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      The crystalline lens is a transparent, avascular structure that focuses light onto the retina. It is suspended within the eye by zonules arising from the ciliary body epithelium (i.e., pars plicata) and attaching circumferentially to the lens capsule at the lens equator. The lens is also held in place posteriorly within a shallow depression in the anterior vitreous (i.e., the patella fossa), and the iris rests against it anteriorly. In many mammals, birds, and reptiles, the lens is biconvex; the degree of convexity (i.e., shape) changes during accommodation due to the elasticity of the capsule and the pliability of the lens fibers. In young mammals, the lens is quite soft, with only a small, central, denser nucleus. The lens grows throughout life, with newly formed fibers added continuously to the outermost cortex, causing compression of the central, older zone of lens fibers. This results in a hardening of the central nucleus (i.e., nuclear sclerosis), which reduces accommodation ability as the lens ages.

      The refractive power of the lens is less than the cornea because the change of refractive index is much greater at the air–cornea interface than at the aqueous–lens and lens–vitreous interfaces. Contraction of the ciliary body muscle reduces tension on the lenticular zonules, changing the shape of the lens and resulting in an alteration of the dioptric power. Of the roughly 60 diopters of total refractive power of the eye, the lens contributes approximately 13–16 diopters in humans. In dogs, the dioptric power of the lens contributes approximately 40 diopters. The remaining refraction is provided by the cornea.

The lens is proportionately larger in domestic animals than in humans. The dog lens has a volume of approximately 0.5 ml and averages 7 mm in thickness at the anteroposterior axis, with 10 mm equatorial diameter. The ratio of lens volume to entire globe volume ranges from 1:8 to 1:10. The equine lens, on the other hand, has a volume of approximately 3 ml, 12–15 mm average anteroposterior axis thickness, approximately 21 mm equatorial diameter, and a lens–globe ratio of 1:20. Lens volumes of sheep, cattle, and pigs fall between these volumes, thicknesses, and diameters. The lens consists of an enveloping basement membrane called the lens capsule, an anterior epithelium, and lens fibers occupying two main zones: the nucleus and the cortex (Figure 1.47).

      Lens Capsule

The lens fibers are completely enclosed within a thick, PAS‐positive capsule, which is the exaggerated basement membrane of the lens epithelium. It has elastic properties but no elastic fibers. The thickness of the capsule varies by region, with the thinnest being the posterior pole. The canine lens capsule thickness is 8–12 μm at the equator, 50–70 μm anteriorly, and only 2–4 μm posteriorly.

Figure 1.47 Composite drawing of the lens, capsule, attachments, and nuclear zones. The lens epithelial cells line the anterior capsule. At the equator, these dividing cells elongate to form lens cortical cells (fibers). As they elongate anteriorly and posteriorly toward the sutures, their nuclei migrate somewhat anterior to the equator and form the lens bow. Zonular fibers (zf) attach to the anterior and posterior lens capsule and to the equatorial capsule, forming pericapsular or zonular lamellae of the lens.

      Anterior Epithelium

      Lining the anterior capsule is a monolayer of lens epithelial cells that continuously produce new basement membrane (i.e., capsule material). The cells are cuboidal to squamous axially at the anterior pole of the lens, become columnar near the equator, and then elongate into slender hexagonal lens fibers. Nuclei are lost as lens fibers mature and move centrally. The lens epithelium lines only the interior aspect of the anterior surface of the capsule postnatally. The cell apices face the outer lens fibers, being attached to the underlying cortical fibers by tight junctions (zonula occludens) and macula adherens. The posterior lens epithelium forms the embryonic primary lens fibers and, thus, is absent under the posterior lens capsule later in life.

      Mature lens fibers become dependent on the anterior epithelium for maintaining a critical level of dehydration, which allows the soluble proteins to be functionally effective, and for providing a healthy level of reduced glutathione. The lens epithelium is highly susceptible to damage caused by factors such as changes in local oxygen concentration, exposure to toxins, X‐ray irradiation, and ultraviolet light damage.

      Lens Fibers

Immediately anterior to the lens equator is a proliferative zone within the epithelium, referred to as the lens bow (Figure 1.48a and b). The cells within this zone begin to mitose at approximately the same time the primary lens fibers form during early fetal development. This zone of mitosis continues throughout life. The most recently formed cells elongate, with the apical portion of the cell extending forward beneath the epithelium and the basal portion posteriorly along the capsule. As these cells transform into lens fibers, small ball‐and‐socket interdigitations begin to develop and the lens fibers become roughly hexagonal in shape. The ball‐and‐socket junctions, which are present along the length of the fibers, are formed only at the six angular regions; in this way, any particular lens fiber is tightly coupled to six other lens fibers, including two older fibers, two of the same generation, and two younger fibers.

Figure 1.48 Young horse lens near the equator. (a) Lens capsule. (b) Columnar lens epithelium at equator. Arrows delineate the formation of the lens bow by the nuclei of the newly formed fibers. Open arrow points rostrally. (Original magnification, 500×.)

The lens fibers elongate toward the anterior and posterior poles, forming a U‐shaped cell. The fibers do not reach the full distance from one pole to the next, much less the entire circumference of the lens; rather, they meet fibers from the opposite side to form the clinically visible anterior and posterior lens sutures. The sutures are simply the junctions from opposite fibers at a given level in the lens. They vary in configuration among species and at different levels within the lens. The sutures usually form a Y‐shaped pattern near the center of the lens, but in older eyes, they become more complex, with branching arms in the more superficial layers (Figure 1.49). The suture patterns extend throughout the depth of the lens, but they are apparent in vivo only at optical interfaces. The sutures in the anterior half are typically in an upright Y‐shaped pattern, whereas those on the posterior half are in an inverted Y‐shaped pattern.

Figure 1.49 Drawing of the embryonal lens (i.e., nucleus) shows the anterior (a) Y suture, posterior (p) Y suture, and arrangement of the lens cells. The lens cells are depicted as wide, shaded bands. Those that attach to the tips of the Y sutures at one pole of the lens (a) attach to the fork of the Y at the opposite pole (p).

      The mammalian adult lens consists of lens fibers formed chronologically throughout life. The oldest portion, formed during embryonic development, is in the center of the lens and known as the embryonic nucleus. It is a small, dark, lucent zone. Extending outwardly, the fetal nucleus, adult nucleus, and cortex are, respectively, encountered. These portions are frequently subdivided clinically into anterior and posterior divisions to further localize lesions.

      To a greater extent than in mammals, lenticular accommodation in birds depends on the ability of the lens

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