Diagram of the role of SSEA4 antigen in corneal limbal homeostasis. Isolation of SSEA4+ limbal fibroblasts and growth in 3D culture system induces stem cell marker expression that in turn leads to sphere formation and subsequent differentiation into corneal epithelium. Abbreviations: LFs, limbal fibroblasts; SSEA4, stage-specific embryonic antigen 4; CD, cluster of differentiation; CK, cytokeratin;BMP4, bone morphogenetic protein 4; RA, retinoic acid; EGF, epidermal growth factor.