Sensory and sympathetic innervation of the mammalian cornea. A retrograde tracing study

The method of retrograde transport of horseradish peroxidase-wheat germ agglutinin (HRP-WGA) was used to study the locations, numbers and somata diameters of corneal afferent and efferent neurons in four different mammalian species. A 2% solution of HRP-WGA was applied to the central cornea of the rat, rabbit, cat and monkey and the animals were perfusion-fixed 48-96 hr later. HRP-WGA-labeled sensory neurons were distributed relatively uniformly throughout the ophthalmic region of the ipsilateral trigeminal ganglion of the rat, cat and monkey. In marked contrast, labeled cells in the rabbit trigeminal ganglion were clustered in a sharply defined longitudinal column located in the midregion of the ophthalmic area. Occasional cells in some cats and monkeys were observed near, and possibly within, the maxillary region of the ganglion. There was no evidence for a dorsoventral somatotopy of corneal afferent neurons in any species. The majority of the labeled afferent somata were small or medium in size, although some larger diameter neurons were also observed. Modest numbers of labeled neurons were observed in the ipsilateral superior cervical ganglion (SCG) of the rabbit and cat; however, only occasional labeled neurons were observed in the SCG of the rat, and none were seen in the monkey. The labeled SCG cells, when present, were concentrated in the rostral half of the ganglion, although many cells in the cat SCG were also found further caudally. No labeled neurons were found in the middle cervical or stellate ganglia of any animal. The results of this study have revealed the existence of subtle interspecies differences in the organization of the mammalian corneal afferent and efferent innervations.