Fig. 9. SEM micrographs from Weber and Schmid (1985) showing longitudinal (L) and cross (X) sections of Polyorchis radial fibres. Note the beaded appearance of the fibrillin microfibrils in the longitudinal section. The microfibrils are the high aspect ratio filaments arranged axially with large regions of overlap. The cross section shows the fibre to be a densely packed bundle of microfibrils, with a volume fraction of 70–80%. It is not possible to determine whether individual microfibrils span the entire length of the fibre, but this seems unlikely. No interfibrillar material is obvious in the SEM, but we cannot rule out the possibility that a matrix might exist that did not stain in Weber and Schmid's preparation. We therefore present two possible composite models of the fibre mechanics: a parallel model in which the microfibrils either span the entire length of the fibre, or are cross-linked such that they behave effectively as if they did, and a series model in which the microfibrils transmit axial loads through interfibrillar shear. Reproduced from Weber and Schmid (1985) with permission from Elsevier. Scale bar, 250 nm.