The Mammalian Ocular Lens in Focus: Development, Anatomy, Physiology, Transparency, Biomechanics, and Age-related Challenges

Summary

The function of the ocular lens to focus light onto the retina depends on tissue transparency, biomechanics, and refractive index. This review highlights the three-dimensional architecture of the lens and discusses the complex mechanisms that maintain tissue homeostasis.

Abstract

The lens is a transparent, ellipsoid organ in the anterior chamber of the eye that focuses light onto the retina. Composed of epithelial cells and fiber cells, this 3-dimensional tissue is highly organized to ensure a clear light path and utilizes multiple strategies to establish and maintain a high refractive index and transparency. In humans, the lens undergoes shape changes to focus light from different distances in a process known as accommodation, and these elastic and biomechanical properties of the lens rely on tissue, cellular, and molecular mechanisms. Cataracts, defined as any opacity in the normally transparent lens, remain the leading cause of blindness in the world. Presbyopia, due to age-related changes in lens stiffness, has been linked to accommodative decline and the need for reading glasses. This review will explore mammalian lens development and lifelong growth, crystallins and cytoskeletal proteins, cell-cell communication and microcirculation, lens biomechanics, and the challenges and novel methods for studying this unique tissue.

Introduction

The ocular lens is, at first glance, a relatively simple tissue composed of two cell types, epithelial cells and fiber cells. The bulk mass of the lens is made up of fiber cells, and there is a monolayer of epithelial cells covering the anterior hemisphere¹ (Figure 1A). The entire tissue is encased in a thin basement membrane, called the lens capsule, that is primarily made of collagen IV and laminin, as described in a previous review². Lacking a blood supply and nerves, the lens is suspended in the anterior chamber of the eye by elastic zonular fibers (aka zonules) connecting the lens capsu....