Ultrastructural Features of TNF–alpha–Induced Optic Neuropathy in a Rat Model

Abstract

Abstract: : Purpose:To describe the ultrastructural features of optic nerve in a rat model of tumor necrosis factor–alpha (TNF–α) –induced optic neuropathy and to provide a generalized hypothesis to its structural pathogenesis. Methods: Adult male Lewis albino rats were injected intravitreously, posterior to the pars plana, with either 10 ng of TNF–α or phosphate–buffered saline. Untreated normal animals were also used as an additional control. Rats were divided into groups of 2 weeks, 1 and 2 months post–injection. At the end of each time point, animals were humanely euthanized. After enucleation, cross–sectional profiles of retrobulbar optic nerve were dissected away, immersion–fixed in 1/2 strength Karnovsky's, post–fixed in osmium tetroxide, then processed for embedding into plastic blocks. Thin sections were cut on a Sorvall MT–6000 ultramicrotome, double–stained with uranyl acetate and lead citrate, then examined on a Zeiss EM–10 transmission electron microscope. Results: All controls presented a normal spectrum of axonal and glial elements. For rats injected with TNF–α, optic nerves demonstrated a mild–to–moderate progression of axonal condensation, demyelination, and degenerated axonal profiles with increasing time points. The number of unmyelinated axons was greatest at 2 months. Many of the oligodendrocytes displayed an increased density and/or fragmentation of chromatin. In these same cells, the integrity of membranes delimiting organelles were often disrupted. The number of oligodendrocytes appeared to have decreased the greatest at 2 months. Astrocytes and macrophages appeared to have increased in number peaking at 2 weeks, then decreasing through the 2 months of the study. Astrocytes and their processes were often found adjacent to and circumscribing areas where demyelinated axons were at their greatest density. Conclusions: In previous studies, TNF–α has been shown to produce both demyelination and axonal degeneration. The increased presence of unmyelinated axons at the 2 month time point in this study may provide the basis for two explanations in this rat model of TNF–α induced optic neuropathy; 1) either the initial site of injury is at the level of oligodendrocytes leading to demyelination, then axonal degeneration, and finally retinal ganglion cell death or, 2) partial axonal degeneration leads to new axonal growth with myelination yet to follow.

Keywords: neuro-ophthalmology: optic nerve • pathology: experimental • cytokines/chemokines