The physiological optics of the lens.

The optical properties of the ocular lens are important to overall vision quality. As a transparent biological tissue, the lens contributes to the overall and dynamic focussing power of the eye, and corrects for optical errors introduced by the cornea. The optical properties of the lens change throughout life. Alterations to the refractive properties and transparency of the lens result in presbyopia and cataract, respectively. However, it is not well understood how changes to lens cellular structure and function initiate these changes in refraction and transparency. Here, we attempt to bridge this knowledge gap by reviewing how the optical properties of the lens are first established, and then maintained at the cellular level throughout the lifetime of an individual. Central to this understanding is the fact that the lens has a microcirculation system that generates a flux of ions and water that circulates through the lens. By supporting ionic and metabolic homeostasis in the lens, the system actively maintains lens transparency, and by regulating the steady state water content of the lens, controls the two key parameters, lens geometry and the gradient of refractive index, which determine the refractive properties of the lens. Thus, water transport is emerging as the critical parameter that links the transparency and refractive properties of the lens at the cellular level, and highlights the need to study how age-related changes in water transport result in presbyopia and cataract, the leading causes of refractive error and blindness in the world today.