Contribution of the epithelium to the stability of the tear film.

The superficial cells of the epithelium make a mucoprotein with long chemical chains and package it in the subsurface vesicles. Subsurface vesicles fuse with the cell membrane and the side chains thus exposed join with the overlying mucus to anchor the mucus to the epithelial layer. The membranes surrounding the subsurface vesicles become incorporated in the cell membrane and provide the extra membrane required for the sprouting of microvilli. The goblet cells secrete the overlying mucus layer.