The intra-ocular portion of the optic nerve in the turtle Mauremys caspica

The cytoarchitecture of the intra-ocular optic nerve was investigated by light and electron microscopy. Glial cells in the intra-ocular portion are disposed among the axons, either scattered or forming columns, and bordering the nervous parenchyma. Astroglia forms the limiting glia that separates the optic nerve from the vitreous body, retina, choroid and sclera. Indentations of choroid-scleral tissue are observed in the limiting glial sheet as well as an abundance of hemidesmosomes in the astrocytic plasma membranes that abut onto the basal lamina. At the scleral level there is no connective tissue structure similar to the mammalian lamina cribrosa. However, astrocytes in this region are regularly arranged in thick columns and their processes cross throughout the nerve perpendicular to the axons, forming a framework through which the nerve fibres pass. Numerous myelinated and unmyelinated axons are present in the intra-ocular portion. Oligodendrocytes are also present in close relation with myelinated axons. A deep connective tissue septum arises from the ventral region of the optic nerve and extends along the intra-ocular portion.