. OPCs constantly survey their surroundings through actively extending and retracting processes that have been termed growth cone like processes. Death or differentiation of an OPC is rapidly followed by migration or local proliferation of a neighboring cell to replace it. In white matter, OPCs are found along unmyelinated axons as well as along myelinated axons, engulfing nodes of Ranvier. Recently, OPCs have been shown to reside in close contact with NG2-expressing pericytes in cerebral white matter, as well. OPCs receive synaptic contacts onto their processes from both glutamatergic and GABAergic neurons. OPCs receive preferred somatic contacts from fast-spiking GABAergic neurons, while non-fast spiking interneurons have a preference for contacting the processes. These inhibitory connections (in mice) occur mainly during a specific period in development, from postnatal day 8 till postnatal day 13. Development OPCs first appear during embryonic organogenesis. In the developing neural tube, Shh (Sonic hedgehog) signaling and