Corpus: Lens

The lens of the human eye is a crystalline body, strongly convex on both sides, located in the bulbus oculi. Its function is to refract the light hitting the eye in order to project a sharp image onto the retina, where it is converted into electrical signals. The lens of the human eye is therefore a converging lens, as it bundles the light and transmits it to the retina.

The lens is located behind the iris and in front of the corpus vitreum (vitreous body). It is suspended in the centre of the circular corpus ciliare (ciliary body) with the help of the fibrae zonulares (zonular fibres). Topographically, a distinction is made between an anterior and posterior lens pole (polus anterior et posterior lentis) and the lateral lens equator, into which the zonular fibres radiate.

The anterior lens pole points in the direction of the pupil. It is surrounded by the iris in a ring shape. The posterior pole of the lens is embedded in a depression in the vitreous body, the hyaloid fossa. Between the two structures is a space filled with aqueous humour, the Berger space. This space is enclosed in a ring by the hyaloideocapsular ligament (Wieger's ligament), which runs from the back of the lens to the anterior vitreous limiting membrane, thus connecting the two structures.

In the newborn, the lens has a diameter of approximately 6.5 mm and a thickness of 3.5 mm. In adults, the diameter is approx. 9.0 mm and the thickness approx. 5.0 mm.

Vascular supply[Bearbeiten]

The lens of the eye has no blood vessels. The lens tissue is supplied with nutrients by diffusion from the aqueous humour.

Several layers of the crystalline lens can be distinguished histologically from the outside to the inside:

• Lens capsule (capsula lentis)
• Lens cortex (cortex lentis)
• Lenticular nucleus (nucleus lentis)

The lens capsule is a basal lamina about 14 to 21 µm thick in the anterior section, which completely encloses the lens.

Under the anterior lens capsule is the lens epithelium, which is composed of flat to isoprismatic lens epithelial cells (LECs). In the pre-equatorial region of the lens, they remain capable of dividing throughout life. Here they develop into lens fibre cells (LFCs) through differentiation, which make up the actual body of the lens and account for more than 99% of the lens cells.

Physiologically, there is no lens epithelium on the posterior surface of the lens - this is where the lens capsule comes into direct contact with the lens cortex.

The lens of the human eye is of ectodermal origin. The primary structure is an ectoderm thickening called the lens placode. It develops through cell proliferation and invagination via the lens dimple to the lens vesicle, which is displaced into the eye cup.

When the lens vesicle separates from the ectoderm, the apical cell poles of the cells are turned inwards so that the original basement membrane of the ectoderm becomes the outer boundary of the lens (lens capsule). The cells under the anterior part of the capsule remain as single-layered cylindrical cells and form the lens epithelial cells, which divide and move in a horizontal direction.

The tissue of the eye lens is largely made up of special, water-soluble structural proteins that increase the refractive power of the lens without reducing its transparency. These so-called crystallins are produced by the lens fibre cells.

The normal lens of an adult contains approx. 65% water - significantly less than the surrounding media. As a result, the lens has a refractive index that differs from the aqueous humour and vitreous humour. There is an ion concentration gradient between the inside of the lens and the aqueous humour. It is maintained by an active potassium pump, which is mainly located in the membrane of the lens epithelial cells.

The transparency of the lens is based on a closely regulated ion and water balance. The "pump-leak" mode of transport refers to the active transfer of ions from the aqueous humour into the lens and passive diffusion via the dorsal lens capsule. The water content of the lens and aqueous humour is in equilibrium.

The healthy lens is the only component of the dioptric apparatus that can adapt its refractive power to the incoming light. It is attached to the side walls of the bulbus oculi by the so-called zonule fibres. The contraction of the ciliary muscle causes the zonular fibres to relax and the lens curves due to its inherent elasticity. The light is therefore refracted more strongly. This process occurs during near vision. As nearby objects cast light onto the eye at a different angle to distant objects, the light must be refracted more strongly in order to produce a sharp image on the retina. This mechanism works in reverse in the case of distance accommodation. The ciliary muscle then relaxes and the zonule fibres flatten the lens by pulling.

The normal accommodative power is around 19 dpt. This value can drop considerably with age, resulting in long-sightedness or presbyopia. This is caused by a decrease in the elasticity of the lens and therefore a lower accommodative power.

The water content of the lens decreases with age. In addition, the concentration of albuminoid, an insoluble lens protein, increases. This results in the loss of elasticity and transparency typical of old age.

The most important disease of the lens of the eye resulting from these degenerative changes is cataract. One form of cataract is the yellowish nuclear cataract, also known as nuclear sclerosis. It describes the clouding process from the inside of the lens. It is found in around 95% of all people over the age of 65.

• Image source podcast: © David Clode / Unsplash