Membrane structure of corneal epithelium: freeze-fracture observation

Cytological details of the corneal epithelium were studied with special emphasis on the structure of the cell membrane. Normal corneas of various animals were fixed in a 4% solution of glutaraldehyde and divided into two portions. One-half was frozen with Freon 22 and freeze-fractured in frontal and sagittal directions using a Balzer instrument. The other half was examined by transmission electron microscopy. The general appearance of the split cell membrane of the corneal epithelium was similar to that of other membranes. Besides the characteristic structure of the fractured cell membrane at desmosomes, gap junctions and hemidesmosomes, the cell membrane demonstrated many "bumps and hollows" type configurations measuring 70-80 nm in diameter. This structure is known to be commonly associated with micropinocytotic vesicles and cellular pores. Careful examination of transmission electron micrographs of the corneas of the animals in this study revealed that the number of micropinocytotic vesicles was far greater than previously noted. Pinocytosis was most abundant in the basal cell and decreased toward the superficial layers. Cluster distribution of pinocytosis was frequently demonstrated in the basal cell. In addition, abundant pinocytosis was frequently demonstrated in the basal cell. Also, abundant pinocytosis was present in the cell membrane surrounding nerve fibers which occasionally reached the superficial layer. It was found that numerous nerve fibers pierce through the cytoplasm of the basal cell, in addition to running through the normal intercellular spaces. These findings may be of certain significance in the explanation of transport of fluid through the epithelium and innervation of the cornea.