Scanning electron microscopy of the normal and experimentally infected ocular surface

Scanning and transmission electron microscopy were used to characterize the normal pup and adult mouse ocular surface. Various fixatives were examined and categorized into those which stabilize and preserve ocular mucus associated with surface corneal epithelial cells and those which do not, but allow good visualization of surface detail for scanning electron microscopy. Desquamation patterns of corneal epithelial surface cells in the mouse pup and adult were examined, compared with each other and with desquamation patterns originally reported for several other animal species. Once the normal cytoarchitecture of the corneal surface and its associated ocular mucus was established for the mouse pup and the adult animal, the murine ocular response to P. aeruginosa infection was studied. Those studies revealed that in the pup, after inoculation of bacteria beneath the eyelid, with no corneal scarification, organisms adhered preferentially to young surface cells and quickly penetrated the corneal and conjunctival epithelium. In contrast, scarification of the corneal surface had to precede topical bacterial inoculation for infection to occur in the adult cornea. In this model, bacteria adhered initially to the wound site and to aged cells but did not preferentially adhere to young surface cells. An extracellular virulence factor, exotoxin A, produced by the bacteria, was also examined to determine its role in P. aeruginosa pathogenesis. Studies using toxin A, have shown that it was capable of inducing epithelial and endothelial cell death and corneal necrosis in the adult scarified cornea. In the pup, where no scarification preceded toxin A challenge, little corneal epithelial damage was produced when compared with the infection model. Nonetheless, as with bacterially challenged pups, experimental toxin A treated animals died within 24 hr following toxin administration.