A third neuroglial cell types. An electron microscopic study

During a study of developing optic nerves, it was necessary to reevaluate the earlier electron microscopic evidence that only two neuroglial types, oligodendrocytes and fibrous astrocytes, are contained within central white matter. A reexamination of adult optic nerves showed that in addition to these two kinds of cells, a third morphologically distinct cell type exists. Cells of this third neuroglial variety are small and their cytoplasmic density is less than that of all forms of oligodendrocytes but slightly more than that of astrocytes. Neither the filaments and glycogen particles characteristic of astrocytes nor the microtubules typical of oligodendrocytes are observed in the third type of neuroglia. Dense, laminar bodies and fat droplets are frequently present within their cytoplasm, and profiles of the granular endoplasmic reticulum consist of long, stringy cisternae. Fetal and early postnatal optic nerves contain a cell type with the same morphological characteristics. This suggests that the third type of neuroglia in adult nerves may represent cells of a similar nature to the multipotential stem cells of other tissues. Some of the characteristics of the third type of neuroglia are reminiscent of the microglia of classical light microscopy, but data from developing nerves indicates that the former are derived from neuroectodermal matrix cells and not from invading mesenchymal elements.