Lens

The lens (Latin: lens) is a double convex, circular structure located anterior to the vitreous body and posterior to the iris. Together with the cornea, the lens transmits light to the retina. By the ability to change its shape, the lens selectively transmits light and adjusts the focus of the eye so that the retina can form a sharp image. This process is known as accommodation.

Structure of the lens

The lens is encapsulated structure swimming in aqueous humor. The lens is divided into anterior and posterior surfaces by the outer margin of the lens, also known as the equator. The central points within the surfaces are called the poles. The imaginary line connecting the poles is called the axis of the lens. The way the lens is convex on the anterior surface is less steep than its posterior convex. The equatorial diameter of the lens is approximately 9-9,5 mm, while the thickness of the lens is 4 mm. The lens tends to get flatter during the first decade of life.

The lens is avascular and has no nerve fibers. No other structures are preset as the lens needs to keep its transparency. The lens surface works as a barrier to protect from invasion by cells or immune systems elements. Due to the absence of the blood vessels, the lens gets nutrients from the aqueous humor. 

Layers of the lens

The lens has three parts: the lens capsule, the lens epithelium, and the lens fibers. The elastic capsule surrounds the outer surface of the lens. The lens epithelium lies below the capsule. The lens fibers are responsible for the substance of the lens. The majority of the lens consists of the lens fibers, which anteriorly are covered by a single layer of epithelium and are adjective by the outer lens capsule.

Lens capsule

The lens capsule is an elastic basement membrane that surrounds the whole lens. The lens capsule originates from the epithelial cells of the anterior lens. The thickness of the lens capsule is different based on its position on the surface of the lens and age. The lens capsule is the thickest on the anterior and posterior surfaces that are close to the equator, while the thinnest area is at the posterior pole. 

In adults, the lens capsule at the anterior pole is around 11-15 micrometers thick, the equator is 7.2 micrometers, but the posterior pole is 4-9 micrometers. The thickness of the lens capsule at the anterior pole increases with age, while the thickness at the posterior pole and equator does not change with age. The inner surface of the anterior part of the lens capsule is associated with the lens epithelium, but the posterior part joins the lens fibers. The lens epithelium forms the basement membrane on the anterior part, while the lens fibers – on the posterior surface. 

The lens capsule consists of type IV collagen, so the lens capsule can be stretched up to around 60 percent of its circumference. The lens capsule is responsible for the shape of the lens as an answer reaction to the zonular fibers being pulled during accommodation. The lens is surrounded by the ciliary processes. 

Small ligament-like bands support the lens in its place. The band extends from the ciliary processes to the equator of the lens. The fibers of this band are called the zonular fibers (zonule of Zinn), but together the fibers are known as the suspensory ligament of the lens. The fibers emerge and form bundles. The larger bundles run straight and towards the lens capsule on the anterior part of the lens. These bundles create the anterior zonular sheet. 

On the other hand, the smaller bundles run posteriorly and form the posterior zonular sheet. Upon reaching the lens, the zonular fibers turn into fine fibers and get embedded in the outer part of the lens capsule. The suspensory ligament of the lens takes part in changing the shape of the lens through the accommodation of the eye.

Lens epithelium

The anterior surface of the lens is covered by one-layered cuboidal epithelial cells - lens epithelium. The epithelial cells are elongated in the anterior and posterior directions. The lens epithelium is present only on the anterior surface. At the level of the equator, the cells elongate and become columnar cells arranged in meridional rows. It is also at the level of the equator where the epithelial cells turn into the lens fiber

Based on the position, the lens epithelium has two different functions. The cells on the equator keep dividing all the time and differentiate into the lens fibers, while more centrally placed cells take part in secreting capsular material and transporting substances from the aqueous humor to the interior area of the lens.

Lens fibers

The lens fibers form the main mass of the lens. The lens fibers are made by differentiation of the lens epithelium at the equator. The lens fibers located near the surface at the equator are nucleated, while the deeper fibers have no nuclei. Due to the different structures and compositions of the fibers, the lens fibers have two zones: a softer cortical zone and a firmer central zone. The cortical zone is made of younger fibers, while the central zone – older nuclei. 

The lens fibers are organized meridionally from the posterior to the anterior surface of the lens. The earliest differentiated lens fibers are found in the center, but the later differentiated ones – in the outer part or cortex of the lens. The lens fibers go through the equator and end at sutures on the anterior and posterior surfaces of the lens. The sutures eradiate away from the poles to the equator. The sutures are junctions between terminating lens fibers

The lens fibers that originate near the central axis of the lens anteriorly end posteriorly on a suture near the periphery and the other way around. The lens fibers contain a few small vesicles, microtubules, microfilaments. The lens fibers are so tightly packed that there is almost no intracellular space. The lens fibers are joined by their adjacent plasma membranes. During their development, the lens fibers get rid of their nuclei, and their organelles start to produce lens proteins - crystallins. Crystallins, together with the absence of blood vessels and other structures, maintain the lens transparent.

Function of the lens

Together with the cornea, the lens transmits light to the retina. By the ability to change its shape, the lens selectively transmits light and adjusts the focus of the eye so that the retina can form a sharp image. This process is known as accommodation.

The suspensory ligament of the lens takes part in changing the shape of the lens through the accommodation of the eye. When the eye is in a resting position (looking far away), the ciliary body keeps the tension on these zonular fibers. As soon as the focus is changed and the eye is out of the resting position, the ciliary muscle contracts to allow the accommodation of the eye when looking at close objects and causes the suspensory ligament of the lens to relax. 

The meridionally positioned fibers pull the choroid and ciliary body forward, while the circular fibers move the ciliary body inward. These movements relieve the tension on the zonule fibers resulting in the elastic lens assuming a more globular shape. At the same time, the sphincter pupillae muscle contracts and cause the pupil to become smaller, allowing only the light rays going through the thickest part of the lens to reach the retina. 

These changes allow an increase in refractive power. The lens can change its dioptric power by changing its shape. This change makes the refractive power of the lens more flexible. Even though the cornea has the highest refractive power, the lens gives around 15 diopters that make the lens an important structure for the maintenance of vision. Its ability to change the dioptric power allows distant and near objects to be focused on the retina.

The lens not only transmits light but also stops UVA radiation from reaching and damaging the retina. UVB is stopped and blocked by the cornea. After birth, the lens has no colour, and it transmits all wavelength types. During lifetime the amount of UVB transmitted decreases.