Essentials of Veterinary Ophthalmology. Kirk N. Gelatt
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between the collagen fibers are filled with hyaluronic acid (HA), which provides viscoelasticity to the vitreous. An increase in the collagen content of the vitreous makes it more solid, or gel‐like, while a decrease in the collagen content makes its consistency more fluid. Species differ in the collagen content of their vitreous, which accounts for variability in its consistency. Generally, the cortical areas of the vitreous contain more collagen, so they are more rigid than other portions. The vitreous contains few cells, termed hyalocytes. Hyalocytes belong to the monocyte/macrophage lineage and derive from bone marrow. Their origin is not from glial cells or retinal pigment epithelial cells, as previously thought. Hyalocytes are important for ECM synthesis, vitreous cavity immunology regulation, and modulation of inflammation.
The embryonic vitreous is very dense and therefore translucent. As an individual matures, however, important structural changes occur in the vitreous. The axial length of the vitreous increases, which is critical for growth of the eye (discussed later). The overall collagen content remains unchanged in the adult, but the HA concentration undergoes a fourfold increase in both cattle and humans. This change in the HA‐to‐collagen ratio contributes to greater dispersal of the collagen fibrils, because the newly synthesized HA molecules push the collagen fibril bundles further apart, thus increasing the optical clarity of the vitreous. These changes in HA–collagen interactions as well as in the GAG contents of the vitreous do not cease upon reaching adulthood. Rather, these alterations continue throughout life, and they are believed to be responsible for the vitreal liquefaction observed as part of the aging process in several species. In humans, rheological (i.e., the gel–liquid state of the vitreous) changes begin in the central vitreous at five years of age and continue throughout life, so that in the geriatric patient, more than 50% of the vitreous is eventually liquefied. As liquefaction progresses, the collagen bundles are packed into the remaining gel fraction, whereas HA molecules are redistributed to the liquid fraction. A common complication of this progressive liquefaction is separation of the posterior vitreous cortex from the retinal inner limiting membrane (posterior vitreal detachment). This detachment can predispose to retinal tears, and has been implicated as a risk factor in rhegmatogenous retinal detachment in dogs.
Vitreous Functions
The vitreous is the largest structure in the eye, occupying approximately 80% of the globe. It contributes to the development, optics, structure, physiology, and metabolism of the eye. The vitreous plays an important role in the growth of the eye by contributing to the increase in globe size. Inserting a drainage tube into the vitreous cavity of chicken embryos lowers intravitreal pressure and effectively stops the growth of the eye, and vitrectomy of rabbit eyes has a similar inhibitory effect. In contrast, vitreal elongation will cause an increase in the axial length of the globe. This lengthens the path of the incoming light, thus providing for greater light refraction. In some aquatic species, such as goldfish, this increased vitreal refractivity is a physiological mechanism that compensates for the loss of refractive power when the cornea is submerged in water. In terrestrial species, the increased refraction by the vitreous leads to myopia. Vitreal elongation resulting in axial myopia has been induced through visual deprivation in a number of species, including nonhuman primates, chickens, and cats. This elongation of the vitreous is affected by the synthesis of collagen. Synthesis, molecular reconfiguration, and hydration of HA molecules likewise change the volume of the vitreous and, hence, of the eye.
Diffusion is slow and bulk flow is limited in a gel such as the vitreous. Therefore, topically administered substances are prevented from reaching the retina and optic nerve and systemically administered antimicrobials are unable to reach the center of the vitreous. This slow change of substance concentrations has been used in humans to determine time of death and aid in postmortem diagnosis in manatees.
The optical transparency of the vitreous is primarily due to a low concentration of structural macromolecules (0.2%, w/v) and soluble proteins. Additionally, the configuration of highly hydrated GAG chains separating small‐diameter collagen fibers aids in the passage of light with minimal scattering. Another important factor in optical clarity is the blood–vitreous barrier; HA is thought to act as a barrier that prevents diffusion of macromolecules and cells into the vitreous, except in cases of trauma or cortex disruption. Inflammatory responses, neovascularization, and collagenase activity are likewise suppressed in the vitreous. As a result of these anatomical and physiological properties, the vitreous transmits 90% of light at wavelengths between 300 and 1400 nm.
In addition to its refractive role, the vitreous appears to have additional functions in the process of accommodation. In both humans and monkeys, imaging has revealed that the vitreous bows posteriorly as the ciliary body contracts. This movement is in proportion to the accommodative amplitude. The vitreous also plays an important role in ocular metabolism. It serves as a storage site for retinal metabolites, including glycogen, amino acids, and potassium. Retinal and lenticular waste products, including lactic acid and free radicals, are absorbed by the vitreous, which thus serves to protect the lens and retina from toxic compounds. In cattle, these molecules (and water) can diffuse across the vitreous through pores that are 400 nm in diameter. HA serves as a barrier to this diffusion process; therefore, molecule size and HA concentration are two of the primary factors affecting the diffusion of molecules through the vitreous. A decrease in HA concentration, which results in vitreous liquefaction, will thus lead to an increase in particle diffusion through the vitreous. Therefore, pathological or aging processes leading to a decreased HA concentration and vitreal liquefaction will affect the nutrient supply, waste removal, and drug delivery in the posterior segment of the eye.
The vitreous also provides some mechanical and structural support to the lens and retina. Furthermore, its viscoelastic properties protect the internal eye structures from trauma and stress, especially during rapid eye movement (REM). Concentrations of collagen and HA, as well as the nature of their cross‐links, contribute to this viscoelasticity. For example, in humans, the concentration of vitreal HA and collagen is twice as high as in the pig, and this corresponds to a 60% increase in the spring constant of human versus porcine vitreous. Woodpecker vitreous differs from human vitreous in that it does not have vitreoretinal attachments. This lack of coupling of the vitreous to the posterior pole, as well as the orientation of the eye with respect to the axis of striking, is thought to reduce relative shearing motions that would be expected to result in ocular trauma from the woodpecker's rapid acceleration–deceleration movements.
Ocular Mobility
Higher‐resolution vision is subserved by a small section of the retina termed the area centralis in animals. Visual acuity and other parameters of vision (e.g., color perception) decrease rapidly in the more peripheral retina outside the area centralis. To keep an object of interest in the center of the visual field, so that its image will stimulate the area centralis (or fovea in birds and primates), vertebrates rely on the actions of six or seven EOMs. Domestic species have four rectus EOMs – dorsal (or superior), ventral (inferior), nasal (medial), and temporal (lateral) – all of which move the eye in those respective directions (see Chapter 1). The oculomotor nerve (CN III) innervates the dorsal, ventral, and medial rectus muscles, and the abducens nerve (CN VI) innervates the lateral rectus muscle. Two oblique muscles that work in conjunction with the rectus muscles are also present. The ventral oblique, which is innervated by the oculomotor nerve, rotates the ventral aspect of the eyeball both nasally and dorsally; the dorsal oblique, which is innervated by the trochlear nerve (CN IV), rotates the dorsal aspect of the eyeball both nasally and ventrally. These muscles keep vision horizontally level irrespective of eye position in the orbit. The retractor bulbi, which is present in most species other than primates and birds, is innervated by CN VI and pulls the globe deeper within the orbit. The EOMs contain both fast (~85%) and slow (~15%) fibers; however, in contrast to noncranial skeletal muscles, they exhibit both very fast contractility and extreme fatigue resistance. Even among the EOMs there is great variation in the composition of each muscle in regard to the myosin heavy chain isoforms that assist with the dynamic physiological properties and CNS control of eye movements. The EOMs are highly aerobic as well as resistant to injury and oxidative stress, with only cardiac muscle having a higher blood flow rate. Additionally,