The capacity of specific regions and fiber tracts of developing rat lumbosacral spinal cord to express proteolipid protein (PLP) mRNA was examined using in situ hybridization. PLP mRNA was undetectable at Day 18 postfertilization but present in the white matter by Postnatal Day 0 (P0). Initially, levels of PLP mRNA were higher in ventral than in dorsal funiculi. Hybridization signal was observed in the corticospinal tract by P10. Appearance of PLP message in gray matter lagged several days behind that in white matter. The expression of PLP mRNA in gray matter rose between P10 and P40 and the abundance of PLP transcript remained high in white matter during the same period. In the adult spinal cord some oligodendrocytes in the Lissauer tract and in the posterior commissure expressed low levels of PLP mRNA, which was undetectable in the substantia gelatinosa. Comparing the time of emergence of PLP mRNA to the histological appearance of myelination gave evidence that the message is expressed in oligodendrocytes that are not many days away from myelin formation. These results provide new information on the topographic distribution of PLP mRNA during myelination of the rat spinal cord.
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