The zonules and the elastic microfibrillar system in the ciliary body.

It has been proposed that elastic fibers occur in some tissues as a three-part interconnecting system. The system includes two sizes of elastin-containing fibers surrounded by tubular microfibrils (elastic microfibrils), besides isolated bundles of tubular microfibrils without elastin (oxytalan fibers). This little-studied system was identified in the bovine ciliary body by light and electron microscopy. Its architecture varied regionally, suggesting different vectors of tractional force in the anterior and posterior ciliary body related to accommodation. Zonular fibers had the staining characteristics of oxytalan fibers, and their fibrils were ultrastructurally similar to the tubular microfibrils around elastic fibers and those composing oxytalan fibers. Antibodies to microfibrillar protein bound to zonules and to tubular microfibrils in all sites. This is the first evidence that tubular microfibrils both with and without elastin share antigenic determinants and confirms the close antigenic relationship of the zonules to this class of proteins.