Fig. 3 Two types of ocular surface failure. In limbal stem cell deficiency, the corneal epithelium is replaced by conjunctival epithelium, while in squamous metaplasia corneal and conjunctival epithelia transform into a keratinized (skin-like) epithelium. In addition to the change in epithelial phenotype, both types of ocular surface failure are also accompanied by the destruction of the basement membrane and the inflammation and scarring of the underlying stroma.