14.9: The Vitreous
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The vitreous is a semifluid and transparent structure composed of 99% water with the remnants composed of collagen fiber matrix and mucopolysaccharide (hyaluronic acid). It is normally avascular and does not contain nerves or lymphatics. The vitreous is attached anteriorly to the zonules and to the posterior lens and peripherally at the pars plana and around the optic nerve.
Primary vitreous (the hyaloid vessel system) forms as the lens vesicle and optic cup ar forming. The hyaloid artery originates from the site of the future optic nerve and extends to the posterior aspect of the developing lens. This system is compressed centrally and gradually atrophies as the secondary vitreous forms. It has usually disappeared completely by 2-3 weeks of age in the dog but portions may persist for a few months in ruminants.
Secondary vitreous is produced by the Muller cells of the retina. As it forms, and the eye enlarges, the primary vitreous becomes proportionally smaller and axially located. The secondary vitreous becomes the adult vitreous.
Tertiary vitreous consists of the ciliary zonules (lenticular zonules) suspending the lens. It forms from the neural ectoderm (surface epithelium) of the ciliary body and condenses into the individual zonules.