Epithelial layers of numerous organs are exposed to injuries. However, they also contain the ability to compensate for tissue loss through wound healing. The cornea offers an excellent model to study wound healing. It forms the external surface of the eye and provides a protective layer for the sensitive ocular machinery. Thus, cornea functions as a physical barrier to pathogens and water loss. It is composed of three layers; epithelium, stroma and endothelium. The epithelium of the cornea makes up the outermost layer of the cornea. Epithelial cells maintain the barrier function of the cornea by adhering strictly to each other through tight junctions1,2,3. An acellular corneal basement membrane, the Bowman's membrane, separates the epithelium from the extensive stroma, which contains refractory keratocytes. Under the stroma, endothelial cells channel nutrients, water, and oxygen to the upper layer.
Corneal abrasions are very common in the clinic4. Injuries to the cornea are diverse, but are largely caused by small particles such as dust or sand, scratches, or other foreign objects. The protocol described here aims at reproducing a clinically relevant type of corneal epithelial abrasion. In doing so, this protocol provides a controllable and seminal method for clinicians and corneal scientists to implement in their own studies. We have performed an in vivo injury repair assay on the murine cornea by abrading the tissue with a dulled ocular burr, the Algerbrush II. Here, we target the abrasion only to the central corneal epithelium and leave the other parts of the organ without damage. Thus, the protocol is ideal to study corneal epithelial cell dynamics or the basement membrane during re-epithelialization, cell migration, proliferation and differentiation in vivo5. Recently, this model was used to analyze progenitor cell dynamics in the murine cornea as well as to unveil the capacity of the differentiated corneal epithelial cells in re-establishing the corneal stem cell niche after injury6,7. Following abrasion, the cornea returns to its normal transparency and tensile strength. Interestingly, an in vitro study indicated that re-epithelialization occurs without increased cell proliferation8. This protocol describes the timeline of uninterrupted healing in the murine cornea. The method is thus applicable to test the effect of drugs on healing patterns and speed.
The cornea has been extensively used for wound healing studies. However, many studies have relied on other models of injury. A well-established model of corneal injury is the alkaline burn that is performed by applying sodium hydroxide (NaOH) with or without filter paper on the corneal surface9. Alkaline exposure results in a large and diffuse injury that affects not only the corneal epithelium, but also the conjunctiva and stroma9,10. Strong alkaline solutions have been shown to induce corneal ulcers, opacification, and neovascularization9. Inflammatory cells invade the stroma typically within 6 h and remain there until 24 h11. Thus, alkaline injury is an advisable method in studies related to stromal activation. Another type of chemical injury can be inflicted by applying dimethyl sulfoxide (DMSO) on the cornea9,10. Other commonly used injury models include incisional wounds that penetrate through the stroma and keratectomy wounds, which are limited to the upper portion of the stroma14,15. These methods are also useful to answer questions regarding stromal wound healing. Different injury models have their own advantages and disadvantages. Abrasion, or debridement, of the corneal epithelium was first developed using dulled scalpels or blades on ex vivo corneas16. This method has later been used in vivo on mouse, rat, and rabbit17,18,19,20,21,22. Using the ocular burr (Figure 1), we remove only a selected region of the epithelium, leaving the rest of the epithelium unaffected. This way, it is possible to precisely target the epithelial removal to different parts of the cornea. In addition, the abrasion size can be assessed with fluorescein staining. Furthermore, here we follow abrasion closure during the healing period.
This method poses several advantages, i) including precise location of abrasion site, which is not possible with chemical injury, ii) the abrasion is quick to perform, and iii) it is non-invasive. Herein, we describe the method using the outbred NMRI mouse as a model, however this could be applied to the vast array of mouse genetic models, as well as to the rat and rabbit, which are common models used to study human corneal disruption.