Aging-US: Age-related changes in eye lens biomechanics09-08-2021
Aging-US published a Special Collection on Eye Disease which included "Age-related changes in eye lens biomechanics, morphology, refractive index and transparency" which reported that life-long eye lens function requires an appropriate gradient refractive index, biomechanical integrity and transparency.
The authors conducted an extensive study of wild-type mouse lenses 1-30 months of age to define common age-related changes. Biomechanical testing and morphometrics revealed an increase in lens volume and stiffness with age. Their results suggest similarities between murine and primate lenses and provide a baseline for future lens aging studies.
Figure 14. Wild-type mouse lenses in the B6 genetic background showed increased volume, nucleus size and overall stiffness, changes in cell morphology and microstructure along with appearance of anterior, cortical and ring cataracts with age. Lens volume and nucleus volume increase steadily with age. The shape and size of lens fiber cells become more disorganized in aged lenses. With age, mouse lenses develop anterior and cortical cataracts. Anterior cataracts are correlated with detachment of the anterior epithelial cells from the underlying fiber cells. Cortical ring opacities in the aged lenses are due to a zone of compaction in the cortical fiber cells leading to an optical discontinuity. While there is a steady increase in lens stiffness with age, resilience, or lens elasticity, is only increased in very old lenses. The maximum refractive index at the center of the lens (nucleus) increases rapidly until 6 months of age and reaches a plateau at 6 months. Lens capsule thickness and fiber cell width remain steady after 4 months of age, while epithelial cell area increases slightly between 4 and 12 months of age. Cartoons not all drawn to scale.
Dr. Velia M. Fowler and Dr. Catherine Cheng said, "The eye lens is required for fine focusing of light onto the retina to form a clear image, and the function of the lens is intimately tied to its shape, biomechanical properties, transparency and refractive index."
The eye lens is required for fine focusing of light onto the retina to form a clear image. It has long been known that age-related changes in these lens properties lead to two major lens pathologies, cataracts and presbyopia. Presbyopia is caused by a reduction in the lens' ability to change shape during focusing (accommodation), and, by extension, the need for reading glasses. Mice offer an opportunity to investigate changes in lens morphometrics, stiffness, transparency and refractive properties with age in a relatively shortened period of time. Little is known about the morphological, mechanical, refractive and cellular changes that occur with advanced age in the lens. The authors demonstrate that age-related changes in mouse lenses mimic some aspects of aging in human lenses.
The Fowler/Cheng Research Team concluded in their Aging-US Research Output, "the increases in lens size and nucleus size are correlated with increase stiffness with age. The addition of new fiber cells at the lens periphery becomes disordered with age, but this does not appear to impact lens biomechanical properties. Cataracts in aged lenses can be due to cell structural abnormalities, including incomplete suture closure, collapse of the lens epithelial cell layer into the suture gap and loss of epithelial-fiber cell attachments and compaction of the cortical lens fiber cells forming a circumferential light scattering ring. GRIN is present in the lens from 2 weeks of age and continues to increase until about 6 months of age, after which the maximum refractive index remains stable. The increase in the area of highest refractive index at the center of the lens is directly correlated with the increase in lens nucleus size, suggesting nuclear compaction drives the maximum GRIN. Whether there is a common molecular mechanism that drives changes in all the measured parameters remains unknown, but further biochemical and cell morphology studies will be needed to determine how subcellular aging affects the whole tissue. Thus, our study provides a baseline for future studies of lens aging by providing quantitative measurements of key parameters and identifying common age-related changes in the overall tissue and in individual cells"
Full Text - https://www.aging-us.com/article/102584/text
Launched in 2009, Aging-US publishes papers of general interest and biological significance in all fields of aging research as well as topics beyond traditional gerontology, including, but not limited to, cellular and molecular biology, human age-related diseases, pathology in model organisms, cancer, signal transduction pathways (e.g., p53, sirtuins, and PI-3K/AKT/mTOR among others), and approaches to modulating these signaling pathways.