The lens is part of the anterior segment of the human eye. In front of the lens is the iris, which regulates the amount of light entering into the eye. The lens is suspended in place by the suspensory ligament of the lens, a ring of fibrous tissue that attaches to the lens at its equator and connects it to the ciliary body. Posterior to the lens is the vitreous body, which, along with the aqueous humor on the anterior surface, bathes the lens. The lens has an ellipsoid, biconvex shape. The anterior surface is less curved than the posterior. In the adult, the lens is typically circa 10 mm in diameter and has an axial length of about 4 mm, though it is important to note that the size and shape can change due to accommodation and because the lens continues to grow throughout a person's lifetime.
The lens has three main parts: the lens capsule, the lens epithelium, and the lens fibers. The lens capsule forms the outermost layer of the lens and the lens fibers form the bulk of the interior of the lens. The cells of the lens epithelium, located between the lens capsule and the outermost layer of lens fibers, are found only on the anterior side of the lens. The lens itself lacks nerves, blood vessels, or connective tissue.
The lens capsule is a smooth, transparent basement membrane that completely surrounds the lens. The capsule is elastic and is composed of collagen. It is synthesized by the lens epithelium and its main components are Type IV collagen and sulfated glycosaminoglycans (GAGs). The capsule is very elastic and so allows the lens to assume a more globular shape when not under the tension of the zonular fibers (also called suspensory ligaments), which connect the lens capsule to the ciliary body. The capsule varies from 2 to 28 micrometres in thickness, being thickest near the equator and thinnest near the posterior pole.
The lens epithelium, located in the anterior portion of the lens between the lens capsule and the lens fibers, is a simple cuboidal epithelium. The cells of the lens epithelium regulate most of the homeostatic functions of the lens. As ions, nutrients, and liquid enter the lens from the aqueous humor, Na+/K+-ATPase pumps in the lens epithelial cells pump ions out of the lens to maintain appropriate lens osmotic concentration and volume, with equatorially positioned lens epithelium cells contributing most to this current. The activity of the Na+/K+-ATPases keeps water and current flowing through the lens from the poles and exiting through the equatorial regions.
The cells of the lens epithelium also serve as the progenitors for new lens fibers. It constantly lays down fibers in the embryo, fetus, infant, and adult, and continues to lay down fibers for lifelong growth.
Pattern of lens fibers (anterior and lateral aspect)
The lens fibers form the bulk of the lens. They are long, thin, transparent cells, firmly packed, with diameters typically 4–7 micrometres and lengths of up to 12 mm long. The lens fibers stretch lengthwise from the posterior to the anterior poles and, when cut horizontally, are arranged in concentric layers rather like the layers of an onion. If cut along the equator, it appears as a honeycomb. The middle of each fiber lies on the equator. These tightly packed layers of lens fibers are referred to as laminae. The lens fibers are linked together via gap junctions and interdigitations of the cells that resemble "ball and socket" forms.
The lens is split into regions depending on the age of the lens fibers of a particular layer. Moving outwards from the central, oldest layer, the lens is split into an embryonic nucleus, the fetal nucleus, the adult nucleus, and the outer cortex. New lens fibers, generated from the lens epithelium, are added to the outer cortex. Mature lens fibers have no organelles or nuclei.
Development of the human lens begins at the 4 mm embryonic stage. Unlike the rest of the eye, which is derived mostly from the neural ectoderm, the lens is derived from the surface ectoderm. The first stage of lens differentiation takes place when the optic vesicle, which is formed from outpocketings in the neural ectoderm, comes in proximity to the surface ectoderm. The optic vesicle induces nearby surface ectoderm to form the lens placode. At the 4 mm stage, the lens placode is a single monolayer of columnar cells.
As development progresses, the lens placode begins to deepen and invaginate. As the placode continues to deepen, the opening to the surface ectoderm constricts and the lens cells forms a structure known as the lens vesicle. By the 10 mm stage, the lens vesicle has completely separated from the surface ectoderm.
After the 10 mm stage, signals from the developing neural retina induces the cells closest to the posterior end of the lens vesicle begin to elongate toward the anterior end of the vesicle. These signals also induce the synthesis of crystallins. These elongating cells eventually fill in the lumen of the vesicle to form the primary fibers, which become the embryonic nucleus in the mature lens. The cells of the anterior portion of the lens vesicle give rise to the lens epithelium.
Additional secondary fibers are derived from lens epithelial cells located toward the equatorial region of the lens. These cells lengthen anteriorly and posteriorly to encircle the primary fibers. The new fibers grow longer than those of the primary layer, but as the lens gets larger, the ends of the newer fibers cannot reach the posterior or anterior poles of the lens. The lens fibers that do not reach the poles form tight, interdigitating seams with neighboring fibers. These seams are readily visible and are termed sutures. The suture patterns become more complex as more layers of lens fibers are added to the outer portion of the lens.
The lens continues to grow after birth, with the new secondary fibers being added as outer layers. New lens fibers are generated from the equatorial cells of the lens epithelium, in a region referred to as the germinative zone. The lens epithelial cells elongate, lose contact with the capsule and epithelium, synthesize crystallin, and then finally lose their nuclei (enucleate) as they become mature lens fibers. From development through early adulthood, the addition of secondary lens fibers results in the lens growing more ellipsoid in shape; after about age 20, however, the lens grows rounder with time and the iris is very important for this development.
Several proteins control the embryonic development of the lens: among these, primarily, PAX6, considered the master regulator gene of this organ. Other effectors of proper lens development include the Wnt signaling components BCL9 and Pygo2.
In many aquatic vertebrates, the lens is considerably thicker, almost spherical, to increase the refraction. This difference compensates for the smaller angle of refraction between the eye's cornea and the watery medium, as they have similar refractive indices. Even among terrestrial animals, however, the lens of primates such as humans is unusually flat.
In reptiles and birds, the ciliary body touches the lens with a number of pads on its inner surface, in addition to the zonular fibres. These pads compress and release the lens to modify its shape while focusing on objects at different distances; the zonular fibres perform this function in mammals. In fish and amphibians, the lens is fixed in shape, and focusing is instead achieved by moving the lens forwards or backwards within the eye.
In cartilaginous fish, the zonular fibres are replaced by a membrane, including a small muscle at the underside of the lens. This muscle pulls the lens forward from its relaxed position when focusing on nearby objects. In teleosts, by contrast, a muscle projects from a vascular structure in the floor of the eye, called the falciform process, and serves to pull the lens backwards from the relaxed position to focus on distant objects. While amphibians move the lens forward, as do cartilaginous fish, the muscles involved are not homologous with those of either type of fish. In frogs, there are two muscles, one above and one below the lens, while other amphibians have only the lower muscle.
In the most primitive vertebrates, the lampreys and hagfish, the lens is not attached to the outer surface of the eyeball at all. There is no aqueous humor in these fish, and the vitreous body simply presses the lens against the surface of the cornea. To focus its eyes, a lamprey flattens the cornea using muscles outside of the eye and pushes the lens backwards.