Morphology and neurochemistry of canine corneal innervation

Purpose. To determine the architectural pattern and neuropeptide content of canine corneal innervation. Method. Corneal nerve fibers in normal dogs eye were labeled immunohistochemically with antibodies against protein gene product (PGP)-9.5, calcitonin gene-related peptide (CGRP), substance P (SP), vasoactive intestinal polypeptide (VIP), and tyrosine hydroxylase (TH). Relative innervation densities and distribution patterns for each fiber production were assessed qualitatively by serial line-drawing reconstructions and quantitatively by computer-assisted analyses. Results. More than 99% of all corneal PGP-9.5-immunoreactive (IR) nerves contained both CGRP and SP, approximately 30% contained TH, and none contained VIP. Distribution patterns of corneal PGP-9.5-, CGRP-, SP-, and TH-IR nerves were indistinguishable, except that TH-IR fibers were absent from the corneal epithelium. Morphologically, canine corneal innervation consisted of a rich anterior stromal plexus, divided on the basis of morphologic criteria into anterior and posterior levels, and a rich epithelial innervation, characterized by large numbers of horizontally oriented, basal epithelial "leash" formations. Leash axons in all quadrants of the corneal epithelium oriented preferentially toward a common locus in the perilimbal cornea. Conclusions. The results of this study demonstrate for the first time the detailed architectural features, distinctive basal epithelial leash orientations, and peptidergic content of canine corneal innervation. The normal innervation pattern described in this study will provide other investigators with essential baseline data for assessing corneal nerve alterations in canine patients with spontaneous chronic corneal epithelial defects (SCCED) and other ocular diseases or injuries.