Demonstration of Efferent Fibers in the Human Optic Nerve

Abstract

Purpose:: Animal studies have demonstrated that while most fibers constituting the optic nerve are retinofugal, there are also a few fibers that run the other way. Leber's Hereditary Optic Neruopathy (LHON) is a genetic condition that selectively obliterates retinal ganglion cells and their axons in the optic nerve. This provides an opportunity for searching and characterizing efferent fibers in optic nerves obtained from LHON subjects.

Methods:: Four eyes and retrobulbar optic nerves were collected at autopsy from two Affected patients who were members of a large pedigree with the 11778 LHON mtDNA mutation. Eyes and nerves were initially immersion fixed in normal buffered formalin within 24 hours postmortem. Eyes and nerves were dissected and processed for paraffin embedding. Nerves were immunohistochemically stained for various enzymes and neurotransmitters consistent with a postganglionic sympathetic pathway as well as for basic neuronal elements that would verify axonal integrity. Tissues were further post-fixed in glutaraldehyde for processing into plastic, sectioned, stained and examined with a transmission electron microscope (TEM).

Results:: Immunohistochemistry (IHC) demonstrated positive staining of small-caliber axons for neurofilament protein without evidence of myelin staining in human LHON optic nerves. Furthermore, these axons were immuno-positive for norepinephrine. Their location was primarily observed in the peripheral zones of the nerve, but a few profiles were also found centrally. TEM evaluation of this tissue revealed a small-caliber unmyelinated fiber with a mean axon diameter of approximately 0.55 um.

Conclusions:: In an LHON optic nerve, within zones of retinofugal fiber depletion, there remain small caliber unmyelinated fibers. These axons are characterized both by IHC and TEM methods that demonstrate a probable sympathetic postganglionic function involving vascular regulation of the optic nerve.

Keywords: optic nerve • neurotransmitters/neurotransmitter systems • immunohistochemistry