Observations on glial cells within myelin sheaths in degenerating optic nerves.

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The occurrence of glial cells surrounded by myelin sheaths was investigated in cat and monkey optic nerves undergoing Wallerian degeneration. Occasionally, collapsed myelin sheaths were closely apposed to oligodendroglia. More often, small glial cells and their processes were completely surrounded by a myelin sheath. On the basis of ultrastructural appearance the cells were identified as being of three different types. One type had ‘light’ cytoplasmic processes and was comparable to astroglia. The cytoplasm of another type, with a dark granular cytoplasm and sparse rough endoplasmic reticulum, had characteristics of cells previously considered to be microglia, multipotential glia, ‘M’ cells or dark oligodendroglia. The third cell type contained numerous rosettes of ribosomes and an extensive rough endoplasmic reticulum, somewhat similar to cells previously described as medium oligodendroglia. The second and third cell types were considered to be oligodendroglia and the existence of a third glial element is questioned.