Critical appraisal of ex vivo expansion of human limbal epithelial stem cells

Abstract

The stem cells (SCs) of the corneal epithelium located in the limbal basal layer are the ultimate source to maintain corneal epithelial homeostasis. Like other adult tissue-specific SCs, self renewal and fate decision of limbal SCs are regulated by a specialized in vivo microenvironment, termed "niche". Loss of limbal SCs or dysfunction of the limbal niche renders corneas with a unique clinical disease labeled limbal stem cell deficiency (LSCD). Besides transplantation of autologous or allogeneic limbal SCs or amniotic membrane, a new strategy of treating LSCD is to transplant a bio-engineered graft by expanding limbal SCs ex vivo. Herein, we conduct a critical appraisal of six protocols that have successfully been practiced in treating human patients with LSCD, and identify issues whether niche regulation has been disrupted or maintained during isolation and expansion. Consequently, we propose a future direction that may circumvent the potential pitfalls existing in these conventional protocols by preserving the interaction between limbal SCs and their native niche cells during isolation and expansion. Such an approach may one day help realize considerable promise held by adult SCs in treating a number of diseases.

Fig. (1). The Limbal Palisades of Vogt

Palisades of Vogt (arrow) are readily recognized in the human limbus (A). Such a unique pigmented structure can be identified on the flat mount preparation of dispase-isolated human limbal epithelial sheets (B, Bar represents 500 µm in A and B). Schematic drawing of the limbal epithelium and the limbal niche shows the cellular components of SC, TAC, PMC (post-mitotic cells), TDC (terminally differentiated cells), M (melanocytes), LC (Langerhan’s cells) and MC (presumed NC) as well as BM (basement membrane), Bo (Bowman’s layer), N (nerves) and BV (blood vessels) (both modified from [39] with permission).

Fig. (2). Clinical and Basic Ophthalmology Literature Regarding to AMT by Year

Over the past decade, there has been a surge of interest in amniotic membrane transplantation (AMT) for ocular surface reconstruction. PubMed search of papers published in Ophthalmology per year are plotted from the PI’s pioneering paper in 1995 [53]). These studies collectively showed that AMT is effective in facilitating epithelial wound healing and reducing stromal inflammation, scarring and unwanted new blood vessel formation.