Surface features of the conjunctiva and cornea.

Guinea pig corneal and conjunctival surfaces were examined by transmission electron microscopy and cytochemistry. Some specimens of cornea and conjunctiva were examined morphologically; others were stained with ruthenium red or tannic acid before examination to enhance the detection of glycoproteins at cell surfaces. The epithelia were covered by microvilli and on the cornea also by microplicae. These surface projections were the shortest (150 nm) over the central cornea, and became progressively longer (approximately 300 nm) over the tarsal and fornical conjunctiva. There was a filamentous cell coat (glycocalyx) on the microvilli and microplicae that was best demonstrated in specimens stained with tannic acid. The glycocalyx extended approximately 300 nm from the tips and lateral surfaces of the microvilli and microplicae. Although there were slight local variations in its thickness, the maximum thickness of the cell coat was uniform over the cornea and conjunctiva. Heavy deposits at the cell surface after ruthenium red staining indicated that the cell coat contained many highly charged polyanions. The density of the ruthenium red stain obscured the fine structure of the filaments in the cell coat. The glycocalyx forms a scaffolding that is believed to bind mucus, with its content of immunoglobulins, by weak chemical interactions to the epithelial surface. Therefore, the microvilli, microplicae, and glycocalyx that were demonstrated in this study provide the structural framework that supports and binds a complex of related factors, including tears, mucus, and immunoglobulins, that have the common function of protecting the eye.