The Eyeball: Lens

The lens of the eye is a soft, transparent, protein-rich, biconvex structure suspended in contact with the posterior face of the iris and the anterior face of the vitreous body. Its function is to bring images into critical focus on the photoreceptor layer of the retina. The crystallins, the major structural proteins of the lens, are considered organ specific and have been characterized in the dog and are known to exist as α-crystallin, β-crystallin, and γ-crystallin subunits. Zeta crystalline, an inactivated form of DAD(H)P/quinine dehydrogenase has also been described in the dog and some other mammalian species. Other major components of the canine lens are phospholipids including ethanolamine plasmalogen, phosphatidylethanolamine, phosphatidylserine, sphingomyelin, and phosphatidylcholine.

In an assessment of lens morphologic examination using ultrasonography, Williams (2004) reported the following mean intraocular measures (N = 50): globe diameter 20 (±1.6) mm, axial lens thickness 6.7 (±1.0) mm, axial lens thickness/globe diameter ratio 0.33 (±0.1).

Images are focused on the retina by the combined refraction of the cornea, aqueous, lens, and vitreous. Most refraction occurs at the anterior face of the cornea. The lens actually alters the path of the light rays only slightly, but is the only structure in the dog eye capable of altering its refractive power. It therefore is solely responsible for changing visual focus (accommodation) in the dog eye. The focal length of the lens is altered by changes in its shape brought about by the action of the ciliary muscle, zonular fibers, and lens capsule. The lens has an extraordinarily high protein content giving it a refractive index substantially higher than the surrounding fluid media and allowing it to refract light effectively. The dog lens has been shown to exhibit negative spherical aberration. In a spherical lens of uniform refractive index, the light rays that pass through the most peripheral aspect of the lens will be brought to a point focus in front of rays that pass through the paraxial region of the lens. This is referred to as positive spherical aberration. In the canine lens, peripheral rays are brought to a point focus at a distance farther from the lens than rays passing through the paraxial region (negative spherical aberration). This occurs because the lens possesses a gradient of refractive indices. In the dog, the more peripheral cortical regions of the lens have a lower refractive index than the more centrally located nuclear regions. The canine lens has also been shown to possess chromatic aberration. Longitudinal chromatic aberration arises when longer wavelengths of light (reds) are refracted less by the lens than shorter wavelengths (blues). Red wavelengths would therefore be focused behind blue wavelengths. The percentage of chromatic aberration with respect to focal length (or power) found for the dog lens is quite high compared with other vertebrates.

The lens contributes significantly to the overall refractive power of the eye. When it is surgically removed during cataract surgery, the eye becomes significantly hyperopic (focused behind infinity). The aphakic canine eye is approximately 14 diopters hyperopic relative to infinity. Intraocular lenses are currently being implanted by many veterinary ophthalmic cataract surgeons to replace the optical power lost by lens removal.

An anterior and posterior pole and equator are described for the lens in a manner analogous to that for the eyeball as a whole. The equator demarcates the anterior and posterior faces. The lens is circular in transverse section but slightly ellipsoidal in sagittal or dorsal section. The superior-inferior and mediolateral diameters average approximately 11 mm, whereas the anteroposterior length along the axis bulbi is approximately 4 mm less. The posterior face is more convex than the anterior. The posterior pole of the lens approximates the equatorial plane of the eyeball as a whole.

The lens is ectodermal, originating from an invagination of the surface epithelium overlying the optic cup, which pinches off to form the lens vesicle by approximately 27 days of gestation. The cells forming the posterior wall elongate until they reach the anterior epithelium obliterating the cavity of the vesicle. These cells then lose their nuclei to become the primary lens fibers of the embryonal nucleus. As a result, there is a cuboidal lenticular epithelium only on the anterior face of the lens at birth. This simple epithelium gradually becomes squamous in the adult.

Throughout life the epithelium continues to proliferate slowly near the anterior equatorial margin. Cells at the equator elongate along the meridians until their apices approach the poles. In section, the nuclei of these elongating cells form an arc, the so-called lens bow, from the equator toward the deeper portions of the lens. As successive layers of cells accumulate, the deeper cells lose their nuclei but remain viable as secondary lens fibers. This manner of growth results in a lens that is distinctly lamellar, resembling an onion in crosssection. In dogs older than 1 year of age, an embryonal, fetal, and adult nucleus and an adult cortex are recognized.

For the normal lenses, the thickness of the anterior lens capsule increases linearly with age by 5-8 µm/year. Because the continued epithelial proliferation is unaccompanied by cell loss, the weight of the lens also increases with age. The increase in weight is most rapid during the first few months of life but continues at a slow rate throughout life. Because individual variation of the growth rate of the lens within a species is usually slight, there is a direct correlation between age and dry lens weight. This method for age determination has been used for several wild carnivores.

The nuclear portion of the lens undergoes progressive dehydration and condensation (nuclear sclerosis). In adults, this shrinkage occurs at a rate roughly equal to the rate of growth at the periphery, so that the lens does not continually increase in size. The progressive nuclear sclerosis results in a much firmer and less elastic lens in older individuals. The development of sclerosis in older dogs has been associated with a myopic shift in resting refractive state. Nuclear sclerosis causes the deeper portions of the lens to appear blue-gray and hazy. This normal age change should not be confused with cataracts, which are pathologic lenticular opacities.

The apices of the lens fibers do not all meet at a single point at each pole, like sections of an orange. Rather, the junctions form distinct linear markings known as the lens sutures. On the anterior face, the lens sutures form an upright letter Y. On the posterior face the Y is inverted. Lens fibers that begin at the tip of one of the arms of the Y on the anterior face end in the crotch between the arms of the Y on the posterior face. This pattern is most obvious in the adult nuclear region. The prominence of the sutures increases with age, beginning at approximately the third year. In the cortex the lens sutures are more stellate in form. The lens sutures are easily visualized in the living dog with a slit lamp and are frequently the site of cataracts. With suitable illumination, a small white dot (Mittendorf dot), a remnant of the hyaloid artery, can be seen in the center of the posterior lens face.

The entire lens is enveloped by the lens capsule. The capsule is a basement membrane secreted by the cells of the lenticular epithelium. It is highly refractile and elastic. The elasticity of the capsule aids in cataract surgery when a prosthetic lens is introduced through a relatively small opening in the anterior capsule. The posterior epithelium is present only in the early fetal period and the posterior capsule remains thin, approximately 4 µm throughout life. The anterior capsule in the dog is approximately 82 µm in thickness, roughly eight times thicker than in humans. Its thickness, which may increase in diabetic patients, reduces anterior rounding of the lens during accommodation.

The zonular fibers, which suspend the lens, insert into the superficial layers of the lens capsule. More and larger fibers insert in the thicker anterior capsule than on the posterior surface (see section on zonule). The lens rests in a depression in the vitreous, the hyaloid fossa. The vitreous is tightly adherent to the posterior capsule, forming the hyaloidlenticular ligament (ligamentum hyaloideo-capsulare), which greatly complicates surgical removal of the lens.

In the adult, the lens, although active metabolically, is avascular. Nutrition is received from, and wastes are eliminated into, the aqueous and vitreous humors. Disease processes, for example diabetes mellitus, that affect lenticular metabolism result in a loss of transparency (cataract).