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Biomineralization in the radular teeth of the chiton Acanthopleura hirtosa

Evans, Louise Anne (1991) Biomineralization in the radular teeth of the chiton Acanthopleura hirtosa. PhD thesis, Murdoch University.

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The mineralized major lateral teeth of the chiton, Acanthopleura hirtosa have been studied to elucidate both the structural organization and chemical nature of the organic matrix and to extend the characterization of the major biominerals present within the teeth.

Light and electron microscopic examination of the tooth ultrastructure has shown that the organic matrix consists of a complex architectural arrangement of fibres which differs according to the particular biomineral which will be eventually deposited with it. In the posterior (magnetite-mineralized) region of the tooth relatively few, fine fibres occur, whereas in the anterior (calcified) region the fibres are highly ordered, appearing as bundles of thick rope-like material. Both unmineralized and demineralized teeth show the same variation in fibre density and arrangement, indicating that mineralization occurs on a structurally complex, preformed organic framework.

The major component of the organic matrix has been identified as the partially deacetylated a-polymorph of the polysaccharide, chitin. The remainder of the organic matrix is composed of proteinaceous material rich in aspartic and glutamic acids, with phosphoserine also being detected in appreciable quantities. The chitin and protein components possess a close spatial relationship, with a number of factors suggesting that the degree of organic matrix-mediated control differs in the different regions of the tooth.

Fourier transform infrared spectroscopy has identified the calcium biomineral as an apatite material containing carbonate and fluoride ions. This apatite material has been further separated into low and high density fractions with X-ray diffraction analysis indicating that the crystallinity of both fractions is intermediate between that of bovine tibia cortical bone and that of human tooth enamel. In addition the iron containing region of the tooth has been characterized using Mossbauer spectroscopy to determine the Verwey transition temperature and degree of non-stoichiometry of the magnetite component as well as the relative abundances of maghemite and other iron phases with low magnetic ordering and/or superparamagnetic transition temperatures.

The iron-storage protein, ferritin, has been used as a probe to further elucidate the nature of the organic matrix particularly at the posterior surface and in the central region of the tooth which is mineralized with lepidocrocite. These two areas demonstrated the greatest accumulation of ferritin molecules, indicating specialized properties of the matrix in these regions.

It is postulated that the structural organization of the organic matrix of the tooth is capable of influencing the formation of architecturally discrete deposits of very different biominerals even in the absence of cellular factors. In addition, it is thought that the physical arrangement of matrix fibres contributes significantly to the mechanical properties of the tooth: the magnetite-mineralized posterior region is hardened appropriately for its role in abrading the rocky substrate, while the matrix-rich apatite and lepidocrocite-mineralized regions are constructed so as to absorb the physical shocks encountered during the feeding process.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): School of Biological and Environmental Sciences
Notes: Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: Thank you.
Supervisor(s): Webb, John and Macey, David
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