The fate of limbal epithelial progenitor cells during explant culture on intact amniotic membrane

PURPOSE. The clinical success of treating corneas with total limbal stem cell deficiency using limbal biopsy explants cultured on intact amniotic membrane ( iAM) relies on ex vivo expansion of limbal epithelial progenitor cells. However, the ultimate fate of limbal epithelial progenitor cells in the explant remains unclear. METHODS. Human limbal explants were cultured on iAM for 2 weeks and then removed and transferred to a new iAM until passage 3. The outgrowth surface area of each passage was measured and compared. For each passage, clonogenicity on 3T3 fibroblasts feeder layers was compared among progenitor cells removed from the outgrowth, the explant surface, and the remaining stroma. Cryosections of the explant and the outgrowth were detected with p63, vimentin, pancytokeratin, and the basement membrane components type VII and IV collagen and laminin 5 antibodies. RESULTS. The outgrowth surface area significantly decreased from passage ( P) 1 to P3. The total number of epithelial cells that were isolated from the explant surface also decreased from before culture ( P0) to P1, became stable from P1 to P2, but was uncountable at P3. Clonogenicity significantly declined from P1 to P3 for the epithelium derived from the explant surface and the outgrowth epithelium; the extent was less in the former than in the latter at P2 and P3. In addition, groups of epithelial cells invaded the limbal stroma of the explants from P1 to P3; p63( +)/ pancytokeratin( -) and p63( +)/ vimentin( +) cells also presented in the limbal stroma. Increasing fibroblast, but not epithelial, colonies were observed from cells isolated from the remaining limbal stroma when seeded on 3T3 fibroblast feeder layers from P1 to P3. CONCLUSIONS. During ex vivo expansion on iAM, some limbal epithelial progenitor cells indeed migrate onto iAM from the explant surface, whereas some also invade the limbal stroma, very likely undergoing epithelial-mesenchymal transition. This new information should be taken into account in formulating new strategies to improve the expansion protocol.