The Lid Wiper Contains Goblet Cells and Goblet Cell Crypts for Ocular Surface Lubrication During the Blink

Purpose: The conjunctival side of the upper and lower inner eyelid borders, termed the lid wiper, has a thickened epithelial lip for apposition to the globe, assumed to distribute the preocular tear film. The human lid wiper structure and its goblet cells are investigated. Methods: Conjunctival whole mounts, including lid margins from 17 eyes of human body donors, were investigated by routine histology and semithin plastic sections, using histology, histochemistry, and immunohistochemistry. Results: In routine histology, the conjunctival lid wiper epithelium regularly showed goblet cells, single and in clusters, at the luminal surface and also deep within the epithelium without apparent surface contact. Semithin sections revealed that the deep goblet cells were often connected to cryptal epithelial infoldings that opened to the surface, hence making their mucins available at the surface. The goblet cells produced mucins of neutral (periodic acid-Schiff) and acidic (Alcian blue) type and stained positive for the gel-forming mucin MUC5AC. Surprisingly, MUC5AC-negative goblet cells were also observed in the lid wiper. Conclusions: Contrary to conventional assumptions, the lid wiper is part of the conjunctiva. It contains previously undescribed goblet cell crypts deep in the epithelium, suitable as an internal lubrication system for reduction of friction between the lid margin and the globe. This provides the first evidence of the morphological basis for the hydrodynamic type of lubrication and a more conclusive understanding of lid-margin lubrication and tear film distribution. It is another strong indication that the lid wiper is that area in apposition with the globe for distributing the thin preocular tear film during the blink.