The vitreous is composed primarily of water (≈98%) and macromolecules (0.15%), including collagen, hyaluronan, and soluble proteins. There are very few resident cells in the vitreous; these are called hyalocytes (see Fig 11-4). In addition to the 2 major structural components, collagen and hyaluronan, several noncollagenous structural proteins and glycoproteins have been identified in the vitreous; these include chondroitin sulfate (versican), opticin, VIT1, and fibrillin. The human vitreous also contains hyaluronidase and at least 1 matrix metalloproteinase (MMP-2, or gelatinase), suggesting that turnover of vitreous structural macromolecules can occur.
At present, 19 types of collagen are known, and the genes for several more have been identified. Tropocollagen, the smallest molecular unit of the various collagen types, is arranged in a specific pattern to create collagen fibrils. Aggregation of fibrils, sometimes of different types, gives rise to collagen fibers. Vitreous collagen fibers are composed of 3 different collagen types (Fig 11-2):
Type II fibrils are the major structural component of the fiber and are also found in cartilage.
Type IX fibrils, found on the surface of the fiber, act to shield type II collagen fibrils and prevent them from fusing together, which can lead to condensation of vitreous collagen.
Type V/XI fibrils, located in the core of collagen fibers, likely participate in the initial stages of fiber formation.
A, Model for the structure of a collagen fiber from the vitreous. Type II collagen (red) forms the major structure of the vitreous, accounting for three-quarters of the total vitreous collagen. Type IX collagen (blue), the second most common collagen found in the vitreous, lies on the surface of the fiber. Type IX collagen is proposed to protect type II collagen from degeneration. Type V/XI collagen (purple) is present in the core of the fibril and functions in fibrillogenesis. B, Vitreous collagen fibrils are organized into bundles surrounded by sodium hyaluronate.
(Part A modified with permission from Schachat AP, Wilkinson CP, Hinton DR, Sadda SR, Widemann P. Ryan’s Retina. 6th ed. Amsterdam: Elsevier; 2018:545. Part B reproduced with permission from Lund-Andersen H, Sander B. The vitreous. In: Levin LA, Nilsson SFE, Ver Hoeve J, Wu SM, eds. Adler’s Physiology of the Eye. 11th ed. St Louis: Saunders; 2011:167.)
The vitreous collagens are closely related to the collagens of hyaline cartilage. They differ from the collagens (types I, III, XII, and XIV) commonly found in scar tissue and in tissues such as dermis, cornea, and sclera.
Collagen fibers are condensed in the peripheral vitreous, which comprises the cortical vitreous and has a thickness of approximately 100–300 μm. The vitreoretinal interface exists between the cortical vitreous and the internal limiting membrane (ILM). Interaction between the collagen fibers of the cortical vitreous (known as the posterior hyaloid over the posterior pole) and the ILM is mediated by laminin, fibronectin, and the proteoglycan chondroitin sulfate, among others (Fig 11-3). The adhesion of cortical vitreous to the ILM is relatively weak in the posterior pole compared with adhesion in the region near the vitreous base, where the fibers are firmly anchored to the peripheral retina and pars plana.
Figure 11-3 Vitreoretinal interface. A, Condensed collagen fibers in the peripheral vitreous form the cortical vitreous, which, over the posterior pole, is also known as the posterior hyaloid. The vitreoretinal interface lies between the collagen fibers of the posterior hyaloid and the internal limiting membrane (ILM). Interaction between the cortical collagen fibers and the ILM occurs via several macromolecules, including laminin and fibronectin. Pharmacologic cleavage of these connections facilitates posterior vitreous detachment. B, Optical coherence tomography scan of the vitreoretinal interface (arrow).
(Part A reproduced with permission from Barak Y, Ihnen MA, Schaal S. Spectral domain optical coherence tomography in the diagnosis and management of vitreoretinal interface pathologies. J Ophthalmol. 2012;2012:876472. Part B courtesy of Vikram S. Brar, MD.)
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Excerpted from BCSC 2020-2021 series: Section 2 - Fundamentals and Principles of Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.