The crystalline lens (lens) is a transparent structure with a roughly spherical, biconvex shape with a more pronounced posterior curvature and a flatter anterior surface. It is located behind the iris and its anterior surface is bathed by the aqueous humour present in the posterior chamber. A very small part of the lens is in contact with the iris and exerts a light, physiological pressure on this latter giving the iris a slight convexity towards the exterior. In the case of luxation, subluxation or removal of the lens, the loss of pressure on the iris leads to instability of the iris, a condition which is called iridodonesis (tremulous iris).
The lens is suspended by the zonular fibres of the ciliary body around its equator. Posteriorly it is in contact with the anterior surface of the vitreous and lies in a saucer-shaped depression called the patellar fossa of vitreous. The anterior surface of the vitreous has a confining membrane that is attached to the posterior capsule of the lens along an annular line called the hyaloid capsular ligament of Wiegert. In a slit lamp examination, this ligament is identified as Egger’s line. The vitreous is firmly attached to the internal surface of the posterior lens capsule except for a small area at the posterior pole where there is the contact with the hyaloid canal.
The centre of the anterior surface is called the anterior pole, while that of the posterior surface is called the posterior pole. The axis is defined as the line that joins the anterior and posterior poles, the meridians are defined as the lines that connect one pole with the other, passing through the surface of the lens, while the equator is represented by the circumference of the lens which has an irregular, indented shape. The lens has no innervation and, after birth, no blood supply either. The main purpose of the lens is to act as a refractory organ in order to focus images correctly on the fundus of the eye.
Microscopically the lens has three main structures: an enveloping basement membrane called the capsule, the anterior epithelium and the cellular elements, called the lens substance (or lens fibre substance). The capsule is an elastic structure that under a microscope appears homogeneous and is intensely stained by Periodic acid Schiff (PAS). It can be considered a true epithelium basement membrane of the lens and regulates the shape of the lens thanks to its elasticity, enabling the insertion of the zonular fibres that make up the suspensory ligament of Zinn. It is formed of collagen fibres, polysaccharides and insoluble proteins and, although not containing elastic fibres, it has elastic properties, such that its rupture causes changes in the lens profile. The lens substance is formed of fibrils organized in laminae and has a basement membrane. During life the capsule continues to thicken, forming new fibrils adjacent to the membrane of the epithelium cells, in this way causing greater compression on the more external fibres. Only these epithelial cells are able to produce new capsular material. The thickness of the capsule differs greatly depending on the part of the lens: normally it is thickest in the region of the anterior pole, where it measures 50-70 µm, while posteriorly it measures 2 - 4 µm and at the equator 8-12 µm.
The epithelium of the lens is located below the capsule, but only at the anterior part and some of the equator of the lens. It is a layer of epithelial cells that is strongly adherent to the overlying capsule. Posteriorly, the epithelium is present only during embryonic life and contributes to form the primary embryonic fibres of the lens, and then disappears from the posterior capsule of the lens after birth. The epithelial cells of the axial part of the lens are wide and relatively flattened and appear polygonal in sagittal section. Moving towards the periphery, the epithelial cells appear smaller and more cuboidal. At the equator, they tend to become cylindrical and small and then gradually become higher and columnar; in this zone they are positioned more obliquely with respect to the anterior capsule, reaching a rotation of about 90° with respect to the epithelium of the lens. In this location the nuclei stretch anteriorly in the cortex contributing to giving the appearance commonly described as the “lens bow”. At the equator the epithelial cells are elongated anteriorly and posteriorly to form processes called lens fibres. As soon as new fibres are generated, the older ones are pushed towards the interior of the lens.