The epithelial basement membrane zone of the limbus

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Abstract
The basement membrane zone of the limbal epithelium adjacent to the cornea was examined by ultrastructural and immunohistochemical techniques to determine whether differences exist between this region and central cornea. In human limbus, the percentage of basal cell membrane occupied by hemidesmosomes was significantly less (14.9±3.5) than that in central cornea 27.9±9.2), whereas the area of basement membrane/100 |im of cell membrane did not differ significantly. In rabbits, both percentage of membrane occupied by hemidesmosomes and area of basement membrane were less in the limbal region. Comparison of laminin and type VII collagen (anchoring fibril collagen) localisation in limbus and in central cornea demonstrated that both matrix proteins had a more convoluted pattern of localisation in the limbus. In addition, short segments of basement membrane with associated anchoring fibrils were present in the zone between the basal cells' basement membrane and blood vessels. These areas of duplicated basement membrane with anchoring fibrils were separated from the epithelium by layers of extracellular matrix that included collagen fibrils. Scanning electron microscopy of the surface topography of human limbal and central corneal basement membrane, prepared by removal of the epithelium with EDTA, demonstrated that in the limbal zone between the Palisades of Vogt and cornea, a very rough undulating surface was present with papillae or 'pegs' of stroma extending upward, and that central cornea lacked such papillae. Rabbit limbal basement membrane surface showed no such papillae, only occasional indentations into the stroma. Compared with central cornea, the differences in adhesion structure pattern in limbus may indicate that this surface has less abrasion pressure from the lids or that the convoluted interdigitation of cytoplasmic processes with the stroma compensates for the smaller number of hemidesmosomes.