A new epithelial cell type in the human cornea.

PURPOSE To study the expression of intermediate filaments in the human cornea. METHODS Light and electron microscopic and immunohistochemical studies were performed on 20 corneas from subjects of various ages. RESULTS A hitherto unrecognized epithelial cell population emerged from the immunohistochemical studies. Epithelial cells were invariably present in the superior cornea, whereas the nasal, temporal, and inferior segments almost lacked these cells. They were situated at the transition between peripheral cornea and limbus, and occurred as small groups in the basal epithelium. On electron microscopy, they were recognized by their marginated nuclear chromatin, large nucleoli, prominent bundles of intermediate filaments, and numerous hemidesmosomes and desmosomes. Immunohistochemistry on frozen sections revealed a unique intermediate filament make-up: ie, strong co-expression of vimentin and cytokeratin 19; other intermediate filaments, including cytokeratins 3, 4, 6, 7, 8, 10, 13, and 18 were negative. Finally, the cells lacked ultrastructural and immunohistochemical features of melanocytes, neuroendocrine cells, Langerhans' cells, and leukocytes. CONCLUSIONS A new epithelial cell type in the human cornea is described with characteristic morphologic and immunohistochemical features. According to their particular segmental distribution, restricted localization at the junction between cornea and limbus, and expression of an "early" intermediate filament profile, it is tempting to speculate that they represent stem cells of the human cornea. Further studies are aimed to characterize their phenotype and function more extensively.