Review
Corneal nerves: structure, contents and function

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Abstract

This review provides a comprehensive analysis of the structure, neurochemical content, and functions of corneal nerves, with special emphasis on human corneal nerves. A revised interpretation of human corneal nerve architecture is presented based on recent observations obtained by in vivo confocal microscopy (IVCM), immunohistochemistry, and ultrastructural analyses of serial-sectioned human corneas. Current data on the neurotransmitter and neuropeptide contents of corneal nerves are discussed, as are the mechanisms by which corneal neurochemicals and associated neurotrophins modulate corneal physiology, homeostasis and wound healing. The results of recent clinical studies of topically applied neuropeptides and neurotrophins to treat neurotrophic keratitis are reviewed. Recommendations for using IVCM to evaluate corneal nerves in health and disease are presented.

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.

References (263)

  • G.J. Burbach et al.

    The neurosensory tachykinins substance P and neurokinin A directly induce keratinocyte nerve growth factor

    J. Invest. Dermatol.

    (2001)
  • L.C. Butterfield et al.

    Cyclic nucleotides and mitosis in the rabbit cornea following superior cervical ganglionectomy

    Exp. Eye Res.

    (1977)
  • O.A. Candia et al.

    Topical epinephrine causes a decrease in density of beta-adrenergic receptors and cathecholamine-stimulated chloride transport in the rabbit cornea

    Biochim. Biophys. Acta

    (1978)
  • K.Y. Chan et al.

    Isolation and culture of corneal cells and their interactions with dissociated trigeminal neurons

    Exp. Eye Res.

    (1982)
  • T. Chikama et al.

    Treatment of neurotrophic keratopathy with substance-P-derived peptide (FGLM) and insulin-like growth factor I

    Lancet

    (1998)
  • T. Chikama et al.

    Up-regulation of integrin alpha5 by a C-terminus four-amino-acid sequence of substance P (phenylalanine-glycine-leucine-methionine-amide) synergistically with insulin-like growth factor-1 in SV-40 transformed human corneal epithelial cells

    Biochem. Biophys. Res. Commun.

    (1999)
  • R.H. Edwards et al.

    Processing of the native nerve growth factor precursor to form biologically active nerve growth factor

    J. Biol. Chem.

    (1988)
  • A.A. Elbadri et al.

    The distribution of neuropeptides in the ocular tissues of several mammals: a comparative study

    Comp. Biochem. Physiol

    (1991)
  • I. Emoto et al.

    Stimulation of neurite growth by epitehlial implants into corneal stroma

    Neurosci. Lett.

    (1987)
  • C.D. Felipe et al.

    Quantification and immunocytochemical characteristics of trigeminal ganglion neurons projecting to the cornea: effect of corneal wounding

    Eur. J. Pain

    (1999)
  • J. Garcia-Hirschfeld et al.

    Neurotrophic influences on corneal epithelial cells

    Exp. Eye Res.

    (1994)
  • J.P. Gilbard et al.

    Tear film and ocular surface changes in a rabbit model of neurotrophic keratitis

    Ophthalmology

    (1990)
  • F. Giraldez et al.

    Response characteristics of corneal sensory fibers to mechanical and thermal stimulation

    Brain Res.

    (1979)
  • M.C. Gnadinger et al.

    Choline acetyltransferase in corneal epithilium

    Exp. Eye Res.

    (1973)
  • S.A. Grando et al.

    A nicotinic acetylcholine receptor regulating cell adhesion and motility is expressed in human keratinocytes

    J. Invest. Dermatol.

    (1995)
  • J.V. Jester et al.

    Corneal stromal wound healing in refractive surgery: the role of myofibroblasts

    Prog. Ret. Eye Res.

    (1999)
  • S. Jing et al.

    GDNF-induced activation of the ret protein tyrosine kinase is mediated by GDNFR-alpha, a novel receptor for GDNF

    Cell

    (1996)
  • S.M. Johnson

    Neurotrophic corneal defects after diode laser cycloablation

    Am. J. Ophthalmol.

    (1998)
  • M.A. Jones et al.

    Peptidergic innervation of the rat cornea

    Exp. Eye Res.

    (1998)
  • P. Keen et al.

    Substance P in the mouse cornea: effects of chemical and surgical denervation

    Neurosci. Lett.

    (1982)
  • G.F. Kieselbach et al.

    Autoradiographic analysis of binding sites for 125I-Bolton-Hunter-substance P in the human eye

    Peptides

    (1990)
  • G.G. Knyazev et al.

    Trophic functions of primary sensory neurons: are they really local?

    Neuroscience

    (1991)
  • R. Adler et al.

    Cholinergic neurotrophic factors: intraocular distribution of trophic activity for ciliary neurons

    Science

    (1979)
  • K.M. Albers et al.

    Overexpression of nerve growth factor in epidermis of transgenic mice causes hypertrophy of the peripheral nervous system

    J. Neurosci.

    (1994)
  • M.G. Alper

    The anesthetic eye: an investigation of changes in the anterior ocular segment of the monkey

    Invest. Dermatol.

    (1975)
  • Excimer laser photorefractive keratectomy (PRK) for myopia and astigmatism

    Ophthalmology

    (1999)
  • K. Araki et al.

    Epithelial wound healing in the denervated cornea

    Curr. Eye Res.

    (1994)
  • K. Araki-Sasaki et al.

    Substance P-induced caused by interrupting the trigeminal nerve at various levels along its course

    Trans. Am. Ophthalmol. Soc.

    (2000)
  • K.S. Baker et al.

    Trigeminal ganglion neurons affect corneal epithelial phenotype. Influence on type VII collagen expression in vitro

    Invest. Ophthalmol. Vis. Sci.

    (1993)
  • G. Barbin et al.

    Purification of the chick eye ciliary neuronotrophic factor

    J. Neurochem.

    (1984)
  • H.J. Beckers et al.

    Ultrastructural identification of trigeminal nerve endings in the rat cornea and iris

    Invest. Ophthalmol. Vis. Sci.

    (1992)
  • H.J. Beckers et al.

    Substance P in rat corneal and iridal nerves: an ultrastructural immunohistochemical study

    Ophthalmic. Res.

    (1993)
  • J.L. Bennett et al.

    Patterned expression of BDNF and NT-3 in the retina and anterior segment of the developing mammalian eye

    Invest. Ophthalmol. Vis. Sci.

    (1999)
  • M. Bibel et al.

    Neurotrophins: key regulators of cell fate and cell shape in the vertebrate nervous system

    Genes Dev.

    (2000)
  • T. Bourcier et al.

    Expression of neurotensin receptors in human corneal keratocytes

    Invest. Ophthalmol. Vis. Sci.

    (2002)
  • S.M. Brown et al.

    Neurotrophic and anhidrotic keratopathy treated with substance P and insulinlike growth factor 1

    Arch. Ophthalmol.

    (1997)
  • H. von Brücke et al.

    Azetylcholin und aneuringehat der hornhaut und seine beziehungen zur nervenversorgung

    Ophthalmolgica

    (1949)
  • S. Campbell et al.

    The effect of growth factors, neuropeptides and neurotransmitters on NGF content in SV-40 transformed human corneal epithelial cells

    Invest. Ophthalmol. Vis. Sci.

    (2001)
  • F. de Castro et al.

    Corneal innervation and sensitivity to noxious stimuli in trkA knockout mice

    Eur. J. Neurosci.

    (1998)
  • H.D. Cavanagh

    Herpetic ocular disease: therapy of persistent epithelial defects

    Int. Ophthalmol. Clin.

    (1975)
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