PNS features of rodent optic nerve axons.

Peripheral nerves undulate together, giving them a wavelike appearance. This axonal pattern is also found in one region of the central nervous system, the optic nerve. Undulations provide a degree of compliance, as when a nerve is stretched, they are pulled straight. In the peripheral nervous system, undulations are thought to be conditional on the presence of extrafascicular collagen, which is also present between fascicles in the mammalian optic nerve. The pattern of undulations is described in the rat optic nerve and is related to the nerve's fascicular configuration and the regions between the fascicles, the extrafascicular matrix. The stage at which the undulations appear is determined to ascertain whether they are an intrinsic feature of optic axons or whether they emerge in association with the development of other events. The waves could be traced across the width of the nerve. Their pattern was not altered at fascicular boundaries, where axon groups are segregated. The periodicity of the undulations was constant between the eye and the middle of the nerve's length. Here fascicular divisions are present. Close to the chiasm, where the glial organisation changes and fascicles disappear, the periodicity of the undulations lengthened. They disappeared in the chiasm. Although collagen is a major component of the mammalian optic nerve's extrafascicular matrix, it was not present in the rat. Hence, the wavelike trajectory is independent of connective tissue. The waves are not present during early development and hence not an intrinsic feature of these axons. They appear behind the eye at late prenatal stages, emerging in association with glia in the extrafascicular matrix. They develop caudally, mirroring glial maturation. It is probable that the glia are maturing astrocytes that provide a scaffold for this feature of optic axon trajectory.