Selective biodegradation of keratin matrix in feather rachis reveals classic bioengineering

Abstract

Flight necessitates that the feather rachis is extremely tough and light. Yet, the crucial filamentous hierarchy of the rachis is unknown-study hindered by the tight chemical bonding between the filaments and matrix. We used novel microbial biodegradation to delineate the fibres of the rachidial cortex in situ. It revealed the thickest keratin filaments known to date (factor .10), approximately 6 mm thick, extending predominantly axially but with a small outer circumferential component. Near-periodic thickened nodes of the fibres are staggered with those in adjacent fibres in two-and three-dimensional planes, creating a fibre-matrix texture with high attributes for crack stopping and resistance to transverse cutting. Close association of the fibre layer with the underlying 'spongy' medulloid pith indicates the potential for higher buckling loads and greater elastic recoil. Strikingly, the fibres are similar in dimensions and form to the free filaments of the feather vane and plumulaceous and embryonic down, the syncitial barbules, but, identified for the first time in 140þ years of study in a new location-as a major structural component of the rachis. Early in feather evolution, syncitial barbules were consolidated in a robust central rachis, definitively characterizing the avian lineage of keratin.

Nanomechanical properties of bird feather rachises: exploring naturally occurring fibre reinforced laminar composites

“…Alternatively, if differences in orientation are subtle to begin with they might not be sufficiently different enough to diffract Busson et al's X-rays, or even show up on our modulus mapping; if the two innermost laminae are at similar orientations to the indenter tip despite being relatively different, they may also have been reported as a single lamina in the second indentation of C. olor. Further work is clearly needed to compare and contrast the two approaches as neither Lingham-Soliar et al [14], nor Busson et al [13] reported the location in the wing from where they sampled feathers. It is possible that the number of rachis laminae also varies along the wing.…”

“…Figure 2a,b does arguably show a thin lamina at the interface although it is more likely that none of the indentations landed directly on this proposed amorphous lamina and this is an artefact of the graphing software's interpolation algorithm. The scanning electron microscope (SEM) image presented by Lingham-Soliar [14] after microbial degradation does show Busson et al's [13] two 'feather' laminae but the nature of the degradation combined with their imaging method make it difficult to distinguish the amorphous lamina from the embedding substantia. It should be noted that we do not observe an amorphous lamina in between fibre laminae on any nano-indentation maps, nor in the CT scan, all of which show an abrupt transition from one fibrous lamina to another.…”

“…fig.). however, relies on the differing orientations of the b-keratin fibres within the laminae identified by Lingham-Soliar et al [14] and their resulting anisotropy for differentiation. This mechanical anisotropy was demonstrated in the modulus results, with the same two laminae identifiable in the longitudinal section as in the cross-sectional approach, but with reversed E r values.…”

“…While we know that feather rachises consist of keratin micro-layers, or laminae, the exact number of these layers remains uncertain. Busson et al [13] reported three laminae (four including a superficial lipid membrane) in a peacock feather and, more recently, Lingham-Soliar et al [14] reported two functional laminae in chicken feathers. It is also not known whether these keratin microlaminae are an artefact of development or whether they evolved as a mechanism to cope with multi-directional stresses.…”

“…We know that b-keratin has a fibrous, microtubular microstructure [14] embedded in a substantia composed of an amorphous protein [17]. Similar to other fibre composite materials, we therefore expect the mechanical response of the feather b-keratin composite to be very different when tested in different directions [18], which provides an opportunity to identify laminae on the basis of this mechanical variation.…”