Crystalline lens
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Santosh Kumar Paidi,1 Qinrong Zhang,2,3 Yuhan Yang,2 Chun-Hong Xia,1,4 Na Ji,2,3,5,6,7 and Xiaohua Gong1,4,7 1School of Optometry, University of California, Berkeley, California 94720, USA 2Department of Physics, University of California, Berkeley, California 94720, USA 3Department of Molecular and Cell Biology, University of California, Berkeley, CA 94720, USA 4Vision Science Program, University of California, Berkeley, California 94720, USA 5Helen Wills Neuroscience Institute, University of California, Berkeley, CA 94720, USA
6Molecular Biophysics and Integrated Bioimaging Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA, CC BY-SA 4.0
The lens, or crystalline lens, is a transparent biconvex structure in most land vertebrate eyes. Relatively long, thin fiber cells make up the majority of the lens. These cells vary in architecture and are arranged in concentric layers. New layers of cells are recruited from a thin epithelium at the front of the lens, just below the basement membrane surrounding the lens. As a result the vertebrate lens grows throughout life. The surrounding lens membrane referred to as the lens capsule also grows in a systematic way, ensuring the lens maintains an optically suitable shape in concert with the underlying fiber cells. Thousands of suspensory ligaments are embedded into the capsule at its largest diameter which suspend the lens within the eye. Most of these lens structures are derived from the epithelium of the embryo before birth. Along with the cornea, aqueous, and vitreous humours, the lens refracts light, focusing it onto the retina. In many land animals the shape of the lens can be altered, effectively changing the focal length of the eye, enabling them to focus on objects at various distances. This adjustment of the lens is known as accommodation (see also below). In many fully aquatic vertebrates, such as fish, other methods of accommodation are used, such as changing the lens's position relative to the retina rather than changing the shape of the lens. Accommodation is analogous to the focusing of a photographic camera via changing its lenses. In land vertebrates the lens is flatter on its anterior side than on its posterior side, while in fish the lens is often close to spherical. Accommodation in humans is well studied and allows artificial means of supplementing our focus, such as glasses, for correction of sight as we age. The refractive power of a younger human lens in its natural environment is approximately 18 dioptres, roughly one-third of the eye's total power of about 60 dioptres. By 25 years of age the ability of the lens to alter the light path has reduced to 10 dioptres and accommodation continues to decline with age.
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