The cornea is the transparent part of the outer anterior part of the wall of the eye and is one of the important components of the eye. Histologically, in order from the outside in, the cornea can be divided into five layers, namely the epithelial cell layer, the anterior elastic layer, the stromal layer, the posterior elastic layer, and the endothelial cell layer.1. Epithelial cell layer: It is the outermost cell layer of the entire cornea and is a compound layer of flat epithelium. Damage to the corneal epithelium is usually seen in clinical settings, such as certain diseases, trauma or accidental damage to the corneal epithelium from wearing keratoplasty contact lenses. After epithelial injury, if it is punctate or small and not infected, it will usually repair itself within 2-3 days without scarring. If the damage is larger, it may cause photophobia, tearing, pain and other symptoms in the patient’s eyes, and will leave a scar if it is not repaired at a later stage; 2, the anterior elastic layer: it is a special membrane from which the corneal nerve endings pass, and cannot be regenerated after being damaged; 3, the stromal layer: it is the thickest layer of the entire cornea, basically accounting for 90% of the corneal thickness, and its shape is a thin plate of multi-layered, parallel arranged collagen fibers. The stromal layer cannot regenerate after injury and is usually replaced by opaque connective tissue if it repairs itself. Corneal clouding occurs in people with corneal trauma, mostly due to problems with the stroma; 4. Posterior elastic layer: The main components are collagen fibers and mucopolysaccharide-like substances, which are the basement membrane of corneal endothelial cells. The posterior elastic layer is strong and has a high regenerative capacity after injury; 5. Endothelial cell layer: The endothelial cell layer is very important and has a structural shape of a single layer of flat epithelium, and the metabolism and functional regulation of the entire cornea depend on endothelial cells. Once the endothelium is damaged, it will cause edema phenomenon and cannot be regenerated, and can only be repaired by the surrounding endothelial cells migrating over. Most corneal transplants do not work well because the endothelium is damaged.