Associations Among the Gradient Refractive Index of the Lens, Axial Length, and Refractive State in the Developing Murine Eye

PURPOSE. To systematically characterize changes in lens geometry, gradient refractive index (GRIN) profile, and refractive state in developing murine eye. METHODS. Optical measurements using synchrotron radiation-based x-ray phase tomography were conducted on murine samples from 1 week to 18 months of age. Lens geometric parameters and three-dimensional GRIN distributions on a micrometer scale were extracted/reconstructed using a custom-developed program. Ocular refractive power and refractive error were calculated using ray-tracing models. RESULTS. The GRIN profile was evident from 2 weeks of age, with the peak index located close to the anterior pole (0.44 mm), and a posterior shift was seen to adult stage (1.20 mm). A substantial increase in the maximum refractive index (from 1.50 to 1.54) occurred at 4 to 12 weeks and remained relatively unchanged thereafter. The axial length and the lens thickness increased rapidly during the early postnatal stage and plateaued after approximately 3 months. The decrease of anterior segment thickness (from 1.06 mm to 1.03 mm at 3 to 4 weeks and from 1.20 mm to 1.18 mm at 6 to 8 weeks) was associated with increased optical power. The period of rapid increase in the maximum index coincided with a decrease in lens posterior segment thickness (from 1.03 mm to 1.20 mm at 4 to 6 weeks) and axial elongation, whereas a period of minimal change in maximum index was associated with an anterior shift of the lens nucleus and minimal axial growth. CONCLUSIONS. These findings suggest that geometrical and optical changes are related, contributing to a coordinated myopic system of the murine eye. The early postnatal stage is a critical period for developing a functional optical visual system.