Background: The corneal epithelium converges at the peripheral zone (limbus) with the conjunctival epithelium, forming a continuous sheet with phenotypically distinct regions--central, limbal, and conjunctival. The epithelial basement membrane (EBM) is important for corneal functions and cell adhesion, but its regional composition is poorly understood. Current literature is controversial as to the occurrence of type IV collagen in the cornea. The aim of this study was to investigate in detail corneal basement membrane (BM) composition and correlate it with the differentiation state of contributing cells.
Experimental design: Adult human corneas (N = 8) were cryosectioned and analyzed by immunofluorescence with antibodies to 15 BM components and to keratin 3, a marker of corneal epithelial differentiation.
Results: A novel type of spatial heterogeneity ("horizontal") in the EBM composition was found between the central cornea, limbus, and conjunctiva. Central EBM had type IV collagen alpha 3-alpha 5 chains, whereas limbal and conjunctival EBM contained alpha 1-alpha 2 chains and also laminin alpha 2 and beta 2 chains. Limbal EBM in addition had alpha 5(IV) chain. Laminin-1 (alpha 1 beta 1 gamma 1), laminin-5 (alpha 3 beta 3 gamma 2), perlecan, fibronectin, entactin/nidogen, and type VII collagen were seen in the entire EBM. Another novel type of BM heterogeneity ("vertical") was typical for the corneal Descemet's membrane: its stromal face had alpha 1(IV) and alpha 2(IV) chains and fibronectin, whereas alpha 3(IV)-alpha 5(IV) chains, entactin/nidogen, laminin-1, and perlecan were present on the endothelial face.
Conclusions: Type IV collagen controversy is the result of the shifts of isoforms in the limbus and conjunctiva. These shifts and the appearance of additional laminins in the limbus may be related to the differentiation state of corneal cells contributing to the EBM formation. Novel types of BM heterogeneity in the human cornea are described: regional (horizontal) in the EBM and vertical in the Descemet's membrane. The first one may be a common feature of converging complex epithelia, whereas the second one may be another unique property of the Descemet's membrane.