Keratocyte reflectivity and corneal haze.

Corneal transparency is a remarkable characteristic that is essential for vision. Biophysical models of corneal transparency have entirely focused on the stromal extracellular matrix and disruption of the regular array of collagen fibres as the main reason for corneal haziness. Therefore, disorder of corneal transparency has traditionally been explained by a combination of three main factors: (1) abnormal water content (i.e. swelling or edema); (2) abnormal collagen fibre diameter, spacing, and orientation (i.e. scar tissue or fibrosis); and (3) abnormal accumulation of macromolecules (proteins, glycosaminoglycans, lipids, etc.) as in many corneal dystrophies. Here, clinical and experimental data are provided to support the concept that corneal keratocytes, which are normally invisible and transmit light, may show intense light scattering in injured corneas. Thus, the existence of a fourth group of corneal transparency disorders is proposed that predominantly are associated with abnormal cellular-based reflections from multiple layers of stromal keratocytes. In this group of patients, the light scattering structures (keratocyte nuclei, cell-body, and cell-processes) cannot be discriminated using standard slit-lamp biomicroscopy but requires a confocal microscopic examination. Despite their importance, almost nothing is known about the physical basis for the invisibility and haziness of the keratocytes. A more comprehensive model to understand corneal transparency is needed and should include the interaction of visible light with the physical structure of the keratocyte and its subcellular constituents.