Review paper Clinical applications of limbal epithelial stem cells (original) (raw)

The cornea is the unvascularised refractive element of the eye responsible for approximately two-thirds of its total optical power. Its integrity and transparency is essential for proper functioning of the visual system. The cornea consist of five layers: the outermost stratified squamous epithelium, the Bowman's membrane, the stroma accounting for about 90% of its thickness, the Descemet's membrane and the innermost endothelium actively pumping water out of the stroma. The limbal epithelial stem cells (LESCs) reside in the basal layer of the transition region between the corneal and conjunctival epithelium which is referred to as the limbus [1, 2]. The stem cell niche is considered to be located in the limbal palisades of Vogt which are radially pointed stromal ridges intersected with epithelial rete pegs more abundantly present in the superior and inferior part of the limbus [3, 4]. The term "palisades of Vogt" was coined approximately two centuries ago while observing these radial linear structures under the slitlamp. Such a location provides LESCs with protection by the lids and blood supply from near vessels [5]. Differences in the composition of the limbal basement membrane in comparison to the rest of the cornea are considered to play a mechanical protective role for resident stem cells [6, 7]. Furthermore, melanin pigmentation safeguard LESCs from potential damage by UV light [8, 9]. The limbal zone as opposed to the central cornea has distinguishing featrures, such as the thicker epithelium which forms pegs consisted of 10-12 layers and absence of the Bowman's layer and the Descemet's membrane [10]. Recently, novel components of the stem cell niche were proposed: limbal epithelial crypt (LEC) [11], limbal crypts (LCs) and focal stromal projections (FSPs) [12]. In support of this findings is observation that LCs and FSPs don't occur in patients with limbal stem cells deficiency (LSCD) and cells within them express high levels of the putative limbal stem cell markers p63α and ABCG2. These structures are predominantly distributed in the superior and inferior corneal limbal quadrants and extend to a varying degree temporally and nasally with no presence in the horizontal meridian. An advance in technology of the observation and identification in vivo stem cells niches is essential in terms of improvement in stem cells harvesting by the targeted