The vitreous of the normal human eye weighs approximately 4 g and occupies a volume of almost 4 mL. The precise weight and volume vary with the age and the size of the eye. The vitreous body is spherical with a depression in the anterior surface, the patellar fossa, corresponding to the posterior surface of the crystalline lens. Larsen¹ ultrasonically measured the axial length of the vitreous body in 926 children. He noted that the mean value in newborns was 10.48 mm in boys and 10.22 mm in girls. A sex difference of 0.23 to 0.38 mm greater mean length was noted in males at birth and persisted throughout the growth period. By the age of 13 years, when the axial growth of the vitreous body is essentially completed, the average length was 16.09 mm in males and 15.59 mm in females. Larsen further noted that the relationship between vitreous length and refraction was already established by 1 year of age.

The vitreous body can be divided into two zones or regions. The more peripheral zone, the cortical vitreous, encases the medullary vitreous. The cortical vitreous consists of a relatively more condensed, fibrillar vitreous. Although the cortical vitreous represents only 2% of the total vitreous volume, it is the metabolic center of the vitreous body, because it contains the hyalocytes.^2,3 Another connective tissue cell, the fibrocyte, is also found in the cortical vitreous. Most of the vitreous body, the medullary vitreous, is essentially a cell-free mixture of collagens and hyaluronan (HA) existing either in a gel or a liquid state depending on the age, refraction, and condition of the eye.

The vitreous body interfaces with several ocular structures through the vitreous cortex (Fig. 11-1). The vitreous cortex extends anteriorly from the vitreous base to form the anterior vitreous cortex and posteriorly to form the posterior vitreous cortex. Recently, the clinical importance of vitreous cortex has become increasingly apparent. The vitreous cortex has been implicated as a primary factor in various vitreoretinal disorders, including retinal breaks, proliferative vitreoretinopathy (PVR), anterior hyaloidal fibrovascular proliferation, macular holes, epiretinal membranes, diabetic retinopathy and age-related macular degeneration.

The anterior vitreous cortex or anterior hyaloid is the anterior surface layer or condensation of the vitreous body. There is no specialized membrane that constitutes the anterior vitreous cortex, but rather, there is a greater density of collagen fibrils. Microscopically, the framework of collagen fibrils that run throughout the vitreous body ends in an interwoven network to form the anterior vitreous cortex. The superficial fibrils tend to run parallel to the surface in this region. The smooth surface and membrane-like appearance are owing to the lamellar distribution of the cortical fibers and the associated highly polymerized mucoproteins.^4,5 Anatomically, the anterior hyaloid forms the posterior limits of the posterior chamber. This portion of the vitreous cortex functions in the physiologic communication between the vitreous cavity and the aqueous humor. The anterior surface of the vitreous body separates from the pars plana approximately 1.5 mm anterior to the ora serrata. It extends medially to contact the lens posterior to the lens equator. Thus, the anterior hyaloid is in contact with the ciliary processes and the lens zonules, as well as the posterior lens capsule. The vitreous attaches to the lens capsule in a ringlike manner, forming the hyaloideocapsular ligament of Wieger. This ligament is believed by some to be synonymous with the attachment ring of the posterior zonular fibers.⁴ The circular area of attachment measures approximately 9 mm in diameter and is especially firm in younger persons or after intraocular inflammation.⁵ In this circular area the anterior hyaloid is thickened.

Central to the attachment of the Wieger ligament (also known as the Egger line), the vitreous lens attachment is less pronounced and appears to be owing to surface tension. This central area contains a potential space within the 9-mm ring known as the Berger space, or the patellar fossa. The anterior hyaloid then turns posteriorly to form the anterior portion of the Cloquet canal in the midportion of the Berger space. The Cloquet canal represents the remnants of the primary vitreous and can sometimes be seen with the slitlamp. It arises from the optic disc in a funnel-shaped manner, in the area of Martegiani, and extends forward to the posterior lens surface. The canal is 1 to 2 mm wide and has a downturn in the central vitreous cavity. The area of contact with the posterior lens capsule can at times be identified by a tag of embryonic tissue, known as a Mittendorf dot, located slightly nasal to the posterior pole of the lens. Similarly, a remnant of the posterior primary vitreous can occasionally be identified on the optic disc. This remnant, representing the embryonic point of exit of the hyaloid vascular system from the optic nerve head, is known as the Bergmeister papilla. The walls of the Cloquet canal are formed by a vitreous condensation rather than a true membrane (Fig. 11-1).

The mechanical relationship between the vitreous and the retina is mediated by the posterior vitreous cortex, which is also called the posterior hyaloid. The posterior vitreous cortex consists of relatively densely packed type II collagen fibrils arranged tangentially to the retina. The retinal basal lamina is the basement membrane of the Müller cells that compose the internal limiting membrane (ILM) of the retina.^6,7 Ultrastructurally, the ILM consists of three layers.⁸ Adjacent to the end feet of the Müller cells is the lamina rara interna. The lamina rara externa is contiguous with the vitreous cortex. In between these layers is the lamina densa. Collagen fibers of cortical vitreous are tangential to the lamina rara externa. The ILM is composed of primarily type IV collagen but also contains fibronectin, laminin, and type I collagen.⁹ The morphology of the ILM varies topographically in the retina. The ILM, and in particular the lamina densa, is thin in the retinal periphery and becomes increasingly thicker and irregular in the posterior retina.^6,8 The ILM thickens from 50 nm at the vitreous base to 300 nm at the equator to 1,900 nm posteriorly. In the foveal region the ILM thins to 10 to 20 nm.¹⁰

The nature of the adhesion between the vitreous cortex and the ILM is enigmatic. This adhesion is strongest at the optic nerve, the macula, the vitreous base, and retinal vessels. Foos⁶ demonstrated attachment plaques between Müller cell cytoplasm and the ILM in the basal and equatorial retina (Fig. 11-2). Vitreous traction mediated by vitreous fibrils may contribute to these adhesions. These attachment plaques are not present posterior to the equator except where the ILM is thinned in the fovea. This anatomic variation in the fovea region may play a role in the pathogenesis of some of the vitreomacular disorders.

The ILM also thins over major blood vessels. Vitreous strands extend through pores in the ILM to surround the vessels¹¹ and have been termed vitreoretinovascular bands.¹² These bands may explain the strong adhesion between the vitreous and retinal vessels.

Despite the presence of these anatomic interfaces between the vitreous cortex and ILM, the adhesion of these structures is still not completely understood. Biochemical investigations suggest that vitreoretinal adhesion involve molecular adhesion mediated by glycoproteins such as fibronectin, laminin, and other glycoconjugates. The ILM consists of type IV collagen, laminin, fibronectin, type I collagen, and as yet uncharacterized glycoproteins. It is believed these substances mediate vitreoretinal adhesion.¹³

The ILM undergoes age-related changes that are anatomic, biochemical, and functional. Anatomically, the ILM thickens with age. Biochemically, the distribution of fibronectin and laminin changes with age. In young eyes, antifibronectin antibody binding has a homogeneous distribution. In older eyes, the posterior ILM shows a segmented bilaminar distribution of both antifibronectin and antilaminin antibodies.¹⁴ An age-dependent distribution of other ILM glycoconjugates has also been demonstrated. Variations in the distribution of these and other glycoproteins may play a role in conditions that are characterized by alterations in vitreoretinal adhesion.¹⁵

Mechanical separation of the vitreous from the retina occurs when the vitreous separates from the ILM of the retina. This separation may be relatively complete, as in posterior vitreous detachment (PVD), or may be limited, as in macular hole formation. PVD occurs when mechanical stress forces acting on the vitreous body overcome the naturally occurring adhesion of the vitreoretinal interface.

PVDs are usually spontaneous events that are age related, but they may also be precipitated by trauma. They occur earlier in association with vitreous inflammation, vitreous hemorrhage, myopia, and diabetes. As the vitreous ages, there is an increase in the water content and subsequent liquefaction of the vitreous, a process that normally begins by the age of 5 years. At about 40 years, the volume of the vitreous gel begins to decrease in parallel with the increase in liquid vitreous until approximately half the vitreous space is occupied by liquid vitreous (Fig. 11-4). This process of liquefaction is called synchysis. Synchysis usually begins in the midvitreous and is recognized with biomicroscopy as optically empty spaces without the membranous appearance of the normal vitreous. This is because liquid vitreous does not contain collagen fibrils.

Once vitreous liquefaction occurs, fluid currents develop within the vitreous. These currents, stemming from the rotational and gravitational forces transmitted to the vitreous body, produce stresses at the vitreoretinal interface that lead to separation at points of weakest attachment. Dark-field slit microscopy studies on human autopsy specimens indicate that persons 21 years or younger demonstrate firm adherence between the ILM and the vitreous cortex in the posterior pole. Older persons demonstrate relatively weak adhesive forces in comparable areas.¹⁵

After liquid vitreous has formed, vitreous body collapse or syneresis can occur. This event is precipitated by the passage of liquid vitreous through the vitreous cortex at either the prepapillary hole or the premacular cortex and into the potential space between the vitreous cortex and the ILM. The liquid vitreous then dissects and fills the potential space. The subsequent volume displacement of liquid vitreous through holes in the vitreous cortex results in vitreous collapse and a PVD. Liquid vitreous may also pass through an intact vitreous cortex which creates a slower detachment occurring as a result of a pathologic condition, such as diabetes.

Defects in the premacular vitreous cortex have been identified in autopsy eyes by Sebag and Balazs¹⁸ and confirmed by Kishi and Shimizu³⁵ using differing techniques. The premacular or prefoveal hole appears to correspond to the posterior aspect of the posterior precortical vitreous pocket (PPVP). In this region liquefied vitreous is immediately adjacent to the vitreous cortex. It is unknown what allows the passage of liquid vitreous through the vitreous cortex to disrupt the vitreous cortex ILM adhesion, but mechanical, hormonal, and biochemical changes have been implicated.

Although vitreous detachment typically results in cleavage between the posterior vitreous cortex and the ILM of the retina, clinical and electron microscopic observations suggest that this is not always the case, particularly in younger persons and in pathologic states. Microscopic studies have confirmed the multilaminar nature of the vitreoretinal interface, including the direct insertion of vitreous cortical collagen fibers into the ILM of the retina. These insertions are particularly firm in areas where the ILM is thinnest, such as the fovea (where the ILM may measure as few as 10 nm) and the basal region (where it again thins to as few as 50 nm).^6,10 In young persons, adhesions between the ILM of the retina and the vitreous cortex may be stronger than the Müller cells themselves, leading to a different plane of separation than between the ILM and posterior vitreous cortex during PVD. Vitreous cortex remnants have been identified at the fovea by scanning electron microscopy after spontaneous vitreous detachment in an autopsy series.³⁶

In addition to the topographic and age-related differences in the vitreoretinal interface, gender-related differences in the rates of vitreous separation also exist. In most clinical series, women are twice as likely to present with symptomatic PVD³⁷ (65%–68%) as men (32%–35%). This is paralleled by gender-related differences in disease rates associated with vitreoretinal separation. Macular holes occur between three and four times more frequently in women than in men, whereas nontraumatic giant retinal tear occurs three times more frequently in men than in women. The reasons for this disparity remain unclear, but these data may reflect gender-related differences in the strength of the vitreoretinal interface at different locations with the eye.

Mechanical separation of the vitreous from the ILM is believed to occur in normal eyes in the absence of any cellular contribution or energy-dependent processes. Under pathologic circumstances, the cortical vitreous gel may also contract rather than separate at the vitreoretinal interface. This leads to the development of vitreoretinal traction and complex forms of retinal detachment in the presence of retinal breaks. Vitreous contraction has long been recognized as a factor in complex retinal detachments.³⁸ However, the biologic basis for the contraction of vitreous gel was not understood until relatively recently.

Machemer³⁹ first emphasized the critical role of cells in the genesis of vitreoretinal traction. Early histopathologic and ultrastructural studies on patients undergoing vitrectomy for PVR demonstrated transformed retinal pigment epithelial (RPE) cells, retinal glia, fibroblasts, and macrophages in membranes that were hypothesized to be contractile. It was proposed that these membranes, composed of densely packed interdigitating cells and ECM, produced tractional forces in a manner analogous to the contraction of syncytial smooth muscle elements as a result of actin and myosin filaments within the cytoplasm, particularly of myofibroblasts. However, clinical observations of the vitreous suggested that vitreoretinal traction could occur by mechanisms other than syncytial hypercellular membranes.

A different mechanism is required to explain the development of vitreous cortical shrinkage that occurs in the region of the vitreous base following vitrectomy. The ability of fibroblastic cells of different proliferative potential to contract a collagen latticework was first described by investigators evaluating the wound healing response in skin.⁴⁰ Subsequent investigations in the dermatologic and ophthalmologic literature corroborated the applicability of this system for the testing of antiproliferative and anticontractile drugs.^41,42 Clinical investigators pointed out the applicability of this experimental system as an explanation for contraction of the vitreous gel by dispersed intraocular cells.^43,44 Various ocular cell types including dermal, scleral, or chorioretinal fibroblasts; pigment epithelial cells; and glial cells, in decreasing order of magnitude, will contract a clear gel of type I or type II collagen to a small fraction of its original volume with resultant opacification. Scanning and transmission electron micrographs of contracted gels reveal similarities with specimens of contracted vitreous taken from experimental animals and humans with PVR (Fig. 11-5). This process, termed hypocellular gel contraction, requires the presence of either serum, specific serum components, or other non–serum-derived growth factors to occur. Thrombin, transforming growth factor-β, platelet-derived growth factor, and fibronectin have all been shown to stimulate this process and can substitute for the serum requirement to variable degrees.^45,46 Similarly, other pharmacologic compounds, which either intrinsically inhibit cellular contractility^47,48,49,50 or interfere with cellular attachment to collagen through fibronectin-mediated binding sites^51,52 can inhibit this process. Conventional and low-molecular-weight heparin also inhibit cellular attachment to collagen and, consequently, contraction. Various heparin forms also inhibit collagen polymerization.^52,53 Antiproliferative compounds that innately inhibit the cellular contractility process include steroids, certain fluoropyrimidines, colchicine, and paclitaxel ⁵³

Quantitative analysis of these effects indicates that human dermal fibroblasts are nearly 20 times more potent contractors of collagen than human RPE cells. For example, 100,000 dermal fibroblasts will contract a collagen gel to <10% of its original volume within 48 hours, contrasted to a negligible change at 48 hours for 100,000 human RPE cells. However, given 120 hours, 1 million human retinal RPE cells will contract a gel to <30% of its original volume⁴³ (Fig. 11-6). Using time-lapse video cinematography, it can be shown that RPE cells will pull in collagen fibrils in a hand-over-hand fashion at a rate of approximately 2.5 μm per minute,⁴⁴ and this effect can be blocked by the addition of a peptide analogue that inhibits fibronectin-mediated binding.⁵¹ Similar effects have been demonstrated for human retinal glial cells, although the rate of contraction is less than that of either pigment epithelial cells or fibroblasts.⁵⁴

In addition to demonstration of these effects in an in vitro model using purified type I and type II collagen, other investigators have demonstrated similar effects in vitro using native bovine vitreous. Although pigment epithelial cells, glial cells, and fibroblasts all produce collagen and vitreous contraction, the effects of capillary endothelial cells are modest, and no contraction is induced by either erythrocytes, blood leukocytes, or activated macrophages.⁵⁵

Histologic and ultrastructural studies of contracted gels disclose consistent morphologic changes that account for these contractile events. Pigment epithelial cells cultured on plastic demonstrate a rounded or cuboidal morphology when subconfluent. It has been previously shown that when pigment epithelial cells are cultured on either vitreous, collagen, or fibrin they assume a more mesenchymal configuration, including the development of dendritic extensions and lamellipodia.⁵⁶ It is believed that the development of these lamellipodia are critical for locomotion of cells and for the generation of the tractional forces. Histologic cross sections of contracted gels reveal that the cells may produce profound volumetric changes in gels without the development of typical cellular membranes. In contracted gels, cells appear to be dispersed at uniform distances with relatively infrequent cell interactions, particularly in the case of fibroblasts (Fig. 11-7). It is believed that the mechanism of contraction in these cases relates to compaction of vitreous fibrils adjacent to cells through membranous interactions between fibronectin and collagen binding sites on the cells and individual collagen fibrils and HA with resultant extrusion of water.

The water extrusion is largely responsible for the volumetric change (shrinkage) and loss of transparency noted clinically and experimentally and can be quantitated either directly by linear measurement or indirectly by volumetric methods such as isotope dilution or water displacement.^40,48,57

The separation of the vitreous cortex from the retinal surface is a relatively frequent and often a clinically significant occurrence. It is important because of the bothersome and at times alarming symptoms and also because of the frequent association of significant ocular pathology such as retinal tears, retinal detachment, and vitreous hemorrhage.