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.