Proposed model of fibrillin microfibrils (adapted with permission from reference [34••], Copyright The American Society for Biochemistry and Molecular Biology, 2007). (a) Rotary shadowed micrographs of guanidine-extracted microfibrils show characteristic repeats of approximately 57 nm (indicated by arrows). (b) A possible model of microfibril structure involves staggered, parallel molecules of fibrillin-1 with the N-terminal halves on the outside of the microfibril and C-terminal halves in the core. Individual fibrillin molecules extend over multiple sequential microfibril repeats. Antibody epitopes (mAb 26, mAb 201, and mAb 69) are labeled, and major collagenase cleavage sites are marked with arrows.