C A Shuttleworth

Fibrillin-rich microfibrils: elastic biopolymers of the extracellular matrix.

Fibrillin-rich microfibrils are evolutionarily ancient macromolecular assemblies of the extracellular matrix. They have unique extensible properties that endow vascular and other tissues with long-range elasticity. Microfibril extensibility supports the low pressure closed circulations of lower organisms such as crustaceans. In higher vertebrates, microfibrils act as a template for elastin deposition and are components of mature elastic fibres. In man, the importance of microfibrils is highlighted by the linkage of mutations in their principal structural component, fibrillin-1, to the heritable disease Marfan syndrome which is characterised by severe cardiovascular, skeletal and ocular defects. When isolated from tissues, fibrillin-rich microfibrils have a complex ultrastructural organisation with a characteristic ‘beads-on-a-strong’ appearance. X-ray fibre diffraction studies and biomechanical testing have shown that microfibrils are reversibly extensible at tissue extensions of 100%. Ultrastructural analysis and 3D reconstructions of isolated microfibrils using automated electron tomography have revealed new details of how fibrillin molecules are aligned within microfibrils in untensioned and extended states, and delineated the role of calcium in regulating microfibril beaded periodicity, rest length and molecular organisation. The molecular basis of how fibrillin molecules assemble into microfibrils, the central role of cells in regulating this process, and the identity of other molecules that may coassemble into microfibrils are now being elucidated. This information will enhance our understanding of the elastic mechanism of these unique extracellular matrix polymers, and may lead to new microfibril-based strategies for repairing elastic tissues in ageing and disease.

Fibrillin-rich microfibrils of the extracellular matrix: ultrastructure and assembly.

Fibrillin-rich microfibrils are a unique class of extensible connective tissue macromolecules. Their critical contribution to the establishment and maintenance of diverse extracellular matrices was underlined by the linkage of their principal structural component fibrillin to Marfan syndrome, a heritable connective tissue disorder with pleiotropic manifestations. Microscopy and preparative techniques have contributed substantially to the understanding of microfibril structure and function. The supramolecular organisation of microfibrillar assemblies in tissues has been examined by tissue sectioning and X-ray diffraction methods. Published findings are discussed and new information reported on the organisation of microfibrils in the ciliary zonular fibrils by environmental scanning electron microscopy. This review summarises microscopy and X-ray diffraction studies that are informing current understanding of the ultrastructure of fibrillin-rich microfibrils.

Scanning transmission electron microscopy mass analysis of fibrillin-containing microfibrils from foetal elastic tissues

We have applied scanning transmission electron microscopy to intact native fibrillin-containing microfibrils isolated from foetal bovine elastic tissues in order to derive new insights into microfibril organisation. This technique provides quantitative data on the mass per unit length and axial mass distribution of unstained, unshadowed macromolecules. Scanning transmission electron microscopy of microfibrils from aorta, skin and nuchal ligament revealed that the beads corresponded to peaks of mass and the interbead regions to troughs of mass. These major features of axial mass distribution were characteristic of all microfibrils examined. Tissue-specific and age-dependent variations in mass were identified in microfibrils that were structurally comparable by rotary shadowing electron microscopy. Increased microfibril mass correlated with increasing gestational age. The additional mass was associated predominantly at, or close to, the bead. Some microfibril populations exhibited pronounced assymetry in their axial mass distribution. These data indicate that intact native microfibrillar assemblies from developing elastic tissues are heterogeneous in composition. Loss of mass following chondroitinase ABC or AC lyase treatment confirmed the presence of chondroitin sulphate in nuchal ligament microfibrillar assemblies.