Extraction of extendable beaded structures and their identification as fibrillin-containing extracellular matrix microfibrils

High molecular weight aggregates were extracted from human amnion using buffers containing 6 M guanidine hydrochloride. Rotary shadowed preparations and negatively stained samples examined by electron microscopy showed that each aggregate appeared to be a string of globular structures joined by fine filaments, giving the appearance of beads on a string. The periodicity of the beads was variable. A mouse monoclonal antibody directed against a previously characterized pepsin fragment of fibrillin was used with gold-conjugated secondary antibody and immunoelectron microscopy to show that the aggregates contained fibrillin. Similar structures were found in non-denaturing homogenates of skin, tongue, ligament, ciliary zonule, cartilage, and vitreous humor. When immunogold-labeled beaded structures were prepared for electron microscopy in the same manner as tissue, the beaded structures could no longer be seen. Instead, gold-labeled microfibrils were found which appeared to be the same as the fibrillin-containing matrix microfibrils observed in connective tissues and often associated with elastin. Thus, standard TEM protocols including fixation, dehydration, and embedding alter the ultrastructural appearance of microfibrils as compared with negative stain or rotary shadowing techniques. When skin was stretched and prepared for electron microscopy while still under tension, beaded filaments were seen in the tissue sections, but were not visible in non-stretched controls. In addition, when stretched ligament was immunolabeled with antibody directed against fibrillin while still under tension, the periodicity of antibodies along the microfibrils increased compared with non-stretched controls. We propose that microfibrils contain globular structures connected by fine filaments composed at lease in part of highly ordered, periodically distributed fibrillin molecules, whose periodicity is subject to change dependent on the tensional forces applied to the tissue in which they are contained.