MORPHOLOGICAL HETEROGENEITY OF RAT OLIGODENDROCYTES - ELECTRON-MICROSCOPIC STUDIES ON SERIAL SECTIONS

The microanatomy of ensheathing and early myelinating rat oligodendroc ytes was analyzed through electron microscopic examination of serial s ections. The study included cells in the spinal cord (SC) ventral funi culus and the corpus callosum (CC), containing early myelinating, pros pective large axons and late myelinating, prospective small axons, res pectively. The results show that ensheathment commences fetal day (F) 19 in the SC and 12 days postnatally (P12) in the CC. By then, multipo lar SC and CC oligodendrocytes provide axons with uncompacted cytoplas mic sheaths. The average number of axons ensheathed by each such cell was 7 in the SC and 13 in the CC. The mean diameter of the ensheathed axons was 0.69 mu m in the SC and 0.36 mu m in the CC. The formation o f compact myelin had clearly been initiated at birth in the SC and at P17 in the CC. At that stage, the mean number of myelinated axons per analyzed oligodendrocyte was 3 in the SC and 15 in the CC. The mean di ameter of the myelinated axons was 1.02 mu m in the SC and 0.54 mu m i n the CC. These observations show that myelin-related rat oligodendroc ytes are morphologically heterogeneous. It also seems that this hetero geneity is related to time of onset of myelination and prospective axo n diameter. Further, the data suggest that some oligodendrocytes reduc e the number of sheaths initially elaborated before formation of compa ct myelin. (C) 1994 Wiley-Liss, Inc.