الفهرس 
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Abstract
Corneal epithelial integrity, and stromal transparency are crucial for vision, the corneo-scleral limbus contains limbal stem cells that provide the source of corneal epithelial renewal and act as a barrier that prevents the conjunctival epithelium from encroaching on the surface of the cornea. Limbal stem cells are present in the pallicades of Vogt. Limbal stem cell micro environment is referred to as the limbal stem cell niche, this microenvironment maintains the state of undifferentiation and the slow cell cycling of the stem cells. On response to corneal epithelial injury limbal stem cells undergo asymmetric division into a daughter stem cell and a transient amplifying cell. The daughter stem cell serves to replenish the stem cell pool, while the transient amplifying cell undergoes further division and differentiation into postmitotic cell and finally corneal epithelium. The role of surface cell markers in the identification of limbal stem cells is still controversial, yet; evidence of the presence of limbal stem cells exist, such as pigment migration studies and corneal epithelial regeneration. A variety of hereditary and acquired disorders may eventually lead to limbal stem cell deficiency, hereditary disorders include congenital aniridia keratitis, keratitis associated with multiple endocrine deficiency, congenital erythrokeratodermia, dyskeratosis congenital and lacrimo-auriculo-dento-digital syndrome.
Acquired disorders include Stevens-Johnson syndrome, ocular cicatricial pemphigoid, chemical, thermal and physical injuries, contact lens induced keratopathies, multiple surgeries or cryotherabies to the limbal region, neuropathic keratopathy, chronic microbial infection, ulcerative keratitis, local application of cytotoxic agents such as 5-fluorouracil and mitomycin C, systemic chemotherapy and vernal keratoconjunctivitis.
Conjunctivalization of the cornea is the hallmark of limbal stem cell deficiency,i other clinical features include stippled fluorescein staining of the area covered by abnormal epithelium, tear film instability, corneal neovascularisation, persistent epithelial defects and fibrovascular pannus formation.
Treatment of limbal stem cell dficiency varied from symptomatic treatment in mild cases to surgical intervention in severe cases. Treatment aims to re-establish the normal anatomic and physiologic environment of the ocular surface by optimizing lids and the tear film, controlling inflammation, and then reconstruction of the corneal and conjunctival epithelium. Conjunctival transplantation and keratoepithilioplasty are no longer considered for the treatment of limbal stem cell deficiency. Currently, the main procedures performed include conjunctival limbal autograft for unilateral limbal stem cell deficiency, using tissue from the fellow eye, living related conjunctival limbal allograft, where a living relative donates conjunctival and limbal tissue and keratolimbal allograft using a cadaveric donor where the peripheral cornea is used to transfer the limbal stem cells. These procedures with the exception of autograft require systemic immune suppression. More recently, ex vivo expanded limbal stem cells have also been successfullyused to reconstruct the ocular surface with or without amniotic membrane transplantation. The use of fibrin glue in limbal stem cell transplantation procedures made the surgery easier with shorter operative time, shorter post operative recovery time, decreased post-operative patient discomfort and decreased post operative complications related to the presence of sutures. These procedures may be combined with, or followed by penetrating keratoplasty to improve the visual outcome. Patients with long standing limbal stem cell deficiency and severely damaged ocular surface interfering with the possibility of performing any of the previously mentioned procedures, still can benefit from implanting a Boston keratoprosthesis and having back some vision. Limbal stem cell transplantation procedures are facing difficulties such as donor
site complications in auto graft and living related allograft procedures, graft rejection and the complications of long term systemic immune suppression, andrecent studies are aiming to overcome these difficulties.