extension, allowing oligodendrocytes to myelinate multiple axons. Multiple pathways contribute to oligodendrocyte branching, but the exact molecular process by which oligodendrocytes extend and wrap around multiple axons remains incompletely understood. Laminin, a component of the extracellular matrix, plays an important role regulating oligodendrocyte production. Mice lacking laminin alpha2-subunit produced fewer OPCs in the SVZ. Deletion of Dicer1 disrupts normal brain myelination. However, miR-7a, and miRNA in OPCs, promotes OPC production during brain development. Controversy The possibility and in vivo relevance of OPC differentiation into astrocytes or neurons are highly debated. Using Cre-Lox recombination-mediated genetic fate mapping, several labs have reported the fate of OPCs using different Cre driver and reporter mouse lines. It is generally held that OPCs predominantly generate oligodendrocytes, and the rate at which they generate oligodendrocytes declines with age and is greater in white matter than in gray matter. Up to 30% of the oligodendrocytes that exist in the adult corpus callosum are generated de novo from OPCs over a period of 2 months. It is not known whether all OPCs eventually generate oligodendrocytes while self-renewing the population, or whether some remain as OPCs throughout the life of the animal and never differentiate into oligodendrocytes. OPCs may retain the ability to differentiate into astrocytes into adulthood. Using NG2-Cre mice, it was shown that OPCs in the prenatal and perinatal gray matter of the ventral forebrain and spinal cord generate protoplasmic type II astrocytes in addition to oligodendrocytes. However, contrary to the prediction from optic nerve cultures, OPCs in white matter do not generate astrocytes. When the oligodendrocyte transcription factor Olig2 is deleted specifically in OPCs, there is a region- and age-dependent switch in the fate of OPCs from oligodendrocytes to astrocytes. Whereas some studies suggested that OPCs can generate cortical neurons, other studies rejected these findings. The question is unresolved, as studies continue to find that certain populations of OPCs can form neurons. In conclusion, these studies suggest that OPCs do not generate a significant number of neurons under normal conditions, and that they are distinct from neural stem cells that reside in the subventricular zone. Function As implied by their name, OPCs were long held to function purely as progenitors to oligodendrocytes. Their role as a progenitor cell type has since expanded to include both oligodendrocytes and some protoplasmic type II astrocytes in grey matter. Later, additional functions were suggested. Adult myelination Remyelination Spontaneous myelin repair