Biological glass: structural determinants of eye lens transparency

Abstract

The purpose of the lens is to project a sharply focused, undistorted image of the visual surround onto the neural retina. The first pre-requisite, therefore, is that the tissue should be transparent. Despite the presence of remarkably high levels of protein, the lens cytosol remains transparent as a result of short-range-order interactions between the proteins. At a cellular level, the programmed elimination of nuclei and other light-scattering organelles from cells located within the pupillary space contributes directly to tissue transparency. Scattering at the cell borders is minimized by the close apposition of lens fibre cells facilitated by a plethora of adhesive proteins, some expressed only in the lens. Similarly, refractive index matching between lens membranes and cytosol is believed to minimize scatter. Refractive index matching between the cytoplasm of adjacent cells is achieved through the formation of cellular fusions that allow the intermingling of proteins. Together, these structural adaptations serve to minimize light scatter and enable this living, cellular structure to function as 'biological glass'.

Chromatin remodeling enzyme Snf2h regulates embryonic lens differentiation and denucleation

“…Retention of lens fiber cell nuclei, normal mitochondrial degradation, and disrupted expression of autophagy regulatory proteins in the Snf2h cKO lens Lens fiber cell denucleation and the degradation of other subcellular organelles, including mitochondria and ER, mark the terminal differentiation of lens fibers (Bassnett et al, 2011). The persistence of nuclei in Snf2h mutant lens fibers (Fig.…”

Deletion of Autophagy-related 5 (Atg5) and Pik3c3 Genes in the Lens Causes Cataract Independent of Programmed Organelle Degradation

“…During embryogenesis, primary fiber cells are differentiated from epithelial cells at the posterior surface of the lens vesicle, whereas secondary fiber cells are subsequently differentiated from newly proliferated epithelial cells at the edges of the anterior epithelium. In the process of terminal differentiation of these fiber cells, membrane-bound organelles such as nuclei, the endoplasmic reticulum (ER), 2 and mitochondria are degraded, forming the organelle-free zone (OFZ) (3)(4)(5). Organelle degradation begins during the embryonic period in primary fiber cells and continues throughout life in secondary fiber cells.…”

Biophysical chemistry of the ageing eye lens

“…The lens sits immediately behind the iris where it can focus incoming light to form a clear image on the retina. The lens exhibits a high transparency and a high refractive index and has flexibility which allows the attached ciliary muscles to alter its curvature (Banh et al 2006;Bassnett et al 2011). As a result, the eye can adjust the focal length of the lens to form a real image of objects at various distances with high quality and minimal light scattering (Purnyn 2013;Trokel 1962).…”