Fibrillin-rich microfibrils are thin filamentous connective tissue assemblies present in virtually all dynamic connective tissues (Fig. 5.1) (Kielty and Shuttleworth, 1995; Handford et al., 2000; Sherratt et al., 2000). In elastic tissues – such as aorta, lung, skin, and elastic cartilage – preformed bundles of microfibrils form a template for tropoelastin deposition during elastic fibre formation and are retained as an outer mantle of mature elastic fibres (Mecham and Heuser, 1991). Microfibril arrays are often abundant in tissues which do not express elastin, such as the ciliary zonules of the eye which hold the lens in dynamic equilibrium (Ashworth et al., 2000).
Several recent studies, based on whole tissues and isolated fibrillin-rich microfibrils, have highlighted their unique elastic properties that are critical to their biological function alone or in association with elastin. The extensibility of lobster aorta was accounted for by microfibril arrays that intersperse medial smooth muscle cells (McConnell et al., 1996). Extracted sea cucumber microfibrils exhibited long-range elastomeric properties (Thurmond and Trotter, 1996). X-ray diffraction, tensile testing, and stress-relaxation tests demonstrated that hydrated mammalian ciliary zonules and microfibril bundles are reversibly extensible in the presence or absence of calcium (Wess et al., 1998a; Wright et al., 1999; Eriksen et al., 2001). Isolated human microfibrils tangled in debris during preparation for electron microscopy can become stretched to periodicities up to ∼ 165 nm (Keene et al., 1991). Extended microfibrils, often observed in matrix metalloproteinase-treated preparations, may reflect pathological loss of elastic recoil (Ashworth et al., 1999a).