This approach can be used when either one or both eyes are affected, providing there is sufficient limbal tissue available (1–2 mm2). A small sample of limbal cells is taken from a healthy part of the eye, and grown in a sterile laboratory to produce a sheet of cells sufficient for transplantation. Once transplanted, they multiply and regrow the corneal epithelium. The manufacturing process is designed to ensure implantation of the right number, size and quality of cells. CLET avoids some of the issues faced by other limbal transplantation procedures and does not pose a threat to the integrity of the donor eye. It also offers the possibility of re-grafting in case of failure of the first graft or need for a further graft. Types
There are three types of clonogenic keratinocytes involved in the generation of the corneal epithelium: holoclones, meroclones and paraclones. 1- Holoclones: as true stem cells, have the greatest growth potential, and give rise to               2-meroclones, which have a much lower proliferative capacity, but frequently divide. 3- Paraclones have even lower proliferative capacity. Both meroclones and paraclones are known as transient amplifying cells and their purpose is to form a stratified squamous epithelium. All three types of keratinocytes are present in the basal layer of the limbus, with holoclones in the least abundance (10%–15%).
Limbal stem cells, also known as corneal epithelial stem cells, are stem cells located in the basal epithelial layer of the corneal limbus. They form the border between the cornea and the sclera. Characteristics of limbal stem cells include a slow turnover rate, high proliferative potential, clonogenicity, expression of stem cell markers, as well as the ability to regenerate the entire corneal epithelium. Limbal stem cell proliferation has the role of maintaining the cornea; for example, by replacing cells that are lost via tears. Additionally, these cells also prevent the conjunctival epithelial cells from migrating onto the surface of the cornea. Medical conditions and treatments
Damage to the limbus can lead to limbal stem cell deficiency (LSCD); this may be primary - related to an insufficient stromal microenvironment to support stem cell functions, such as aniridia, and other congenital conditions, or secondary – caused by external factors that destroy the limbal stem cells, such as chemical or thermal burns, radiation, surgery, infection, use of contact lenses, or certain drugs. Signs and symptoms include: conjunctivalisation, corneal vascularisation, edema, ocular discomfort or pain, visual impairment, and blindness, which are likely associated with failure in the process of regenerating the corneal epithelium. Immediate management aims to limit traumatic or chemical damage to the limbus, control inflammation, and help achieve a healthy corneal epithelium. Initial treatment after trauma/injury includes preservative-free artificial tears, topical steroids, ‘bandage’ contact lenses, and autologous eye drops (eye drops manufactured from the patients own blood serum and plasma).