Characterization, isolation, expansion and clinical therapy of human corneal epithelial stem/progenitor cells.

Stem cells can be defined as cells that have the capacity to self-renew and the ability to generate differentiated progeny or multiple cell lineages. True stem cells can turn into any type of cells, while progenitor cells are more or less committed to becoming cell types of a particular tissue. Human corneal epithelial stem cells (CESCs) represent a great example and model of adult stem or progenitor cells. Human CESCs have been identified to locate in the basal epithelial layer of the limbus, and thus also referred as to limbal stem cells. We would like to use the both terms, stem and progenitor cells in this chapter based on previous use in the literature for more than two decades. Although the CESCs have been identified to reside at the limbus and many stem cell markers have been proposed, there is no consensus to date regarding the definitive markers for CESCs, and identification and isolation of these cells are still challenging. Based on evaluation of a variety of proposed markers, we have characterized that the CESCs located in the basal layer of human limbal epithelium are small primitive cells expressing three patterns of molecular markers, which represent a unique phenotype of putative corneal epithelial stem or progenitor cells. Based on adult stem cell criteria and the putative limbal stem cell phenotype, our group has attempted to enrich for human CESCs through novel approaches including cell-sizing, adhering to extracellular matrix collagen type IV, and cell sorting for side population or for expression of ABCG2 or connexin 43 cell surface markers. The 5 clonogenic populations isolated from limbal epithelium and its cultures by different methods show the properties that are characteristics of adult stem/progenitor cells: 1) relatively undifferentiated, 2) high proliferative potential, 3) self-renewal. Expansion and cultivation of corneal epithelial progenitor cells have been achieved using different methods, such as limbal tissue explant culture, and limbal epithelial cell suspension co-culture with mouse 3T3 fibroblast feed layer. To avoid the use of xeno-components, two cell lines of commercial human fibroblasts have been identified that support human corneal epithelial regeneration, and have potential use in replacing mouse 3T3 cells for corneal tissue bioengineering. The concept of CESCs has formed the basis for identifying a class of blinding diseases that display features of corneal epithelial stem cell deficiency or limbal stem cell deficiency (LSCD), where the limbal epithelium is damaged. LSCD is characterized by persistent or recurrent epithelial defects, ulceration, corneal vascularization, chronic inflammation, scarring, and conjunctivalization (conjunctival epithelial ingrowth). Only transplantation of CESCs can restore vision. Due to an increasing shortage of corneal donors, corneal tissue engineering is becoming an important discipline that holds great promise for corneal reconstruction. CESCs and optical substrates are known to be the most important factors for corneal tissue bioengineering in regenerative medicine. Our team has recently explored the utilization of natural donor corneal stroma in corneal tissue engineering. In combination with fresh limbal epithelium containing stem cells, and the donor corneal stroma, a great source of natural optical substrate, we developed a native-like corneal equivalent construct with proliferative potential. This corneal construct provides a new clinical cell therapy for corneal reconstruction.