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The mammalian brain is probably the most complex organ generated by evolution so far. It is composed of more distinct cell types than any other organ (1), including those that protrude myriads of processes to make the connections that define brain function. Brain development requires that differentiated cells become dispersed and positioned correctly, which demands that progenitor cells migrate—often over very long distances—from sites of origin in the early neural tube to final sites of differentiation in the multilayered brain tissue. Oligodendrocytes, the cells that myelinate axons, are perhaps the most migratory of all the brain's cell types, but their paths have remained ill defined (2). On page 379 of this issue, Tsai et al. (3) report that oligodendrocyte precursor cells (OPCs) migrate along blood vessels, and define a signaling pathway involved in the process. The findings are exciting not only for their pathological implications, but because they add to the emerging picture that blood vessels do much more than provide oxygen and nutrients to the developing or regenerating tissue—a concept sometimes referred to as angiocrine signaling (4, 5).