Cellular incursion into Bowman's membrane in the peripheral cone of the keratoconic cornea.

Analysis of corneal tissue from normal and keratoconic donors has revealed differences which may represent early signs in the pathogenesis of keratoconus. Peripheral areas of keratoconic tissue obtained from transplant surgery were targeted to ascertain cellular disposition and morphological changes which may be masked within the extensive damage of the central keratoconic cone. Peripheral keratoconic corneae exhibited discrete incursion of fine cellular processes into Bowman's membrane. These processes originated from keratocytes and were often observed in conjunction with a defined indentation from the basal epithelium. Comparison of the lysosomal enzymes cathepsin B and G with constitutively expressed cytoplasmic esterase determined that both cathepsins were elevated within keratocytes of keratoconic tissue compared with normal tissue. Some clusters of keratoconic keratocytes had elevated levels of cathepsin exceeding all others. Cathepsin-rich keratocytes localized with morphologically compromised regions of Bowman's membrane. The presence of cell nests deeper within the stroma indicated that the catabolic changes, which are visible within the acellular Bowman's membrane, are probably also occurring deeper within the stroma, but are masked and not readily detectable.