The innervation of the rabbit's anterior eye segment: a retrograde tracing study

Abstract

The differential innervation of the distinct elements of the anterior segment of the rabbit eye, i.e. cornea, conjunctiva, ciliary body, iris, and trabecular meshwork, was studied using horseradish peroxidase-wheatgerm agglutinin (HRP-WGA) as a tracer. HRP-WGA was injected into the anterior chamber, the conjunctiva, or the cornea, and the animals were killed after 6 to 96 hr. The HRP-WGA localization was studied at the light and electron microscopic level. Injection into the anterior chamber led to an ubiquitous spread of the tracer in all elements of the anterior segment, resulting in a consistent retrograde labelling of neurons in the trigeminal (sensory), superior cervical (sympathetic) and ciliary (parasympathetic) ganglion. Neurons in the proximal part of the pterygopalatine ganglion (parasympathetic) were labelled only when the injected tracer was present in the conjunctiva. Varying survival times after corneal injection were used to establish restricted corneal uptake. Up to 16 hr after corneal injection, HRP-WGA was limited to the cornea and led to the exclusive labelling of neurons in the trigeminal ganglion, indicating that the central cornea has an almost exclusive sensory innervation. The trigeminal sensory neurons projecting to the cornea were restricted to a small antero-medial part of the trigeminal ganglion. Electron microscopy revealed four different cell types in the trigeminal ganglion. However, distinct elements of the anterior segment did not project to specific cell types.