When chicks are exposed to constant light during growth, their corneas become flatter and lighter in weight, and their anterior segments become shallower than those of chicks exposed to cyclical periods of light and dark. These effects have been correlated with constant light suppression of cyclical changes in melatonin levels. The question of whether light directly influences corneal growth (e.g. via cryptochromes in the cornea) or whether it acts remotely via the suppression of the melatonin rhythm has not yet been answered. Retinoic acid, an ubiquitous morphogen, also causes non-functional flattening during corneal growth, but its effect in vivo has not been correlated with light regimes.
We wished to characterize and distinguish between hormonal and light effects on corneal growth. We used organ culture to study the direct effects of light regimes, melatonin, and retinoic acid, and compared these results with those of parallel in vivo experiments.
In this study, eye drops containing melatonin or retinoic acid were applied to corneas exposed to constant light in vivo or in organ culture, and effects on corneal mass and hydration were measured. We applied a melatonin blocker, luzindole, to chick corneas in normal light/dark conditions to confirm that the observed melatonin effects are mediated at the cell membrane. Anterior chamber depth and refraction in vivo were measured.
We found that, during constant light exposure, combined application of melatonin and retinoic acid eye drops increase the depth of the anterior segment in vivo, (p = 0.003) and interestingly, both also reduced the hyperopia of constant light exposure after two weeks (p = 0.002), thus partially reversing the effects of constant light. Retinoic acid increased corneal hydration in vivo (p = 0.030) but not in organ culture. Melatonin had no effect on corneal hydration in vivo, but in organ culture, melatonin significantly decreased hydration (p <0.001).
We found no evidence for a direct effect of light on corneal hydration in growing chick corneas in culture. Melatonin is required for normal corneal growth in vivo, and together melatonin and retinoic acid, or retinoic acid alone, affect the regulation of water content within the chick cornea. Melatonin also affects corneal hydration in vitro, but retinoic acid does not.