Essentials of Veterinary Ophthalmology. Kirk N. Gelatt

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Essentials of Veterinary Ophthalmology - Kirk N. Gelatt

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called the lens capsule, an anterior epithelium, and lens fibers occupying two main zones: the nucleus and the cortex (Figure 1.47).

      Lens Capsule

Schematic illustration of composite drawing of the lens, capsule, attachments, and nuclear zones.

      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

Photo depicts young horse lens near the equator. Schematic illustration of drawing of the embryonal lens shows the anterior (a) Y suture, posterior (p) Y suture, and arrangement of the lens cells.

      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

Photos depict zonular attachments to the lens in a dog.

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