During formation of the eye, the primary vitreous contributes the hyaloid artery, which nourishes the developing anterior segment and lens. Failure of the vitreous to regress following this stage leads to pathology of the anterior and/or posterior segment. See BCSC Section 6, Pediatric Ophthalmology and Strabismus, and Section 12, Retina and Vitreous, for further discussion of persistent fetal vasculature (also called persistent hyperplastic primary vitreous). The secondary vitreous consists of a gel matrix representing the largest structure of the eye and is routinely seen on clinical examination. The tertiary vitreous gives rise to the zonular fibers. See Chapter 4 for additional discussion of development of the vitreous.
In adulthood, the vitreous is less dynamic than during ocular development and acts as a conduit for nutrients and other solutes between the lens and the vitreous and for fluid to and across the retina (Fig 11-1). It occupies a volume of 4 mL and has an osmotic pressure and index of refraction (1.334) similar to those of the aqueous humor. Its viscosity, however, is almost twice that of water. The basic physical structure of the vitreous is that of a gel composed of a collagen framework interspersed with molecules of hydrated hyaluronan, also known as hyaluronic acid. The hyaluronan contributes to the viscosity of the vitreous humor and is thought to help stabilize the collagen network.
The relative amounts of collagen determine whether the vitreous is a liquid or a gel. The rigidity of the gel is greatest in regions of highest collagen concentration: the peripheral (cortical) vitreous and the vitreous base. The collagen fibrils confer resistance to tensile forces and give plasticity to the vitreous; the hyaluronan resists compression and confers viscoelastic properties. Degeneration of these fibrils occurs in most of the population and occasionally leads to retinal pathology.
Figure 11-1 Fluid transport in the eye facilitated by aquaporin channels. The red arrows indicate exchange between the lens and the vitreous, as well as flow of fluid to and eventually across the retina, where this movement contributes to retinal adhesion.
(Reproduced with permission from Forrester JV, Dick AD, McMenamin PG, Roberts F, Pearlman E. The Eye: Basic Sciences in Practice. 4th ed. St Louis: Saunders; 2016:233.)
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.