The crystalline lens is a biconvex structure which is suspended between the iris and the vitreous by the zonular apparatus. The human crystalline lens is the only structure within the eye, which keeps on growing throughout life. The adult human crystalline lens measures 9–10 mm in diameter and 4–5 mm in thickness. The antero-posterior thickness can vary greatly with the accommodative state of the eye. The anterior surface of the lens is less convex than the posterior surface with a radius of curvature of approximately 9 mm whereas the posterior surface has a radius of curvature of approximately 5.5 mm. The two surfaces meet at the equator which lies about 0.5 mm within the ciliary processes. The center of the anterior and posterior surfaces is known as the anterior and posterior pole of the lens respectively. Figure 1.1 details the structure of fully developed human crystalline lens. The structures comprising the lens are:
- The capsule
- The anterior epithelium
- The cementing substance
- The lens fibers
FIGURE 1.1: In the adult crystalline lens, the nucleus layers are representative of all stages of development from the embryonic nucleus to adult nucleus. The posterior epithelium is lost during development and the only active cells left are the anterior and the equatorial epithelial cells.
THE CAPSULE
The capsule is a highly elastic transparent bag which, on the anterior side, is lined on the inside by the anterior epithelial cells. The posterior capsule has no such lining as the posterior epithelial cells were used up in filling the embryological lens vesicle. The anterior capsule is thicker than the posterior capsule although the maximum thickness of both the anterior and the posterior capsule is in the periphery just within the equator. The central anterior capsule is approximately 14 microns whereas the thickest part measures 22–23 microns. The central posterior capsule thickness is approximately 4 microns (Figure 1.2). The capsule gets progressively thicker with age and also loses its elasticity with the passage of time. A lamination of the capsule has also been described.
FIGURE 1.2: The capsule of the lens is thinnest at the posterior pole (approx. 4 microns) and the maximum thickness of approximately 23 microns is in the posterior midperiphery.
THE ANTERIOR EPITHELIUM
It consists of a single layer of cubical cells with a considerable level of inter-digitation. The central anterior epithelial cells are more cubical in shape whereas the peripheral cells near the equator assume a more columnar shape and are further elongated posteriorly to form the lens fibers. While forming the lens fibers, the cells undergo an anterior rotation whereby the base of the cell, which was initially located anteriorly, forms the posterior part of the lens fiber.
THE CEMENTING SUBSTANCE
It glues the lens fibers to each other and also divides the lens into shelves into which the ends of the lens fibers are inserted. It is present subcapsularly both on the anterior and posterior sides and is present as a central strand. The central strand runs from the anterior to the posterior pole. Three extensions of this amorphous material from the central strand to the equator divide the lens into sectors and these extensions when viewed from the anterior side are Y-shaped, with the anterior Y upright and the posterior Y inverted. These are known as the anterior and posterior sutures of the lens.
THE LENS FIBERS
They are elongated hexagons tapering from the equator towards the poles. The first fibers are formed from the posterior epithelium and come to lie the innermost whereas the later fibers are derived from the equatorial epithelial cells and are laid external to the older fibers; thus giving the lens a laminated structure. The most superficial or youngest fibers are nucleated, the nuclei become elongated as the fiber gets more and more centralized with the addition of more fibers, and eventually disintegrates into granules so that the older fibers are non-nucleated. The first formed fibers run from pole to pole whereas the lens acquires more volume, the later fibres do not do so. The fact that oldest fibers are centralized and cannot be shed off results in a constant growth of the lens volume throughout life, something which is offset by shrinkage of the older fibers. This process of laying down of fibers leads to formation of a peripheral softer cortex and a harder, sclerosed central nucleus.4
EMBRYOLOGY OF THE CRYSTALLINE LENS
- Lens placode
- Lens pit
- Lens cup or pouch
- Lens vesicle
By about the 28th day of gestation, the surface ectoderm opposite the primary optic vesicle thickens with the cells assuming a columnar form. This thickened structure is the lens placode. Within the next few days, the placode develops a pit or furrow, which deepens to form the lens cup or pouch. This pouch closes by about 5 weeks to form the lens vesicle (Figure 1.3). This lens vesicle moves away from the surface ectoderm and at this stage the primary optic vesicle invaginates to form the optic cup. Initially the optic cup is almost completely filled by the lens vesicle. The lens vesicle at this stage has an inner lining of epithelial cells covered by a basal lamina which eventually forms the capsule of the crystalline lens. The cells in the posterior part of the lens vesicle start elongating by about 7 weeks and fill the lens vesicle to form the ‘embryonic nucleus’. These elongated cells form the primitive lens fibers and this is the only stage when lens fibers run from one pole to the other (Figure 1.4). New fibers formed by the equatorial epithelial cells are then layered around the filled in vesicle thus imparting a laminated structure. At the two ends of the lens mass, the thicker ends of the fibers end in the anterior and posterior Y-sutures. The formation of these sutures is evident as early as two months but becomes prominent by 14 weeks of gestation (Figure 1.5). This is the stage of the ‘fetal nucleus’. At birth, most of the lens volume is made up by the embryonic and fetal nucleus. Cortical fibers are then added on throughout life by elongation of the equatorial epithelial cells. At birth, the lens measures approximately 6.5 mm in diameter and grows rapidly to attain 90% of adult size by the end of the first year of life. Thereafter, further growth occurs slowly and adult size proportions are attained by the 4th year.
FIGURE 1.3: The crystalline lens develops from the surface ectoderm. The surface ectoderm thickens to form the lens placode which invaginates to form the lens pit and subsequently the lens vesicle by the fifth week.
FIGURE 1.4: The embryonic nucleus, which develops by the sixth week, is formed by elongation of primary lens fibers derived from the primitive posterior epithelium. These primary lens fibers elongate anteriorly to fill the lens vesicle.
FIGURE 1.5: The fetal nucleus develops from secondary lens fibers from the equatorial anterior epithelial cells which stretch anteriorly and posteriorly around the embryonic nucleus and join to form the anterior and posterior Y sutures.
At the stage of the optic cup, the hyaloid artery, a branch of the ophthalmic artery, can be made out. It divides rapidly and forms a network of vessels covering the posterior surface of the lens –the tunica vasculosa lentis. The hyaloid system of vessels starts disappearing by the fifth month so that at birth, none of the remnants of the tunica vasculosa lentis are visible except in cases of persistent hyperplastic primary vitreous.6
The laminated structure of development of the lens makes it possible to time the insult in cases of lenticular abnormalities. In utero insults lead to abnormalities in the central part of the lens involving the embryonic and fetal nucleus and are usually stable. On the other hand, in developmental cataracts, the central part of the lens is usually clear and the cataract assumes a lamellar appearance.
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