The Vitreous Humor

Abstract

The vitreous body, also termed the vitreous humor, vitreus, or vitreous, is a clear and transparent mass (gel or liquid or a mixture of both) that fills the posterior cavity of the eye in vertebrates, between the lens and the retina. The human vitreous body is a hydrogel with a very high water content which provides an adequate support for the retina, allows the diffusion of metabolic solutes, and allows the light to reach the retina. There are currently two differing concepts on the nature of vitreous body. A significant amount of evidence supports the view that the vitreous body is basically an extracellular matrix. Another model has been developed in which the vitreous body is considered as a specialized, but simple, connective tissue. The two concepts are not yet reconciled, therefore the structure and role of the vitreous body are usually regarded from both points of view. It is accepted that the vitreous body possesses a unique macromolecular organization, a double-network system consisting of a scaffold of randomly spaced rod-like collagen fibers filled and entangled with a network of very large coiled-up macromolecules of hyaluronic acid (hyaluronan). The latter is present in the form of its sodium salt (sodium hyaluronidate). The double-network model explains satisfactorily most of the properties of the vitreous body, as well as its remarkable stability, although it probably overestimates the importance of hyaluronan. The natural vitreous body displays true viscoelastic properties which enable it to resist sudden compression shocks, offering much the best protection for the retina against contusion trauma. It is believed that the hyauronan network imparts the latter feature, while the collagen network is responsible for the plasticity and tensile strength of the vitreous body.