ReviewCorneal nerves: structure, contents and function
Introduction
The cornea is one of the most densely innervated tissues in the body and is richly supplied by sensory and autonomic nerve fibres. Published descriptions of the anatomy and physiology of the mammalian corneal innervation are numerous, yet many aspects of corneal nerve architecture and function remain incompletely understood. The subject of corneal innervation has taken heightened importance in recent years due to the observation that corneal nerves are routinely injured following modern refractive surgical procedures or following certain corneal diseases. This damage can lead to transient or chronic neurotrophic deficits. In addition, in the past several years neuropeptides synthesized by corneal nerve fibres have been used successfully to promote corneal wound healing in clinical corneas resistant to conventional therapies. The purpose of this review article is to summarize our current understanding of corneal nerve distribution and morphology, and to provide a new schematic of corneal nerve architecture in humans based on recent information. The mechanisms by which corneal nerves and associated neurotrophins maintain a healthy cornea and promote wound healing after corneal injuries will also be described.
Section snippets
Methods of studying the corneal innervation
Our current understanding of corneal nerve architecture and morphology is based primarily on light microscopic observations of gold chloride-, acetylcholinesterase-, and immunohistochemical-stained corneas (Martinez, 1940, Zander and Weddell, 1951, Rozsa and Beuerman, 1982, Schimmelpfennig and Beuerman, 1982, Jones and Marfurt, 1991, Jones and Marfurt, 1998, Marfurt et al., 2001). Several elegant, detailed descriptions have been made by staining tangential sections or epithelial sheets with
Origins of corneal nerves
Most corneal nerve fibres are sensory in origin and are derived from the ophthalmic branch of the trigeminal nerve; however, in some cases the inferior cornea receives some of its innervation from the maxillary branch of the trigeminal (Vonderahe, 1928, Ruskell, 1974). The results of retrograde nerve tracing studies in experimental animals suggest that mammalian corneas receive their sensory supply from a modest number (50–450) of trigeminal neurons (Morgan et al., 1978, Marfurt et al., 1989,
Distribution and ultrastructure of corneal nerves
Nerve bundles enter the cornea at the periphery in a radial fashion parallel to the corneal surface (Fig. 1, Fig. 2). The nerve bundles lose their perineurium and myelin sheaths within approximately 1 mm of the limbus (Fig. 5(D)) continue into the cornea surrounded only by Schwann cell sheaths (Fig. 3, Fig. 4), and then subdivide several times into smaller side branches (Fig. 1, Fig. 2). The absence of a myelin sheath on central corneal axons is necessary to maintain corneal transparency. The
Confocal microscopy of corneal nerves
Reflected light IVCM has become an increasingly popular method of imaging clinical corneas (Minsky, 1957, Cavanagh et al., 1990, Petroll et al., 1998, Jester et al., 1999, Tervo and Moilanen, 2003). Commercial microscopes are based mainly on the confocal slit principle (Masters and Thaer, 1994), whereas tandem scanning microscopes (TSCM) (Petran et al., 1968) utilize a rotating disc (Cavanagh et al., 1990). There are only minor differences between these technologies in terms of efficacy of
Corneal nerve density
IVCM studies of corneal nerves in control individuals show an average of 6–8 nerve bundles per image (Rosenberg et al., 2000, Vesaluoma et al., 2000a, Vesaluoma et al., 2000b, Oliveira-Soto and Efron, 2001). Given the assumption that an IVMC image is about 0·1 mm2 in size, and that the total surface area of the human cornea is approximately 90 mm2, it can be calculated that approximately 5400–7200 nerve bundles are present in the human subbasal plexus. Because each subbasal nerve fibre bundle
The architecture of the subbasal plexus in human corneas differs in some ways from that of other mammals
In 1951, Zander and Weddell published a landmark paper on the innervation of the mammalian cornea. This paper is often cited as the definitive work on corneal nerve architecture and provides an exhaustive description of corneal nerve anatomy in the rabbit; however, it offers relatively limited information on human corneal nerves. Recent studies based on IVCM of human eyes have demonstrated that the directional orientations of basal epithelial leashes in human corneas differ from those in other
A new schematic of the human corneal innervation
In 1984, David Maurice published a three-dimensional schematic of the human corneal innervation (Fig. 1, Fig. 2) based largely on the observations of Beuerman and co-workers in rabbits (Rozsa and Beuerman, 1982). This elegant schematic provided a clear demonstration of corneal nerve distribution in the stroma and subepithelial plexus. The leashes were thought to be oriented in a uniformly radial fashion, which does not hold true for the architecture of the nerve fibres in the human corneal
Neurochemistry of corneal innervation
Sensory nerves. Corneal sensory nerves express a variety of biologically active neurochemicals. Large numbers of these nerves contain substance P (SP) and/or calcitonin gene-related peptide (CGRP) (Tervo et al., 1982, Stone and Kuwayama, 1985, Stone and Mc Glinn, 1988, Ueda et al., 1989, Jones and Marfurt, 1991, Beckers et al., 1993, Jones and Marfurt, 1998, Felipe et al., 1999, Marfurt et al., 2001, Müller and Klooster, 2001). Many corneal nerves that express SP (Fig. 4(D) and (F)) and/or CGRP
The role of nerves in the maintenance of a healthy cornea
Corneal nerve fibres exert important trophic influences on the corneal epithelium and contribute to the maintenance of a healthy ocular surface. Since the earliest experimental studies of Magendie (Magendie, 1824), it has been confirmed repeatedly that dysfunction of the corneal innervation produces a degenerative condition known as neurotrophic keratitis. Most clinical cases of neurotrophic keratitis (Fig. 8, Fig. 9) are caused by herpetic viral infections of the ocular surface, or by
Pathogenesis of neurotrophic keratitis
Many theories have been proposed to explain the pathogenesis of neurotrophic keratitis, including, desiccation of the corneal surface due to diminished lacrimal secretions, impaired corneal sensitivity leading to diminished protective blink reflexes, abnormal epithelial cell metabolism with subsequent failure to resist the effects of trauma, drying, and infection, and the loss of trophic influences supplied by corneal nerve fibres (Paton, 1926, De Hass, 1962, Duke-Elder and Leigh, 1965, Heigle
Corneal nerves release soluble trophic substances
The mechanisms by which corneal nerve fibres maintain a healthy cornea and promote wound healing after eye injuries is currently under active research in several laboratories. The results of in vitro co-culture studies suggest that neurons and corneal epithelial cells support one another trophically through the mutual release of soluble substances. For example, trigeminal neurons release diffusible factors (e.g. neurotransmitters and neuropeptides) that stimulate corneal epithelial cell growth,
Neuropeptides and neurotransmitters as stimulators of wound healing
Substance P. Several lines of evidence suggest that SP is important for regeneration and wound healing of the corneal epithelium. First, SP is present in physiologically relevant concentrations in the normal cornea (Table 1) and SP-specific (NK1) receptors are abundantly expressed on native and cultured corneal epithelial cells (Kieselbach et al., 1990, Denis et al., 1991, Nakamura et al., 1997c). SP is also constitutively expressed in normal tears (Table 1) and SP tear concentration decreases
In vivo confocal microscopy (IVCM) as a clinical tool for evaluation of corneal innervation
IVCM of corneal nerves in refractive surgery. PRK is followed by an immediate loss of nerves and nerve endings due to photoablation (Trabucchi et al., 1994), whereas after LASIK it takes a few days for the nerves to disappear from the flap (Linna et al., 2000, Lee et al., 2001, Tervo and Moilanen, 2003). Moreover, after LASIK some fibres located near the hinge remain intact (Linna et al., 2000, Lee et al., 2001). Recovery of the subbasal plexus can be observed as early as a few days after PRK (
Acknowledgements
The authors thank Anneke de Wolf and Ton Put from the Netherlands Ophthalmic Research Institute for their support with the drawings and photographs. Grants by The University of Helsinki, EVO grant by The State of Finland, and Sokeiden Ystävät (The Friends of the Blind) (to T.T.) are also gratefully acknowledged. Dr Chris Murphy, University of Wisconsin, generously provided the dog tissue illustrated in Fig. 10.
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