Essentials of Veterinary Ophthalmology. Kirk N. Gelatt
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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 to change shape. The avian lens is generally softer and more flexible than the mammalian lens, and consequently is more readily deformed during contraction of the ciliary body and peripheral iris musculature. As the anterior uveal muscles contract, it is theorized that the ciliary body pushes against the mid‐equatorial region of the lens, while the peripheral edge of the iris presses against the anterior equatorial surface. As an evolutionary adaptation to this activity, the avian lens has an annular pad (i.e., “ringwulst”), which consists of lens fibers that are relatively enlarged and arranged radially instead of concentrically. The size of the annular pad appears to relate directly to the degree of accommodative ability.
Zonular Attachment
The lens is circumferentially suspended from the ciliary body by fibers called zonules. Zonular attachment is achieved by a complex arrangement of fibers that insert onto the lens capsule in a zone encompassing the equator and a short distance both anterior and posterior to the equator (Figure 1.50a and b). Each zonular fiber is made of numerous small fibrils, which are visible under SEM as they attach to the lens capsule. The zonular fibers spread out near the equator and terminate into smaller bundles. Each of these bundles also fans out and forms a network that ramifies over the surface of the lens capsule, approximately 1.5–2.0 mm away from the lens equator.
Figure 1.50 Zonular attachments to the lens in a dog. (a) SEM shows that zonules (Z) extend from the ciliary body onto the equator of the lens (L) in a ringlike manner, covering each ciliary process. (Original