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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