The Ultrastructure of Astrocytes, Oligodendrocytes, and Microglia in the Optic Nerve of Urodele Amphibians (A. Punctatum, T. Pyrrhogaster, T. Viridescens)

Recent ultrastructural studies which indicate a single types of glia cell in the amphibian optic nerve are contradicted by the results of the present investigation demonstrating three types of glial cells in the optic nerve of adult newts T. viridescens and T. pyrrbogaster, and the neotenous salamander A. punctatum. Well differentiated astrocytes, oligodendrocytes and microglia are the principal glial constituents with cytological characteristics corresponding to glial elements in the optic nerve of mammals. Immature astrocytes and oligodendrocytes are also present in the nerve indicating continuing production of cells from neuro-ectodermal precursors. Astrocytes constitute approximately 80% of the total. Processes of the large multipolar elements divide axons into bundles and extend to the pial surface to form the glia limitans. A distinct inter-species variability (6--17%) in oligodendrocytes is apparently related to differences in the incidence of myelinated axons. Microglia are the least numerous element constituting only 2--6%. They arise, in part, from monocytic cells near the pial surface.