Anatomy of the Lens

Done by: Othman Al-Abbadi, M.D

Introduction

• It is a transparent, biconvex structure situated behind the iris and the pupil and in front of the vitreous body.

• Anteriorly (radius of curvature=10mm) less convex than posteriorly (radius of curvature=6mm).

• The center points on its anterior and posterior surfaces are referred to as the anterior and posterior poles, respectively; a line joining the poles forms the axis of the lens; the marginal circumference of the lens is called the equator.

• In the adult the lens measures approximately 9-10 mm in diameter (6.5 mm at birth) and 4 mm thick (3.5mm at birth & 5mm at extreme age).

• Its weight inc with age from 135mg to 255mg.• The equator of the lens is encircled by the ciliary

processes of the ciliary body and lies 0.5 mm away from them.

• The lens, which has considerable flexibility, is kept in position by the suspensory ligaments.

Ant. pole

Post. pole

Axis of the lens

Equator

10 mm

4 mm

• The lens contributes about 15-16 diopters to the total refracting power of the eye.

• The importance of the lens is that it can change its dioptric power.

• The range of accommodative power is reduced with age, being 14-16 at birth, 7-8 at 25, & 1-2 at 50 years of age.

• The lens has a refractive index of about 1.36 in the periphery and l.4 in the center.

Structure

• The lens is made up of:1. Capsule,2. Epithelium, 3. Fibers.

Capsule • It is a thin, transparent, hyaline elastic membrane

that envelops the entire lens.• Thicker ant than post.• Thickest close to the equator (20 μm), and

thinnest at the posterior pole (~3 μm).• The inner surface of the anterior part of the

capsule is in direct contact with the lens epithelium, while posteriorly in contact with the superficial lens fiber cells.

• The thick basement membrane is formed by the lens epithelium anteriorly and by the superficial lens fibers posteriorly .

• Under the electron microscope it is seen to consist of 40 lamellae. Each lamella resembles a unit basal lamina and measures about 40nm.

• It consists of type IV collagen fibrils embedded in a matrix of glycoproteins and sulfated glycosaminoglycan.

• The elastic capsule can be stretched up to 60% of its circumference without tearing.

• It functions in molding the shape of the lens in response to the pull of the zonular fibers during accommodation, & as a diffusion barrier to the large colloidal particles.

Epithelium

• It’s a cuboidal unicellular and lies beneath the Anterior capsule.

• At the equator, these cells elongate and form columnar cells, which become arranged in meridional rows.

• Functions;– Centrally transport of substances between the

aqueous humor & the lens interior.– Equatorially mitotic division & differentiation into

lens fibers.

Fibers

• It constitute the main mass of the lens.• The fibers are formed by the multiplication

and differentiation of the lens epithelial cells at the equator.

• As the basal part of the cell elongates, the process moves along the internal surface of the capsule in a posterior direction.

• As the apical part of the cell elongates, it slips beneath the internal surface of adjacent lens cells.

• At first, the nucleus remains intact; but later it fragments and disappears (This process continues throughout life).

• Previous generations of cells are repeatedly pushed into the lens substance. As the cell progressively elongates anteriorly, the nucleus moves anteriorly, so that it takes up a position anterior to the nuclei of the more superficial cells. This anterior movement of the nuclei as the fibers pass deeper produces the nuclear pattern known as the lens bow.

• The lens fiber is a hexagonal prism in cross-section.

• The fibers run meridionally from the posterior to the anterior lens surface in U-shape.

• The earliest formed fibers are those in the center or nucleus of the lens; the later fibers form the outer or cortex of the lens.

• The lens nucleus is divided into:– Embryonic nucleus: up to 3 months of

gestation,– Fetal nucleus: 3 months of gestation till

birth,– Infantile nucleus: birth to puberty,–Adult nucleus: after puberty

• In the fetus, the ends of opposing lens fibers in the same layer abut (border) in a manner producing patterns known as sutures.

• The anterior suture is an erect Y-shape, while the posterior is an inverted Y. As the lens increases in size, the lens fibers are unable to stretch the antero-posterior distance, so that progressively more complicated suture patterns are formed.

• The size of the embryonic and fetal nucleus remains constant, while that of the adult nucleus is always increasing. The area surrounding the adult nucleus, containing the recently formed nucleated fibers is referred to as the lens cortex

• The fibers are tightly packed, there being very little intercellular space. The lens fibers are held together by the interlocking of their adjacent plasma membranes as ball-and-socket type of interdigitation.

• It is interesting to note that the interdigitations are less complicated in the superficial zones of the lens, and this may permit molding of the lens shape during accommodation.

• Lens fibers exhibit numerous gap junctions which may explain how deep lens fibers can survive some distance from the surface, and away from a source of nourishment

• During development, the lens fiber cells lose their nuclei and the cytoplasmic organelles become specialized for the production of lens proteins (crystallins); they’re at least of two types, alpha and beta.

• Crystallins constitute up to 60% of the lens fiber mass, which is the highest protein concentration in the body.

• The differing concentrations of the crystallins in different parts of the lens produce regional differences in the refractive index.

• This probably compensates for the spherical and chromatic aberrations that might exist if the concentrations of the crystallins were uniform throughout the lens!!

Suspension

• The lens is held in position by a series of delicate, radially arranged fibers collectively known as the suspensory ligament of the lens, or zonule.

• The zonule fibers arise from the epithelium of the ciliary processes and run toward the equator of the lens.

• The fibers fuse to form about 140 bundles .

• The larger bundles are straight and reach the lens capsule in front of the equator.. Together they form the anterior zonular sheet.

• The smaller fibers curve backward and are attached to the posterior surface of the lens to form the posterior zonular sheet.

• The third group passes from the tip of the ciliary processes almost directly inward to be inserted at the equator

• As the zonular fibers reach the lens, they break up into fine fibers that become embedded in the outer part of the lens capsule.

• When the eye is at rest (meiosis), the elastic lens capsule is under tension, causing the lens constantly to attempt a globular rather than a discoid shape.

• The pull of the radiating fibers of the zonule tends to keep the elastic lens flattened, permitting the eye to focus on distant objects.

Accomodation • To accommodate the eye for near objects, the ciliary

muscle contracts. The meridional fibers pull the choroid and ciliary body forward, and the circular fibers, acting as a sphincter, move the ciliary body inward. This relieves the tension on the radiating fibers of the zonule.

• This process allows the elastic lens to assume a more nearly globular shape. At the same time, the sphincter pupillae muscle contracts, so that the pupil becomes smaller and only the light rays going through .. the thickest, central part of the lens impinge on the retina.

• With advancing age, the lens becomes denser and less elastic, and, as a result, the ability to accommodate is lessened (presbyopia).