Corneal Repair and Regeneration: Current Concepts and Future Directions

Abstract

The cornea is a unique tissue and the most powerful focusing element of the eye, known as a window to the eye. Infectious or non-infectious diseases might cause severe visual impairments that need medical intervention to restore patients' vision. The most prominent characteristics of the cornea are its mechanical strength and transparency, which are indeed the most important criteria considerations when reconstructing the injured cornea. Corneal strength comes from about 200 collagen lamellae which criss-cross the cornea in different directions and comprise nearly 90% of the thickness of the cornea. Regarding corneal transparency, the specific characteristics of the cornea include its immune and angiogenic privilege besides its limbus zone. On the other hand, angiogenic privilege involves several active cascades in which anti-angiogenic factors are produced to compensate for the enhanced production of proangiogenic factors after wound healing. Limbus of the cornea forms a border between the corneal and conjunctival epithelium, and its limbal stem cells (LSCs) are essential in maintenance and repair of the adult cornea through its support of corneal epithelial tissue repair and regeneration. As a result, the main factors which threaten the corneal clarity are inflammatory reactions, neovascularization, and limbal deficiency. In fact, the influx of inflammatory cells causes scar formation and destruction of the limbus zone. Current studies about wound healing treatment focus on corneal characteristics such as the immune response, angiogenesis, and cell signaling. In this review, studied topics related to wound healing and new approaches in cornea regeneration, which are mostly related to the criteria mentioned above, will be discussed.

Keywords: angiogenesis; biomaterials; cornea; immune privilege; limbus; regenerative medicine; tissue engineering; wound healing.

Figure 1

Current strategies for the repair and regeneration of damaged cornea.

Figure 2

The hierarchical structure of the cornea showing that it is basically composed of three composite regions. A fourth composite region, Descemet's membrane, is included for completion. The macroscopic, microscopic, and nanoscopic features are emphasized (from left to right) to help illustrate the various interactions between the tissue components. Bowman's layer is essentially a random fibril, woven-mat composite, which maximizes multi-axial stiffness and strength. The underlying anterior third of the stroma proper is a lamellar interwoven fabric composed of unidirectionally (UD) fibril-reinforced lamellae. This architectural hierarchy is more rigid against z-axis deformations compared to non-woven UD-laminates. In the human body, the corneal structure is most similar to that of the pericardium, which serves to mechanically prevent the formation of aneurysms in the heart. The posterior two-thirds of the stroma is essentially a non-woven, UD-fibril-reinforced lamellar composite, which maximizes longitudinal x- and y-axis stiffness and strength, but has only weak transverse z-axis stiffness and strength. In the human body, its structure is most similar to that of the annulus fibrosis of the intervertebral disk, which functions efficiently as a cushioning mechanism for the spine, but is prone to chronic biomechanical failure. The UD-orientation of collagen fibrils in each lamella is vital because this arrangement prevents fibril undulation and thus maximizes the initial axial tensile strength of each fibril. Descemet's membrane forms a hexagonal lattice. Taken together these composite-like regions are responsible for the overall stiffness, strength, extensibility, and toughness of the cornea. They also help explain how the cornea behaves biomechanically after surgery, disease, or injury. Reprinted with permission (Kaufman et al., 2011).

Figure 3

(A) Boston keratoprosthesis Type I with titanium back plate. Reprinted with permission from Dohlman et al. (2014). (B) Osteodental-acrylic complex with polymethyl methacrylate optical cylinder. (C) Both core and skirt are shown in this picture. The skirt is white due to collagen incorporated in the pores. The clear ring between the center and the skirt shows interdigitation between the two components. (D) Front profile of the KeraKlear keratoprosthesis demonstrating the 18-hole peripheral design with 4.0 mm central optic. Reprinted with permission from Cortina and De La Cruz (2015). (E) MICOF KPro is composed of two parts: a titanium frame and a central PMMA cylinder. Reprinted with permission from Huang et al. (2012). (F) MiroCornea UR keratoprosthesis. Reprinted with permission from Duncker et al. (2014).

Figure 4

Immune and angiogenic privilege besides limbus structure play a pivotal role in corneal transparency. While inflammatory reaction, neovascularization and limbus deficiency endanger corneal transparency. Reprinted with permission from Ellenberg et al. (2010) and Haagdorens et al. (2016).

Figure 5

Effect of PEDF+DHA treatment on wound healing in diabetic corneas. The right eyes of 16 mice with hyperglycemia for 10 weeks were injured and divided randomly into two groups and treated for 1 or 2 days with PEDF+DHA or vehicle. (A) The wounded corneas were stained with 0.5% methylene blue and photographed with a surgical microscopy through an attached digital camera. (B) Wounded area. Data is expressed as mean ± SD (^*p < 0.05, n = 4 mice/group). Reprinted with permission from He et al. (2017b).

Figure 6

Mechanical injury was induced in murine corneas by scraping the epithelium and either topical recombinant hepatocyte growth factor (HGF) was applied twice daily. Normal corneas without injury or injured corneas receiving MSA served as controls. Corneas were harvested at day 3 after injury. (A) Representative immunofluorescent images of sections of mouse cornea stained for Ki-67 (green) showing proliferating cells (scale bar, 100 mm). (B) Representative immunofluorescence images of corneal cross sections showing higher expression of CD45 (green) in MSA-treated controls, compared to HGF-treated eyes (scale bar, 50 mm). Reprinted with permission from Omoto et al. (2017).

Figure 7

The evolution pathway of ocular surface reconstruction investigations which started with autologous conjunctival transplantation in a patient with bilateral alkali burn in 1977 and have been continued with other methods specially limbus regeneration over the past four decades. Partially reprinted with permission from Nakamura et al. (2016).

Figure 8

The characteristics of ultra-thin amniotic membrane. (A) The thickness change of amniotic membrane after digestion with collagenase type IV for different time durations. (B) HandE and Masson trichrome staining of IAM, DAM, and UAM tissues (Bar: 100 μ m). (C) Macroscopic views of IAM, DAM, and UAM were evaluated by photography scanning in moist form and light microscope in freeze dry form. (D) Hoechst whole mount staining of IAM, DAM, and UAM (Bar: 100 μ m). Reprinted with permission from Zakaria et al. (2014).

Figure 9

(A) Detail of electrospun outer ring with 1.2 cm of diameter, (A′) SEM micrograph of a section of the electrospun scaffold showing a horseshoe electrospun micropocket. (B) Stability and transparency of cultured RAFT (Ortega et al., 2013b). (B′) SEM image of bioengineered limbal crypts on the RAFT surface. Scale bar 200 μm. Reprinted with permission from Levis and Daniels (2016).