A clear view

eye lens is composed of cells of only one type — lens fibre cells — which derive from a monolayer of undifferentiated epithelial cells covering the anterior surface of the lens. In the bulk of the lens, these cells are arranged in concentric sheets, much like the layers of an onion. In order to minimise light scatter in the lens, the fibre cells have to be precisely aligned. The scanning electron micrograph at top right shows the parallel arrangement of the ribbon-like fibre cells in the bovine lens. Each diagonal 'stripe' represents part of an elongated fibre cell (viewed en face from the lens surface). In cross-section, lens fibre cells appear as flattened hexagons. At their narrow edges, the cells are interlocked with their neighbours by extensive 'ball-and-socket' structures, or interdigitations, as shown in the micrograph at bottom right. In this image, the lens has been fractured perpendicular to the surface, revealing the narrow edges of fibre cells in subsequent layers of the lens. The interdigitations prevent the cells from moving against each other during the lens shape changes that occur while focusing. Such movement could disrupt the highly ordered fibre cell arrangement in the lens, leading to its opacification — the condition known as cataract. The image on the left shows the intricate pattern of membrane folds found typically on the surfaces of mature lens fibre cells. Like the interdigitations, these folds are believed to help interlock the fibre cells and maintain lens organisation. During lens cell differentiation, cells undergo an extreme elongation and degrade all intracellular organelles, producing the optically homogeneous fibre cell cytoplasm crucial to lens transparency. Disrupted clearance of organelles is a feature of some forms of congenital cataract. For more details, see Dahm et al.,