Vitreous

The vitreous cavity occupies four-fifths of the volume of the globe. The transparent vitreous humor is important to the metabolism of the intraocular tissues because it provides a route for metabolites used by the lens, ciliary body, and retina. Its volume is close to 4.0 mL. Although it has a gel-like structure, the vitreous is 99% water. Its viscosity, however, is approximately twice that of water, mainly because of the presence of the mucopolysaccharide hyaluronic acid (Fig 2-49).

At the ultrastructural level, fine collagen fibrils (chiefly type II) and cells have been identified in the vitreous. The origin and function of these cells, termed hyalocytes, are unknown, but they probably represent modified histiocytes, glial cells, or fibroblasts. The fibrils at the vitreous base merge with the basal lamina of the nonpigmented epithelium of the pars plana and, posteriorly, with the ILM of the retina, the vitreoretinal interface.

The vitreous adheres to the retina peripherally at the vitreous base (Fig 2-50), which extends from 2.0 mm anterior to the ora serrata to approximately 4.0 mm posterior to it. Additional attachments exist at the optic nerve head margin, at the perimacular region surrounding the fovea, along the retinal vessels, and at the periphery of the posterior lens capsule (Fig 2-51). See Chapter 11 for further discussion of the vitreous.

Figure 2-49 Vitreous. Gross photograph of the vitreous with the sclera, choroid, and retina removed from the eye of a 9-month-old child.

(Modified from Sebag J. Posterior vitreous detachment. Ophthalmology. 2018;125(9):Fig 1.)

Figure 2-50 Vitreous. The vitreous is most firmly attached to the retina at the vitreous base, which straddles the ora serrata. Additional adhesions exist at the posterior lens capsule (hyaloideocapsular ligament; also known as ligament of Weiger), along the retinal vessels, at the perimacular region, and at the optic nerve margin. A prominent area of liquefaction of the premacular vitreous gel is called the premacular bursa, or precortical vitreous pocket.

(Illustration by Mark M. Miller.)

Figure 2-51 Posterior vitreous attachments. OCT image of the fovea and overlying vitreous. Note the adhesion of the vitreous at the margins of the optic nerve (arrows) and fovea (perimacular), with overlying premacular bursa (*).

(Courtesy of Vikram S. Brar, MD.)

• Lund-Andersen H, Sander B. The vitreous. In: Levin LA, Nilsson SFE, Ver Hoeve J, Wu SM. Adler’s Physiology of the Eye. 11th ed. Philadelphia: Elsevier/Saunders; 2011:164–181.

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.