12 The vitreous is a transparent gel that fills the posterior globe, bordering the posterior lens, and the retina and optic nerve head. It is the largest structure of the eye in both size and volume (about two‐thirds of the eye). Its volume varies depending of the size of the eye and the species; for instance, dog (1.7 ± 0.86 mL) and horse (26.15 ± 4.87 mL). The majority of the vitreous is formed during the development of the eye, and in the adult is normally devoid of any vasculature. It primarily consists of water (99%), collagen fibers combined with hyaluronic acid (which form into a gel), infrequent hyalocytes, and few migrating macrophages. It is attached to the ocular structures it borders near the ora ciliaris retinae (the vitreous base) and the posterior lens capsule (ligamentum hyaloideocapsulare). The vitreous changes considerably during embryogenesis and development of the eye. It initially consists of what is referred to as the primary vitreous, which is formed prenatally and consists mainly of the hyaloid artery system. This system regresses postnatally leaving minimal remnants, Mittendorf’s dot (former attachment immediately caudal of the central posterior lens capsule), and Bergmeister’s papilla (former attachment to the optic nerve head). The adult gel is called the secondary vitreous and the tertiary vitreous is the lens zonules. The vitreous gel over time and aging undergoes variable syneresis (or liquefaction). Diseases of the vitreous are mostly extensions from other adjacent tissues (inflammatory cells, foreign bodies, parasites or infectious agent, and hemorrhage). Congenital abnormalities of the vitreous are the results of persistence, hyperplasia, or proliferation of the primary vitreous (hyaloid vasculature). The extent of the opacity created in the posterior lens capsule, lens, and anterior vitreous impacts the effect on vision. Hyaloid remnants are the remaining hyaloid vasculature which initially provided the prenatal blood supply to the posterior lens (Figure 12.1; see also Figure 11.5). In the latter part of the prenatal and early postnatal eye development, after the production of aqueous humor has begun, atrophy of hyaloid vasculature occurs. Hyaloid remnants are not usually associated with visual impairment unless very extensive. Figure 12.1 Hyaloid remnants in a young puppy. Located at the posterior pole of the lens, these hyaloid remnants appear as a white strand attached to the axial posterior lens capsule. Hyaloid remnants consist of either small fibrous remnants located at the axial posterior lens capsule or larger axial cataracts involving the posterior cortex and posterior lens capsule with small but functional persistent hyaloid blood vessels that extend from the optic nerve head to the posterior lens capsule. In the former, the clinical appearance is a small, dense, round to oval, posterior capsular opacity that does not progress, and can reduce in size and density during the first 2 years of life. In the latter, the persistent hyaloid vasculature appears as prominent red vessels within the posterior lens capsule and the posterior lens cortex, with varying amounts of opacification of the lens in the same area. If the axial cataract is not large, direct inspection as well as ophthalmoscopy reveal a fibrous band or perfused red blood vessels extending from the optic nerve head to the posterior lens surface. Intralenticular hemorrhage can also occur with persistent hyaloid blood vessels.
Canine Vitreous
Congenital Abnormalities
Persistent Hyaloid Remnants
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