Rotary shadowing of elastic system microfibrils in the ocular zonule, vitreous, and ligamentum nuchae

Rotary shadowing of zonular fibrils in human and bovine eyes revealed a "string of beads" configuration with multiple interconnecting filaments, identical to that recently reported in fibrils of unknown type within the vitreous. These 29 nm beaded fibrils were the only macrostructures present in zonular samples, showing ultrastructural features correlating with both the macro and microperiodicity of zonular fibrils in tissues. Interbead periodicity varied from 30-57 nm and interbead filaments appeared capable of stretching even further, possibly explaining the inherent elasticity of zonular fibrils. The junctions between outer filaments and beads were fibrillin-positive. Similar beaded fibrils were found in the human and bovine anterior vitreous along with type II and IX collagen fibrils, proteoglycan filaments and other unidentified fibrils. After collagenase and elastase digestion, bovine ligamentum nuchae showed type VI collagen fibrils and clumps of beaded fibrils like those in zonule and vitreous. This distribution indicates that the beaded fibril is the microfibril which constitutes the basic unit of the elastic system.