February 27th, 2012 | Category: Mathematics, Seminars | Leave a comment

Self-organization and self-assembly in biological materials: the example of nacre

This Friday (2nd March) Professor Julyan Cartwright (Universidad de Granada) will be giving the following seminar:

“Self-organization and self-assembly in biological materials: the example of nacre.”

The seminar will be held from 10:00 – 11:00 in Newman D.

Abstract:

How does an organism assemble itself? After the cells make the molecules that are to form part of the structure, how do they form into tissue; flesh and bone? This is the problem of supramolecular assembly in biology, which is as yet in general unresolved. One prediction we may make is that liquid crystals will in many instances turn out to be involved.

One hint that this is so is in the large number of tissues that have a structure of fibres set in a matrix in a similar fashion to artificial fibrous composites like fibreglass or plywood. To name just a selection, bone and cornea, arthropod cuticle and eggshell, and plant cell wall are all examples of such natural fibrous composites. The morpologies of these composites strongly suggest a link to liquid crystals: although these materials are solids, the hypothesis is that they must have self-assembled during an earlier mobile phase before solidification to produce this liquid-crystalline organization [1,2]. It is clear that fluid flow must be involved in these liquid crystallization processes, although as yet the means of both the initial liquid-crystal formation and of its subsequent solidification are in most cases unknown [3].

One such structure that has been researched in more detail than most is nacre, or mother of pearl. Many species of mollusc secrete a mineral shell about themselves. Fluid flow in the liquid filled extrapallial space between the mantle and the shell of molluscs is of a peculiar nature. Liquid containing the components of the shell is continually excreted from the cells of the mantle and self-organizes into the shell structure. In the case of those species that produce a nacreous coating of the interior surface of the shell, at least, this process of self organization, gelification and solidification into nacre involves an intermediate state involving a cholesteric liquid-crystal structure of chitin crystallites that is subsequently coated with protein and mineralized [4,5]. This may be the first instance in which we are able to understand the process of liquid crystallization and solidification of a fibrous composite.

Nacre is of interest because it is in the intersection of fibrous composites with a second group of supramolecular biological structures:  it is also a biomineral. Biominerals like bone and teeth, shell and carapace have long been studied [6]. In those instances, like that of nacre formation, in which the biomineralization process has been examined in detail, liquid crystals have been found to be involved. Such is the case with bone and in fish otolith formation, and it is likely that further examples will come to light as our knowledge of physics and chemistry of the so-called `soft matter’ involved in biological mineral deposition improves [7].

[1] Bouligand, Y (1972) “Twisted fibrous arrangements in biological materials and cholesteric mesophases.”  Tissue Cell 4, 189–217.

2] Neville, AC (1993). Biology of Fibrous Composites: Development Beyond the Cell Membrane, Cambridge University Press, Cambridge.

[3] Cowin, S (2004). “Do liquid crystal-like flow processes occur in the supramolecular assembly of biological tissues?” J. Non-Newtonian Fluid Mech. 119, 155–162.

4] Cartwright, JHE, and Checa, AG (2007). “The dynamics of nacre self- assembly.” J. R. Soc., Interface 4, 491–504.

[5] Cartwright, JHE, Checa, AG, Escribano, B, and Sainz-Diaz, CI (2009). “Spiral and target patterns in bivalve nacre manifest a natural excitable medium from layer growth of a biological liquid crystal ”. Proc. Natl Acad. Sci. USA 106, 10499–10504.

[6] Bouligand, Y (2004). “The renewal of ideas about biomineralisations.” Comptes Rendus Palevol 3, 617–628.[7] Cartwright, JHE, Piro, O, and Tuval, I. (2009). “Fluid dynamics in developmental biology: moving fluids that shape ontogeny”,  HFSP J. 3, 77–93.

[7] Cartwright, JHE, Piro, O, and Tuval, I. (2009). “Fluid dynamics in developmental biology: moving fluids that shape ontogeny”,  HFSP J. 3, 77–93.

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