Density and organization of free nerve endings in the corneal epithelium of the rabbit

Abstract

Many questions regarding the organization and the density of corneal intraepithelial terminals remain unanswered in spite of the extensive work that has been done on corneal innervation. This study used light microscopic observations of Au-impregnated, serial cross-sections of rabbit cornea to document the assumed, but hitherto uncalcalated, high density of intraepithelial terminals. The innervation density of corneal epithelium was .apprx. 300-600 times that of skin and 20-40 times that of tooth pulp. The density of epithelial innervation decreased from the center of the cornea to the periphery. Serial cross-sections and flat mounts were used in the 3-dimensional reconstruction of the organizational design of corneal terminals. From the subepithelial plexus, axons were seen to penetrate the epithelium, forming terminals that either diverged or branched vertically and horizontally. The vertical axons extended toward the outer cell layer. Horizontal axons developed into families of leashes with disorganized terminal branches and endings. The density and organizational geometry of corneal terminals, together with the correlations between their anatomical uniqueness and the types of sensations elicited by their stimulation, suggest that the cornea is a useful model for the study of experimental acute pain.