THE CHRONOLOGY OF OLIGODENDROCYTE DIFFERENTIATION IN THE RAT OPTIC-NERVE - EVIDENCE FOR A SIGNALING STEP INITIATING MYELINATION IN THE CNS

In order to determine the signals that initiate axon myelination in th e CNS, we have chronicled the differentiation of oligodendrocytes in t he rat optic nerve and related this to the time course and spatial gra dient seen for optic axon myelination. By using markers specific to th e varying stages of oligodendrocyte differentiation we found that olig odendrocyte progenitor cells, present throughout the length of the ner ve at postnatal day 2, mature into GC+ oligodendrocytes in a chiasm to eye progression. This gradient along the nerve of oligodendrocyte dif ferentiation continues with oligodendrocytes near the chiasm expressin g the genes and encoded proteins to MBP and PLP 3 d before oligodendro cytes near the eye, Although oligodendrocyte differentiation and matur ation occurs in a chiasm to eye gradient along the nerve, optic axon s egments near the eye are ensheathed with myelin before segments near t he chiasm, This suggests that the myelination of optic axons is initia ted by a signaling step that is independent of oligodendrocyte differe ntiation and is stronger near the eye than the chiasm region of the ne rve, By examining proposed axonal signals, we found that the onset of myelination is independent of the electrical activity of an axon but c an be correlated to the size of an axon.