Vitreous: the resplendent enigma

Abstract

Arguably the most enigmatic of ocular structures, vitreous has long been unappreciated with respect to its role in health and disease. The classic anatomists and histologists of the 18th (Demours, 1741; Zinn, 1755) and 19th (Sir William Bowman, 1848; Virchow, 1885) centuries considered vitreous a tissue. However, in 1930, Duke-Elder1 pointed out that vitreous is not a tissue but a cell product. This is consistent with our current concept of vitreous as an extended extracellular matrix composed primarily of water (98%), collagen and hyaluronan. The exact cells and processes responsible for the synthesis of these macromolecules are, as yet, unidentified. Also lacking is a comprehensive understanding of the supramolecular organisation of vitreous, limiting the ability of investigators and clinicians to study vitreous experimentally and evaluate it clinically. Indeed, even the roles of vitreous in ocular physiology are not universally established. One probable function of vitreous, however, is the maintenance of transparency within the eye (fig 1). This minimises light scattering and allows the unhindered transmission of photons to the retina for photoreception. Towards this end, vitreous possesses biochemical properties that inhibit cell migration and proliferation.23 While essential for vision, the invisibility of vitreous has obliged generations on both sides of the slit lamp and ophthalmoscope to merely look through vitreous, unable to adequately look at it. Figure 1 Human vitreous. The sclera, choroid and retina were dissected off the vitreous body, which remains attached to the anterior segment. Although placed on a surgical towel and exposed to room air, the shape of the vitreous body is spherical, and the gel is intact. The exquisite transparency of the vitreous body is evident. Specimen courtesy of the New England Eye Bank. The absence of effective diagnostic techniques with which to evaluate vitreous, both scientifically and clinically, has also hindered our ability …