Structural control of corneal transparency, refractive power and dynamics

Abstract

The cornea needs to be transparent to visible light and precisely curved to provide the correct refractive power. Both properties are governed by its structure. Corneal transparency arises from constructive interference of visible light due to the relatively ordered arrangement of collagen fibrils in the corneal stroma. The arrangement is controlled by the negatively charged proteoglycans surrounding the fibrils. Small changes in fibril organisation can be tolerated but larger changes cause light scattering. Corneal keratocytes do not scatter light because their refractive index matches that of the surrounding matrix. When activated, however, they become fibroblasts that have a lower refractive index. Modelling shows that this change in refractive index significantly increases light scatter. At the microscopic level, the corneal stroma has a lamellar structure, the parallel collagen fibrils within each lamella making a large angle with those of adjacent lamellae. X-ray scattering has shown that the lamellae have preferred orientations in the human cornea: inferior-superior and nasal-temporal in the central cornea and circumferential at the limbus. The directions at the centre of the cornea may help withstand the pull of the extraocular muscles whereas the pseudo-circular arrangement at the limbus supports the change in curvature between the cornea and sclera. Elastic fibres are also present; in the limbus they contain fibrillin microfibrils surrounding an elastin core, whereas at the centre of the cornea, they exist as thin bundles of fibrillin-rich microfibrils. We present a model based on the structure described above that may explain how the cornea withstands repeated pressure changes due to the ocular pulse.

Fig. 1. Arrangement of collagen and proteoglycans in the human cornea.

A Transmission electron micrograph of corneal collagen fibrils in cross-section stained with phosphotungstic acid and uranyl acetate. B Longitudinal section of corneal collagen fibrils counterstained with cuprolinic blue to visualise proteoglycans. C The collagen and proteoglycan organisation in the corneal stroma. Fibrils contain collagen molecules staggered axially to produce a periodic banding along the fibril axis. The protein cores of the proteoglycans (green) are associated with the fibrils at specific binding sites. Attached to the protein cores are either keratan sulphate glycosaminoglycans (orange) or dermatan/chondroitin sulphate glycosaminoglycans (red). A, B supplied by Dr Rob Young and Prof Andrew Quantock (Cardiff University) and (C) reproduced from Meek and Hayes [39].

Fig. 2. Light scattering from corneal collagen fibrils.

A (i) Each fibril in a regular array of collagen fibrils (green) will scatter parallel wavefronts incident from the left in all directions (only the forward and backward directions are shown in ii and iii). (ii) The secondary wavefronts (red and grey circles) interfere constructively in the forward direction and destructively in other directions. (iii) These wavefronts are shown superimposed on the full intensity field. (iv) The intensity field of forward and backscattered radiation. B (i) A disordered array of fibrils. (ii) The secondary wavefronts from this array, (iii) The secondary wavefronts superimposed on the full intensity field, showing significant constructive interference in the backward direction in addition to the forward direction. (iv) The intensity field of forward and backscattered radiation from the disordered array of collagen. Animated versions of this figure are presented in Supplementary Videos 3 and 4.

Fig. 3. Light scattering from cells in the corneal stroma.

A Predicted transmission (based on modelling of Mie scatter) from the anterior cornea uniformly populated with keratocytes (solid line) or fibroblasts (broken line) [9]. B Anterior corneal haze following photorefractive keratectomy (image reproduced with permission from https://crstodayeurope.com/articles/july-august-2021/corneal-haze-after-prk-enhancement-of-prior-lasik/).

Fig. 4. Organisation of corneal lamellae.

As observed by electron microscopy the arrangement of posterior stromal lamellae may be likened to the structure of plywood (inset) where the grain runs in different directions in adjacent layers.

Fig. 5. Predominant collagen orientations in the cornea and sclera.

X-ray scatter data shows the preferred directions of collagen lamellae in different parts of the cornea and sclera (courtesy of Dr. Craig Boote, Cardiff University). Superimposed is our simplified model of the proposed overall preferential directions of lamellae, based on the x-ray data [13]. These preferential directions are believed to help support the curvature of the cornea and maintain its shape under the pull of the extraocular muscles (inset).

Fig. 6. Anterior lamellar interweaving.

Second harmonic generated composite image showing lamellar interweaving in the anterior stroma above the dashed line [40]. The stromal interweaving supports corneal curvature similar to interweaving of fibres in a bird’s nest (inset image reproduced with permission by Neerav Bhatt via Flickr) (www.flickr.com/photos/neeravbhatt/https://).

Fig. 7. Possible response of the cornea to fluctuations of IOP.

A, B Model showing how the cornea is displaced forward when the IOP is increased. The shape of the cornea remains the same and the displacement occurs in the limbal region where the annular meshwork of collagen lamellae opens. This displacement would necessitate stretching of any interconnecting lamellae. C We propose that this stretching is facilitated by the uncrimping of the crimped collagen in the limbus (image reproduced with permission from [27]). An animated version of this model is shown in Supplementary video 5.

Fig. 8. Transmission electron micrograph of an elastic fibre [29].

The elastic fibre in transverse section shows an amorphous elastin core surrounded by fibrillin-rich microfibril bundles. Scale bar: 200 nm.

Fig. 9. SBF SEM rendered images showing the three-dimensional arrangement of elastic fibres in different regions of the human cornea.

A The limbus (limbal elastic sheets/fibres are rendered in gold and the trabecular elastic fibres are rendered in green) showing reticulated elastic fibre sheets. B The peripheral cornea (elastic fibres are rendered in gold and Descemet’s membrane is coloured blue) showing elastic fibres dividing as they progress towards the centre of the cornea. C Narrow fibrillin-rich microfibril bundles in the centre of the human cornea (microfibril bundles rendered in gold, Descemet’s membrane coloured blue).

Fig. 10. The proposed elastic fibre network in the cornea.

Elastic fibre sheets are associated with the pseudo-circumferential limbal elastic fibres. From these, ‘true’ elastic fibres traverse the peripheral cornea. These split into narrower bundles of fibrillin-rich microfibrils as they approach the centre of the cornea, where most have lost their elastin core [36].

Fig. 11. Model to explain how the elastic sheets and elastin-rich fibres in the limbal and peripheral stroma may provide the force required to restore the cornea to its normal position during the ocular pulse.

The elastic fibres are proposed to act like springs (A) with a function analogous to the springs in a trampoline (B).