Читать книгу Essentials of Veterinary Ophthalmology - Kirk N. Gelatt - Страница 101

Physically, the vitreous is a hydrogel that consists of >98% water and fills the large posterior cavity of the eye. Collagen comprises the framework of the vitreous and provides its plasticity. Despite the low protein content, a diverse array of >1200 soluble proteins have been identified in the vitreous. Spaces 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.