Corneal surface and superficial cells as viewed by scanning electron microscopy and impression cytology sampling

Purpose: To objectively compare sizes of corneal epithelial surface cells visualized by scanning electron microscopy (SEM) or collected by impression cytology (IC). Methods: Corneas of recent postmortem sheep eyes were either glutaraldehyde fixed for SEM or IC samples taken, glutaraldehyde fixed, and then stained with Giemsa. The corneal epithelial surface was examined at •200, with the borders of all cells marked and dimensions and areas measured. For SEM, the cells were classified as having light, medium, or dark electron reflex and by the number of cell sides. Results: SEM revealed a mosaic of polygonal features with different electron reflexes according to the density of cell surface microplicae. The cell area and longest dimension were predictably dependent on the number of cell sides. Compared with the overall average cell area of 1598 μm2 (range 200-3900 μm2), the dark reflex cells were predominantly the largest, with an average area of 2435 μm2 and having the average longest dimension of 59.9 mm. Cells collected by IC had a range of areas from 1150 to 4800 μm2 but were predominantly large and had an average area of 2802 μm2 with the average cell longest dimension averaging 64.4 mm. IC-sampled cells had nucleocytoplasmic ratio values consistent with their having a squamous phenotype. Conclusions: Cells sampled by IC from the corneal epithelial surface are predominantly those that are enlarged (and flattened out) in preparation for desquamation, a feature also attributed to the dark reflex cells seen in SEM.