الفهرس 
Anatomy of the ocular surfaceOnly 14 pages are availabe for public view
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Abstract
This essay is a review of literature on the role of limbal stem cell transplantation in the management of ocular surface disorders with special emphasis on the anatomy of the ocular surface, stem cell concept and pathophysiology of ocular surface wound healing, indications of stem cell transplantation and the different limbal stem cell transplantation techniques used in the management of ocular surface diseases.
The limbal epithelium that separates the corneal epithelium and conjunctival epithelium is made up of a nonkeratinizing stratified squamous epithelium but is much thicker than the corneal epithelium (up to ten cell layers). It is thought to contain the source of the stem cells (SCs) that provide the source of corneal epithelial renewal and also provide a barrier preventing the conjunctival epithelium from encroaching onto the corneal surface. These cells are known as corneal epithelial SCs, or limbal stem cells (LSCs) which settle in stromal folds called palisades of Vogt (PV).
The role of the limbal epithelium being the stem cell provider for the regeneration of the corneal epithelium has been discussed including the stem cell concept. When reviewing the data concerning the pathology of corneal regeneration to demonstrate the amount of limbal stem cells that is sufficient for maintenance of the corneal epithelium, it was found that lesions involving more than two-thirds the limbal tissue together with peripheral and central epithelium resulted in delayed wound healing with subsequent vascularization of the corneal surface and the resulting epithelium exhibited areas of recurrent epithelial breakdown. Microscopically, the epithelium was indistinguishable from conjunctival epithelium and contained numerous goblet cells.
Failure of ocular surface may be due to limbal stem cell deficiency (LSCD), the major type of ocular surface disorders (OSD) or due to squamous metaplasia (the hallmark of dry eye disorders).
Limbal stem cell deficiency (LSCD) results from the loss or dysfunction of LSC, most often because of injury or inflammation. LSCD or dysfunction can result from severe chemical and thermal burns to the surface of the eye, inflammatory diseases (such as Stevens–Johnson Syndrome and ocular cicatricial pemphigoid), and long-term contact lens wear. There are also various iatrogenic causes of LSCD, which include extensive limbal surgery or cryotherapy and therapeutic radiation. Exposure of the limbus to cytotoxic agents such as mitomycin C has also been known to cause LSCD. Hereditary causes of LSCD include aniridia and ectodermal dysplasia. It is probably in these cases that the niche for LSCs is altered and this result in subsequent LSC dysfunction and loss.
Limbal stem cell deficiency disorders have been categorized according variety of factors including the degree of limbal stem cell (SC) loss, partial or total LSCD; the extent of conjunctival disease; and presence and etiology of conjunctival inflammation; Laterality of the disease, unilateral or bilateral.
Treatment of OSD varied from symptomatic treatment in mild cases to surgical intervention in severe cases. The goal of treatment for severe LSCD is to re-establish the anatomic and physiologic environment of the ocular surface by optimizing lids and the tear film, controlling inflammation and the management of glaucoma preoperatively then reconstruction of the corneal and conjunctival epithelium.
A preoperative staging system for disease severity has been demonstrated where staging was determined by the status of both the limbal stem cells and conjunctiva.
The different surgical techniques used in limbal stem cell transplantation have been discussed including sequential sectoral conjunctival epitheliectomy, conjunctival limbal autograft, living-relative conjunctival limbal allograft, keratolimbal allografts, and the adjunctive use of amniotic membrane transplantation and ex vivo expanded limbal transplantation.
Penetrating keratoplasty is often a component of the therapeutic plan in most patients with ocular surface failure. It can be performed either as a combined procedure with limbal stem cell transplantation using tissue from the same donor, or as a sequential procedure in which stem cell transplantation is performed prior to penetrating keratoplasty.
Results of limbal stem cell transplantation are almost promising unless failure occurs which may be early (that occurs less than 12 months from the time of stem cell transplantation) or late (that occurs greater than 12 months after limbal stem cell transplantation). Each type has its causes and methods to overcome it.
Strategies to improve outcome of limbal stem cell transplantation procedures should include the preoperative assessment and proper management of the risk factors that will affect success. These include conjunctival inflammation, abnormal eyelid architecture and severe tear abnormalities. Systemic and topical immunosuppression are also mandatory during the postoperative period to suppress inflammation which lies at the root of most causes early and late stem cell transplant failure.