For comparing cell densities, all quantification was performed by an investigator blinded to the treatment condition. Cells expressing PLP mRNA were quantified using unbiased stereologic morphometric analysis (12) (Stereologer System from Systems Planning and Analysis, Inc., Alexandria, VA). Analysis was restricted to the corpus callosum region from the midline and extending laterally to below the cingulum in 15-Km-thick coronal sections. Using the Stereologer System, the specimen thickness contributes to the sampled volume so that measurements reflect cells/mm 3 . The unbiased stereologic method could not be used appropriately for conditions with relatively few cells of interest in any chosen category. Therefore, quantification of cells in the corpus callosum expressing PDGF>R or Ki-67 required counting all labeled cells and using the Spot 2 CCD camera and software to measure the area sampled, resulting in density units of cells/mm 2 (12) .