Morphological and immunohistochemical characteristics of the equine corneal epithelium

Objective The morphology of the corneal epithelium in two age groups of horses is described. Distribution patterns of proliferation-, differentiation-, stem cell-associated markers and cell junction proteins were assessed. Methods Corneal samples from 12 horses (six foals and six adult horses) were analyzed after H&E staining and immunohistochemistry using the following antibodies: E-cadherin, beta-catenin, Connexin 43 (Cx43), tight junction protein 1 (TJP1), cytokeratin (CK) 14, CK 19, CK 3, CK 10, vimentin, Ki67, p63, nerve growth factor (NGF), ABCG2, and epithelial growth factor receptor. Semiquantitative analysis of crypt, limbal, peripheral, and central zone was performed. Semithin and ultrathin sections were used for ultrastructural evaluation of the epithelium. Results The height of the epithelium varied between age groups and crypts were consistently present. In the peripheral and central epithelium, three types of basal cells resembling a pseudostratified epithelium were characterized. Potential stem cell markers (CK 14, p63, NGF, and ABCG2) were present in all zones with decreasing frequency toward the center. Cornea-specific differentiation marker CK 3 was not expressed in the most basal cell layer of the limbal epithelium. E-cadherin, beta-catenin, and Cx43 revealed a similar apico-lateral signal pattern throughout the entire epithelium; only TJP1 was additionally seen at the basal surface. Conclusions This study presents a systematic semiquantitative evaluation of the equine corneal epithelium, showing the presence of crypts as potential stem cell niche with CK 14, p63, NGF, and ABCG2 as relevant markers for cells with regenerative capacity. The pseudostratified arrangement of the basal layer was a unique finding.