AbstractOligodendrocytes are myelinating cells in the central nervous system. Recent studies demonstrated that oligodendrocyte progenitor cells are generated from a restricted region in the ventricular zone. In the rodent spinal cord, progenitor cells appear from narrow and bilateral longitudinal columns in the ventral ventricular zone, and then migrate dorsally. This ventral‐specific appearance of oligodendrocyte progenitors may be controlled along the dorso‐ventral axis in the spinal cord by extrinsic signals secreted from both the dorsal and ventral cords. The combined action of the Notch signaling pathway and a basic helix‐loop‐helix class of transcription factors may modulate this early specification of spinal oligodendrocytes and also be involved in multiple steps of oligodendrocyte differentiation. Microsc. Res. Tech. 52:740–745, 2001. © 2001 Wiley‐Liss, Inc.