The Limbal Niche and Regenerative Strategies

Abstract

The protective function and transparency provided by the corneal epithelium are dependent on and maintained by the regenerative capacity of limbal epithelial stem cells (LESCs). These LESCs are supported by the limbal niche, a specialized microenvironment consisting of cellular and non-cellular components. Disruption of the limbal niche, primarily from injuries or inflammatory processes, can negatively impact the regenerative ability of LESCs. Limbal stem cell deficiency (LSCD) directly hampers the regenerative ability of the corneal epithelium and allows the conjunctival epithelium to invade the cornea, which results in severe visual impairment. Treatment involves restoring the LESC population and functionality; however, few clinically practiced therapies currently exist. This review outlines the current understanding of the limbal niche, its pathology and the emerging approaches targeted at restoring the limbal niche. Most emerging approaches are in developmental phases but show promise for treating LSCD and accelerating corneal regeneration. Specifically, we examine cell-based therapies, bio-active extracellular matrices and soluble factor therapies in considerable depth.

Keywords: corneal epithelium; limbal epithelial stem cells; limbal stem cell niche; mesenchymal stem cells; ocular surface regeneration.

Figure 1

Limbal niche. Illustration of the limbal niche, focusing on the palisade of Vogt. The palisades of Vogt form crypts in the limbal epithelium, allowing for close contact between LESCs and supportive cells, including melanocytes, keratocytes, mesenchymal stem cells and Langerhans cells. These cells, along with the basement membrane and neurovasculature, provide growth factors, nutrients and structural support to promote proper LESC proliferation and differentiation. At the border of the limbal and corneal basement membranes, LESCs divide into progenitor cells or transient amplifying cells (TAC). The TACs divide into postmitotic cells (PMCs) and migrate centrally. These PMCs differentiate into terminally differentiated epithelial cells (TDCs) to replace lost cells on the corneal surface. Use of illustration permitted by [5].

Figure 2

Limbal stem cell interactions. (A) Illustration demonstrating the interaction between limbal MSCs and LESCs. Use of illustration permitted by [5]. (B) Representative picture of human Limbus MSCs using brightfield microscopy. (C,D) Human limbus mesenchymal stromal cell in suspension expressing markers CD 90 and CD73.

Figure 3

Representative immune staining of whole-mount corneas from mouse model of LSCD that are treated with three conditioned media. Corneas treated with human limbal fibroblast-conditioned media showed consistent expression of K12 (red, (a1)) and lower expression of K8 (green, (a2)) which shows the therapeutic effect of this media. However, corneas treated with DMEM or human skin-conditioned media as a negative control illustrated low expression of K12 ((b1,c1) respectively) and high expression of K8 (green, (b2,c2) respectively). Mouse model of LSCD treated with conditioned media. This study created a mouse model of LSCD through limbus to limbus scraping. Mice were then treated with three weeks of limbal fibroblast-conditioned media (A–F), DMEM (G–I) or skin-conditioned media (J–L). K8-positive cells fluoresce green, while K12 positive cells fluoresce red. M and N are magnified and stained with DAPI [55].

Figure 4

Exosomes from cornea MSCs promote corneal epithelial wound healing. In vivo images using cornea MSC-derived exosomes in corneal epithelial debridement model show the epithelium healing faster when treated with exosomes versus control [125] * Statistically significant.