Читать книгу Essentials of Veterinary Ophthalmology - Kirk N. Gelatt - Страница 102

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.