Cited by 853Corneal surgery and disordersCorneal Surgery and TreatmentsCollagen: Extraction and Characterization

The DOI matches its Crossref record; 167 of the 167 references it cites were found in a registry or an index; 55 of the 152 sources its claims cite were checked for support.

Corneal structure and transparency

Open access
Meek, Keith M., Knupp, Carlo·2015·Progress in Retinal and Eye Research

Abstract

The corneal stroma plays several pivotal roles within the eye. Optically, it is the main refracting lens and thus has to combine almost perfect transmission of visible light with precise shape, in order to focus incoming light. Furthermore, mechanically it has to be extremely tough to protect the inner contents of the eye. These functions are governed by its structure at all hierarchical levels. The basic principles of corneal structure and transparency have been known for some time, but in recent years X-ray scattering and other methods have revealed that the details of this structure are far more complex than previously thought and that the intricacy of the arrangement of the collagenous lamellae provides the shape and the mechanical properties of the tissue. At the molecular level, modern technologies and theoretical modelling have started to explain exactly how the collagen fibrils are arranged within the stromal lamellae and how proteoglycans maintain this ultrastructure. In this review we describe the current state of knowledge about the three-dimensional stromal architecture at the microscopic level, and about the control mechanisms at the nanoscopic level that lead to optical transparency.

Overview

AI snapshot

Written from the paper’s full text.

TL;DR

This review synthesizes current knowledge of corneal stromal architecture and transparency, emphasizing that transparency arises from collagen fibril uniformity and short-range order rather than lamellar arrangement alone. It integrates X-ray scattering, electron tomography, nonlinear microscopy, and theoretical modelling to explain how proteoglycans and type V collagen regulate fibril spacing and diameter.

Key findings

  • The stroma comprises nearly 90% of corneal thickness; the central human cornea has approximately 200 lamellae, with anterior lamellae highly interwoven and posterior lamellae less interlaced. Anterior lamellar insertions into Bowman’s layer average about 19°.
  • Transparency depends on narrow collagen fibrils with uniform diameters and restricted centre-to-centre spacing. Fibrils contain about 300–400 triple-helical molecules per cross-section and about 70 microfibrils, with axial periodicity reduced from 67 nm to closer to 65 nm.
  • Maurice calculated that independent scattering would leave only about 6% of incoming light to reach the retina; destructive interference from ordered fibrils cancels scattering in all directions except forward. If 20% of fibrils have twice the normal diameter, backward radiation arises and transparency is impaired.
  • Type V collagen limits fibril diameter, while proteoglycans maintain interfibrillar spacing. Across over 40 animal species, the volume fraction occupied by hydrated fibrils and interfibrillar GAGs was constant. Human corneal transmittance exceeds 90% between 500 and 1300 nm, reaches 80% at 380 nm, and drops at 1430 and 1950 nm due to water absorption; transmittance decreases outside the central 4 mm.

Methods

  • Narrative review of corneal stromal structure and transparency, synthesizing evidence from X-ray scattering, electron tomography, second harmonic generation and other nonlinear microscopy, spectrophotometry, and theoretical modelling.
  • Evidence base includes human, bovine, rabbit, chicken embryo, mutant mouse, and comparative data from over 40 animal species, plus unpublished results from the authors’ group.
  • Main analysis is qualitative synthesis of structural and optical studies and theoretical models; no systematic review or meta-analysis is described.

Limitations

  • Stated in the paper: X-ray scattering cannot directly image lamellae and must infer arrangement from diffraction patterns; it provides bulk averages and cannot resolve adjacent lamellae. Multiple scattering is not fully modelled, and photonic band structure theory has been applied only to a perfect hexagonal lattice. The existence of Dua’s layer/pre-Descemet’s layer remains controversial, and some cited findings are unpublished.
  • Evident from the text: as a narrative review, it does not report a systematic search strategy or quantitative synthesis, and it relies substantially on animal models and unpublished or preliminary data rather than new primary experimental data.

Model-written summary. Check it against the paper before relying on it.

Integrity signals

Registry

Crossref · 10.1016/j.preteyeres.2015.07.001

Cites

167 of 167 references verified0 citation errors · 0 not checkable

Statistics

Nothing to recompute.

Figures

Fig. 1 Simplified model of the principal lamellar orientations in the human cornea. (a) and (c) are based on X-ray data; (b) and (d) are based on second harmonic generated (SHG) microscopy. (a) and (b) are en face views from the front of the cornea. The X-ray data (a) show the model (black lines) superimposed on the distribution of preferentially aligned collagen lamellae and indicates preferred lamellar orientations in the inferior-superior and nasal-temporal directions, predominantly in the posterior stroma. Many of these seem to become part of a circum-corneal annulus at the limbal region. In addition, anchoring lamellae with larger collagen fibrils are thought to enter the stroma in alignment with the extraocular musculature. The lines in the model show the predominant orientation of lamellae, not the actual course of individual lamellae. SHG microscopy from the posterior stroma (b) reveals an interwoven lattice of lamellae, such that adjacent lamellae are not orthogonal; thus no preferred orientation of collagen is obvious in the central cornea using this technique. (c) and (d) are views through the thickness of the stroma. In (c) the X-ray data are shown on the right. The corresponding model on the left suggests that lamellae are more interwoven anteriorly, particularly in the central 4 mm zone. Outside this zone, interweaving is more prominent in deeper stromal layers also. At the limbus (indicated by broken red line), the circumcorneal annulus appears in cross-section (represented by black circles) and anchoring lamellae appear to enter the stroma from the deeper layers and gradually move towards the surface. SHG microscopy (d) also demonstrates the highly interwoven anterior lamellae, which are often referred to as fibres using this imaging modality. A 3-D reconstruction shows bow spring fibres (blue), anchoring fibres inserting from the limbus (green), and the highly intertwined anterior fibre meshwork (teal) near Bowman's layer (gold). (d) is reproduced from Winkler et al. (2011) with permission of the copyright holder.
Fig. 1 Simplified model of the principal lamellar orientations in the human cornea. (a) and (c) are based on X-ray data; (b) and (d) are based on second harmonic generated (SHG) microscopy. (a) and (b) are en face views from the front of the cornea. The X-ray data (a) show the model (black lines) superimposed on the distribution of preferentially aligned collagen lamellae and indicates preferred lamellar orientations in the inferior-superior and nasal-temporal directions, predominantly in the posterior stroma. Many of these seem to become part of a circum-corneal annulus at the limbal region. In addition, anchoring lamellae with larger collagen fibrils are thought to enter the stroma in alignment with the extraocular musculature. The lines in the model show the predominant orientation of lamellae, not the actual course of individual lamellae. SHG microscopy from the posterior stroma (b) reveals an interwoven lattice of lamellae, such that adjacent lamellae are not orthogonal; thus no preferred orientation of collagen is obvious in the central cornea using this technique. (c) and (d) are views through the thickness of the stroma. In (c) the X-ray data are shown on the right. The corresponding model on the left suggests that lamellae are more interwoven anteriorly, particularly in the central 4 mm zone. Outside this zone, interweaving is more prominent in deeper stromal layers also. At the limbus (indicated by broken red line), the circumcorneal annulus appears in cross-section (represented by black circles) and anchoring lamellae appear to enter the stroma from the deeper layers and gradually move towards the surface. SHG microscopy (d) also demonstrates the highly interwoven anterior lamellae, which are often referred to as fibres using this imaging modality. A 3-D reconstruction shows bow spring fibres (blue), anchoring fibres inserting from the limbus (green), and the highly intertwined anterior fibre meshwork (teal) near Bowman's layer (gold). (d) is reproduced from Winkler et al. (2011) with permission of the copyright holder.CC BY · PubMed Central
Fig. 2 Structural hierarchy in corneal collagen (not to scale). Three helical alpha chains are supercoiled to produce the collagen triple helix molecule (top right). These molecules self-assemble in a staggered axial array (bottom right) to form microfibrils consisting of five molecules which in turn coil together to form the 30 nm diameter collagen fibrils seen in the electron microscope. The micrograph bottom left is reproduced from Ottani et al. (2002), with permission of the copyright holder, and shows the coiled microfibrils within the collagen fibril; the micrograph bottom middle is reproduced from Baldock et al. (2002), with permission of the copyright holder, and shows the microfibrils in cross-section within the collagen fibril.
Fig. 2 Structural hierarchy in corneal collagen (not to scale). Three helical alpha chains are supercoiled to produce the collagen triple helix molecule (top right). These molecules self-assemble in a staggered axial array (bottom right) to form microfibrils consisting of five molecules which in turn coil together to form the 30 nm diameter collagen fibrils seen in the electron microscope. The micrograph bottom left is reproduced from Ottani et al. (2002), with permission of the copyright holder, and shows the coiled microfibrils within the collagen fibril; the micrograph bottom middle is reproduced from Baldock et al. (2002), with permission of the copyright holder, and shows the microfibrils in cross-section within the collagen fibril.CC BY · PubMed Central
Fig. 3 Transmittance through the human cornea as a function of wavelength. Reproduced in part from Boettner and Wolter (1962) with permission of the copyright holder.
Fig. 3 Transmittance through the human cornea as a function of wavelength. Reproduced in part from Boettner and Wolter (1962) with permission of the copyright holder.CC BY · PubMed Central
Fig. 4 Wave interference of two one-dimensional sinusoidal waves. The two interfering waves are at the top of each panel and the resulting wave, which is a point by point sum of the interfering waves, at the bottom. In (a) the interfering waves are in phase. In (b) out of phase and in (c) they are shifted with respect to each other.
Fig. 4 Wave interference of two one-dimensional sinusoidal waves. The two interfering waves are at the top of each panel and the resulting wave, which is a point by point sum of the interfering waves, at the bottom. In (a) the interfering waves are in phase. In (b) out of phase and in (c) they are shifted with respect to each other.CC BY · PubMed Central
Fig. 5 Secondary waves from a single (a) and pair (b) of collagen fibrils. The right hand side insets are a magnified view of the fibril arrangement contained in the rectangle in the main panels. The bar, 1 μm, is in common to (a) and (b).
Fig. 5 Secondary waves from a single (a) and pair (b) of collagen fibrils. The right hand side insets are a magnified view of the fibril arrangement contained in the rectangle in the main panels. The bar, 1 μm, is in common to (a) and (b).CC BY · PubMed Central
Fig. 6 Secondary waves from a collagen fibril distribution presenting short-range order. Primary incoming light is travelling from left to right (yellow arrow), with a wavelength of 500 nm. Collagen fibrils in transverse sections are represented by brown circles. All fibrils have the same diameter of about 31 nm, and no collagen fibrils can be closer than 62 nm. Only the intensity of the secondary radiation arising from the fibrils is shown in blue. No backwards secondary radiation can be seen in the figure. Bar 500 nm.
Fig. 6 Secondary waves from a collagen fibril distribution presenting short-range order. Primary incoming light is travelling from left to right (yellow arrow), with a wavelength of 500 nm. Collagen fibrils in transverse sections are represented by brown circles. All fibrils have the same diameter of about 31 nm, and no collagen fibrils can be closer than 62 nm. Only the intensity of the secondary radiation arising from the fibrils is shown in blue. No backwards secondary radiation can be seen in the figure. Bar 500 nm.CC BY · PubMed Central
Fig. 7 The effect of increased fibril diameters on light transmission. Secondary waves from the same collagen fibril distribution shown in Fig. 6. As before, primary incoming light is travelling from left to right (yellow arrow), with a wavelength of 500 nm. Collagen fibrils in transverse sections are represented by brown circles. 20% of the fibrils were selected at random and their diameter was doubled to 62 nm. The intensity of the secondary radiation arising from the fibrils is shown in blue. Backwards secondary radiation is evident in the figure (white arrows). Bar 500 nm.
Fig. 7 The effect of increased fibril diameters on light transmission. Secondary waves from the same collagen fibril distribution shown in Fig. 6. As before, primary incoming light is travelling from left to right (yellow arrow), with a wavelength of 500 nm. Collagen fibrils in transverse sections are represented by brown circles. 20% of the fibrils were selected at random and their diameter was doubled to 62 nm. The intensity of the secondary radiation arising from the fibrils is shown in blue. Backwards secondary radiation is evident in the figure (white arrows). Bar 500 nm.CC BY · PubMed Central
Fig. 8 The effect of fibril voids on light transmission. Secondary waves from the same collagen fibril distribution shown in Fig. 6. As before, primary incoming light is travelling from left to right (yellow arrow), with a wavelength of 500 nm. Collagen fibrils in transverse sections are represented by brown circles. Regions devoid of fibrils are now present (lakes). The intensity of the secondary radiation arising from the fibrils is shown in blue. Even in this case, backwards secondary radiation is evident in the figure (white arrow). Bar 500 nm.
Fig. 8 The effect of fibril voids on light transmission. Secondary waves from the same collagen fibril distribution shown in Fig. 6. As before, primary incoming light is travelling from left to right (yellow arrow), with a wavelength of 500 nm. Collagen fibrils in transverse sections are represented by brown circles. Regions devoid of fibrils are now present (lakes). The intensity of the secondary radiation arising from the fibrils is shown in blue. Even in this case, backwards secondary radiation is evident in the figure (white arrow). Bar 500 nm.CC BY · PubMed Central
Fig. 9 Disordered Fibonacci quasi-crystal arrangement of fibrils. This type of quasi-crystal is deterministic over a long range. The radial distribution function (inset) plots the number of fibrils per unit area (normalized by dividing by the fibril number density in the image) separated by the distance shown on the horizontal axis. This is very similar to that seen from electron micrographs from the cornea (cf. Fig. 12b inset). Modified from Doutch (2009) with permission of the copyright holder.
Fig. 9 Disordered Fibonacci quasi-crystal arrangement of fibrils. This type of quasi-crystal is deterministic over a long range. The radial distribution function (inset) plots the number of fibrils per unit area (normalized by dividing by the fibril number density in the image) separated by the distance shown on the horizontal axis. This is very similar to that seen from electron micrographs from the cornea (cf. Fig. 12b inset). Modified from Doutch (2009) with permission of the copyright holder.CC BY · PubMed Central
Fig. 10 Three-dimensional corneal stroma reconstruction.(a) Longitudinal section. (b) Transverse section. In both panels the collagen fibrils are depicted in orange and proteoglycans in yellow. Bars 100 nm. Modified from Lewis et al. (2010) with permission of the copyright holder.
Fig. 10 Three-dimensional corneal stroma reconstruction.(a) Longitudinal section. (b) Transverse section. In both panels the collagen fibrils are depicted in orange and proteoglycans in yellow. Bars 100 nm. Modified from Lewis et al. (2010) with permission of the copyright holder.CC BY · PubMed Central
Fig. 11 The electrostatic restoring force mechanism which acts on fibrils to maintain the lattice organization. The mechanism is illustrated for the case where all fibrils in the lattice are held fixed in their lattice positions and a single fibril is displaced relative to its lattice position. (a) Shows a set of fibrils in a regular lattice arrangement with their associated GAG chains and a distribution of mobile ions. In this situation, which corresponds to normal physiological conditions, the osmotic pressure is essentially uniform. (b) Depicts the osmotic pressure in each subcell around the undisturbed fibril. The osmotic pressure in each subcell exerts a force on the fibril but because the osmotic pressure is uniform, all six forces balance and the net force is zero, as indicated by the red dot. (c) Shows a fibril displaced from its lattice position. The GAG chains attached to the fibril move with the fibril and the GAG fixed charge density and mobile ion concentration increase in advance of the fibril displacement and reduce behind it. (d) Indicates the resulting osmotic pressure in each of the subcells, which is higher where GAG fixed charge density has increased and lower where it has reduced. The forces exerted by each of the subcells now has changed magnitude with the result that a net force acts on the fibril with a direction oriented towards the original lattice position. This restoring force is shown as a red arrow. Figure and caption reproduced from Cheng and Pinsky (2013) with permission of the copyright holder.
Fig. 11 The electrostatic restoring force mechanism which acts on fibrils to maintain the lattice organization. The mechanism is illustrated for the case where all fibrils in the lattice are held fixed in their lattice positions and a single fibril is displaced relative to its lattice position. (a) Shows a set of fibrils in a regular lattice arrangement with their associated GAG chains and a distribution of mobile ions. In this situation, which corresponds to normal physiological conditions, the osmotic pressure is essentially uniform. (b) Depicts the osmotic pressure in each subcell around the undisturbed fibril. The osmotic pressure in each subcell exerts a force on the fibril but because the osmotic pressure is uniform, all six forces balance and the net force is zero, as indicated by the red dot. (c) Shows a fibril displaced from its lattice position. The GAG chains attached to the fibril move with the fibril and the GAG fixed charge density and mobile ion concentration increase in advance of the fibril displacement and reduce behind it. (d) Indicates the resulting osmotic pressure in each of the subcells, which is higher where GAG fixed charge density has increased and lower where it has reduced. The forces exerted by each of the subcells now has changed magnitude with the result that a net force acts on the fibril with a direction oriented towards the original lattice position. This restoring force is shown as a red arrow. Figure and caption reproduced from Cheng and Pinsky (2013) with permission of the copyright holder.CC BY · PubMed Central
Fig. 12 Collagen fibril positions and corresponding radial distribution function (inset) from (a) a computer simulation and (b) a transmission electron micrograph of a rabbit cornea. The radial distribution function plots the number of fibrils per unit area (normalized by dividing by the fibril number density in the image) separated by the distance shown on the horizontal axis.
Fig. 12 Collagen fibril positions and corresponding radial distribution function (inset) from (a) a computer simulation and (b) a transmission electron micrograph of a rabbit cornea. The radial distribution function plots the number of fibrils per unit area (normalized by dividing by the fibril number density in the image) separated by the distance shown on the horizontal axis.CC BY · PubMed Central
Fig. 13 Calculated light transmission in the rabbit cornea as a function of wavelength for the collagen fibril distributions represented in Fig. 12a (grey curve) and Fig. 12b (black curve). Both curves were calculated using the direct summation of fields method (Freund et al., 1986), using other parameters taken from rabbit cornea (corneal thickness 360 microns, refractive index of collagen 1.355, refractive index of ground substance 1.420, collagen radius 19.4 nm, fibril density 220 fibrils per square micron).
Fig. 13 Calculated light transmission in the rabbit cornea as a function of wavelength for the collagen fibril distributions represented in Fig. 12a (grey curve) and Fig. 12b (black curve). Both curves were calculated using the direct summation of fields method (Freund et al., 1986), using other parameters taken from rabbit cornea (corneal thickness 360 microns, refractive index of collagen 1.355, refractive index of ground substance 1.420, collagen radius 19.4 nm, fibril density 220 fibrils per square micron).CC BY · PubMed Central

Verification

Claims the paper makes

50 No full text of the sources · 29 Own interpretation · 26 No basis given · 20 Partly supported · 16 Supported · 10 Sources not found · 8 Supported so far · 8 Own result · 3 Couldn’t confirm · 2 Not checked yet · 2 Own method · 1 Mixed support · accuracy not yet measured

Show all 95 claims

  • Probably not a substantial claim. 26
  • It may state two separate claims as one. 65
  • The quote only partly states this claim. 2
80 of 80
  • The authors state this as an assumption.

    The secondary electromagnetic radiation scattered by any collagen fibril does not interact with other fibrils of the cornea but travels undisturbed to the retina.

    “The explanation for cornea transparency presented above is based on the approximation that the secondary electromagnetic radiation scattered by any collagen fibril will not interact with other fibrils of the cornea, but will travel undisturbed to the retina.”
  • The paper relies on this without support.

    If the cornea swells much above its physiological hydration, it begins to scatter significant quantities of light and lose transparency.

    “If the cornea swells much above its physiological hydration it begins to scatter significant quantities of light and to lose its transparency.”
  • The paper relies on this without support.

    Keratocytes themselves are transparent except for their nuclei.

    “Keratocytes themselves are transparent except for their nuclei.”
  • The paper relies on this without support.

    Structural data from real corneas can now be collected using modern techniques such as three-dimensional block face scanning electron microscopy.

    “Structural data from real corneas can now be collected using modern techniques such as three-dimensional block face scanning electron microscopy”
  • The paper relies on this without support.

    Modern computers are fast enough for transparency predictions to be obtained by solving Maxwell's equations for the collected data.

    “modern computers are fast enough for transparency predictions to be obtained by solving Maxwell's equations for the collected data”
5 of 5

Claims other papers make about this one

Each is checked against this paper’s own text: does it say what the citing paper says it does?

Citations

Cites

Show all 166 references

Research works printed but not linked: 12

  1. Dohlman, 1971, The function of the corneal epithelium in health and disease, Invest. Ophthalmol., 10, 383
  2. Feynman, 1963, vol. 1
  3. Harding, 1980, Corneal collagen: a review, Colloq. Int. Cent. Rech. Sci. Natl., 287, 51
  4. Kolmer, 1936, Handb. Der mikr, Anat. Des. Menschen (v. Möllend.), III, 2
  5. Maurice, 2001, Graft transparency is in the eye of the beholder, Invest. Ophthalmol. Vis. Sci., S282
  6. Meyer, 1948, Mucopolysaccharides and mucoids of ocular tissues, vol. 2, 71
  7. Ranvier, 1898, Influence histogénétique d’une forme antérieure, à propos de la regeneration de la membrane de Descemet, C. R. Acad. Sci. (Paris), 126, 23
  8. Rispoli, 1978, Apparecchiatura a fibre ottiche per la misura oggettiva della variazione di trasparenza corneale, Boll. Ocul., 55, 63
  9. Virchow, 1905, vol. 1, 1
  10. Von Fleischl, 1880, vol. 3, 47
  11. Weiner, 1912, Allgemeine Sätze űber die Dielektrizitätskonstanten der Mischkörper, Abh. Sachs. Ges. Akad. Wiss. Math. Phys. Kl., 6, 574
  12. White, 2014, 3D structural studies of the cornea, Ophthalmic Physiol. Opt., 34, 693

Cited by

Sorted by citations

Items 401–500 of 853

Metadata

Authors
Meek, K. M., & Knupp, C.
Affiliations
Cardiff University (GB)
Corresponding author
Meek, Keith M.
Venue
Progress in Retinal and Eye Research
Publisher
Elsevier BV
Published
2015-11
Volume
49
Pages
1-16
ISSN
1350-9462 (ISSN-L)
OpenAlex
W1954007937
Cited by
853
References
166
Citations by year
2026: 104 · 2025: 109 · 2024: 101 · 2023: 94 · 2022: 99 · 2021: 88 · 2020: 87 · 2019: 75 · 2018: 45 · 2017: 32 · 2016: 17 · 2015: 1
FWCI
5.05
Type
journal article
Language
en
Open access
Locations
  • Progress in Retinal and Eye Research (publishedVersion, cc-by)
  • PubMed Central (publishedVersion, cc-by)
  • PubMed (publishedVersion)
  • ORCA Online Research @Cardiff (Cardiff University) (acceptedVersion)
Funders
  • MRC Programme (503626)
  • Diamond Light Source
  • ESRF
Topics
  • Health Sciences › Medicine › Ophthalmology › Corneal surgery and disorders
  • Health Sciences › Medicine › Ophthalmology › Corneal Surgery and Treatments
  • Physical Sciences › Materials Science › Biomaterials › Collagen: Extraction and Characterization
Keywords
corneal stroma, corneal transparency, collagen fibrils, collagen lamellae, proteoglycans, corneal ultrastructure, optical transmittance, corneal refractive index
MeSH terms
Collagen, Cornea, Corneal Stroma, Crystallins, Humans, Microscopy, Electron, Proteoglycans, Extracellular Matrix Proteins

About these checks

Registry, retraction, Cites and statistics results are computed, not judged, with one exception: a printed reference that no index lists is read once by a model before it counts as a citation error. Claims are extracted by a model and kept only when their quote appears verbatim in the text; each cited source is then checked, with a model’s help, for whether it says what the claim relies on. Those verdicts can be wrong — read the claim’s quote and the cited source before relying on one.

A language model drew the claims from the paper on 2026-11-02, under version 3 of the claim definition (model deepseek-v4.1-flash, prompt extract-v3).

A second model labelled each claim on 2026-11-02 (model jev-1.13-20260917, definition revision gold-guideline-v0.2).

Cited sources are checked with a language model’s help (gpt-6-sol).

Statistics and arithmetic are recomputed, not judged.

Confidence bands are shown only for fields calibrated on our annotated test set. None is calibrated yet, so no band is shown.

What counts as a claim ›How we measure confidence ›How we check this ›

Are you an author, or do you see a problem with this page? Request a review