Читать книгу Essentials of Veterinary Ophthalmology - Kirk N. Gelatt - Страница 55

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.