Forming a functional vertebrate nervous system requires intricate interactions amongst various cell populations, including neurons and glia. Diverse populations of glial cells, such as myelinating glia, i.e., oligodendrocytes of the central nervous system (CNS) and Schwann cells and motor exit point (MEP) glia of the peripheral nervous system (PNS), and perineurial glia, which form the blood-nerve barrier, comprise the nervous system. ii