Innervation of the mouse cornea during development.

The vertebrate cornea is a transparent tissue at the anterior segment of the eye that is densely innervated by sensory and autonomic nerves. The majority of the cornea nerves are sensory1 and derive from the neural crest component of the trigeminal ganglion.2 Sympathetic and parasympathetic nerves, which respectively originate from the superior cervical and ciliary ganglia, contribute moderately to the cornea.1,3–5 The dense population of corneal nerves responds to irritation and pain,6 thus playing a critical role in protecting the cornea and the rest of the eye from the potentially harmful external environment. In addition, cornea nerves induce blinking reflexes that maintain proper hydration and also secrete neuropeptides that have a mitogenic effect on epithelial cells.7–9 Absence of corneal nerves results in neurotrophic keratitis, a clinical condition characterized by corneal anesthesia and desiccation, and abnormal epithelium metabolism (reviewed by Muller et al.10). Innervation of the adult cornea has been widely studied in mammals.10–13 These studies revealed that there are no major differences in adult corneal innervation between species including mouse, rabbit, dog, cats, and humans. Nerve bundles traverse from the sclero-limbo region of the eye, enter the cornea periphery, and radially innervate the anterior third of the stroma and epithelium. Within the stroma, nerve bundles bifurcate into several smaller branches along the distance between the cornea periphery and center, as they project toward the surface epithelium. On penetrating the epithelial basal lamina, nerve bundles ramify into several smaller nerves (leashes) that form the subbasal plexus. Individual nerve endings project perpendicularly from the subbasal plexus and densely innervate the superficial epithelial layers. Despite the detailed description of adult mouse cornea innervation, there is a paucity of literature on how it is innervated during development. It is often inferred that development of mouse corneal innervation is similar to the pattern described in avian models.14 In the chick, cornea innervation begins when growth cones of presumptive corneal nerves arrive at the ventrotemporal side of the developing eye between E4 and E5. At this point, the nerves appear to be repelled such that instead of directly entering the cornea, they extend both dorsally and ventrally, forming a pericorneal nerve ring around its entire circumference.14,15 On completion of the nerve ring, axons begin to branch at regular intervals, radially innervating the stroma in the cornea periphery. Stromal nerves repeatedly bifurcate as they project toward the cornea center and epithelium.16,17 In this study, we examined innervation of the mouse cornea at various stages of development. Our results provide a detailed analysis of the development of mouse corneal innervation and reveal a novel pattern of cornea innervation that does not involve the formation of a pericorneal nerve ring, as seen in the chick embryo. The results provide a platform for future studies of corneal nerves in the mouse, a species often used as a model for human ocular defects.