Development of colour-producing beta-keratin nanostructures in avian feather barbs
Original Entry: Tom Dimiduk APPHY 225 Fall 2010
Many organisms owe their color to coherent light scattering from wavelength sized nanostructures. In many many arthropods, these are are believed to be generated by sophisticated biochemical synthesis. The authors propose that in the case of many bird feathers, these structures are created through a self-assembly process involving phase separation. They show evidence that some feathers use spinodal decomposition and others use a nucleation-growth process. They provide evidence for their conclusions in the form of electron micrographs of fully developed feather structures compared simple physical structures known to be caused by spinodal decomposition and nucleation-growth. They also show micrographs of development and propose explanations of the feater growth process in the context of their hypothesis.
Soft Matter Discussion
These feathers show two different kinds of demixing which have noticably different products. Fig 8 shows how the authors quantitatively establish the two kinds of demixing. In Fig 8a the spatial frequencies of the feathers line up remarkably well with those of a polymer spinodal decomposition (low frequency deviations are due to imaging artifacts), while in Fig 8b there is a noticeable peak at a higher frequency representing the relatively uniform spherical structures grown from nuclei. Presumably there would be an even more obvious distinction in a 2d spectrum where the circles would be evident in b while a lacks any rotational symmetry.
The extent of growth in the case of either spinodal decomposition or post nucleation growth is controlled by polymerization of the keratin. There is a competition of the phase separation of keratin monomer from cytoplasm with polymerization of keratin that tends to lock the structure in place. The authors believe that differences in these rates lead to structure changes that explain slight color differences between members of the species. The authors do not explain what triggers the start of phase separation. Presumably there is a continuous synthesis of keratin, that would eventually cross a boundary allowing phase separation, but it would have to pass nearly through the top of the parabola in the phase diagram of fig 3 in order to have spinodal decomposition, so likely the cells accumulate a larger amount of keratin and then change something such that decomposition begins.
They observe that air gaps in the feathers are formed by the coffee stain effect, the cells die and dry out, flow to the pinned interface drags all the cell contents to their exteriors. The authors hypothesis that the drying rate is controlled to allow diffusion of some material back, which they believe may be part of setting the stage for the demixing that causes self assembly.
The authors believe that in both cases this is an example of true self assembly, not some biologically controlled effect. Metabolism sets up the initial conditions for the decomposition, but once those conditions are in place, the assembly is purely a physical process which does not require any biological activity.