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Stem Cells of the Eye

Stem Cells of the Eye



Introduction


Two prerequisites are essential for good vision. The first is a clear optical focusing system. The main components of this (the cornea, lens and intraocular fluids) are designed to bring visual images to a focus on the retina, with the cornea contributing most of the refractive power of the eye. The second is an intact neural system, which detects and transmits images in a coherent fashion from the retina, through an intricate series of neural pathways, to the visual cortex, where they are perceived and interpreted. Ocular stem cell research has concentrated primarily on these two major components of the ocular system that are essential for maintaining clear vision: the ocular surface (comprising the cornea, limbus and conjunctiva) and the retina

. Ocular Surface Stem Cells


The greatest advances in ocular stem cell biology and treatment have been in the area of ocular surface stem cells. The ocular surface is a complex biological continuum responsible for maintenance of corneal clarity and elaboration of a stable tear film for clear vision, as well as protection of the eye against microbial and mechanical insults. The ocular surface epithelium comprises corneal, limbal and conjunctival epithelium (Fig. 1). The cornea is a highly specialized tissue designed to provide the eye with a clear optical surface for vision. The corneal epithelium consists of a stratified squamous non-keratinizing epithelium that is approximately 5 layers thick. The limbus is a 1.5 to 2 mm wide 
area that straddles the cornea and bulbar conjunctiva, extending 8–10 layers in thickness. 


Conclusions



 Our knowledge and understanding of ocular surface stem cells has led to improvements in the management of ocular stem cell deficiency. Conjunctivalization, recurrent corneal epithelial defects, vascularization and inflammation are hallmarks of patients with severe limbal stem cell deficiency. Conventional penetrating keratoplasty has been shown to have uniformly poor results in view of the hostile milieu of these diseased eyes, and our understanding of limbal stem cell biology explains the poor outcome of conventional penetrating keratoplasty in the management of ocular surface diseases. Limbal stem cell transplantation has revolutionized the treatment of these difficult cases, by replacing the depleted stem cell population, but the long-term viability of allograft transplantation remains in question. Although much of the ocular stem cell research has been focused on understanding the physiology and homeostasis of these cells, understanding the “niche”microenvironment where stem 
cells reside is equally important

  Much remains to be learnt about the structural and biochemical components of the stem cell niche, and the regulatory mechanisms involved in the differentiation of stem cells. The development of cellular markers will also greatly improve our understanding of stem cells and their behavior in normal and diseased states, and an enhanced understanding of the complex interactions leading to stem cell survival and maintenance will allow us to develop new and more successful treatment options for ocular disorders.
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