Dr. Steve Weiner. Biomineralization Examples of Recent stadies

During mollusk shell formation, the mineral phase forms within an organic matrix composed of b-chitin, silk-like proteins and acidic glyco-proteins rich in aspartic acid. The matrix is widely assumed to play an important role in controlling mineralization. Thus understanding its structure is of prime importance. Cryo-TEM studies of the matrix of the bivalve Atrina embedded in vitrified ice, show that the interlamellar sheets are composed mainly of highly ordered and aligned b-chitin fibrils. The silk, which is quantitatively an important component of the matrix, could not be imaged within the sheets. Organic material was however observed between sheets. We infer that this is the location of the silk. As this material reveals no regular structure, we suggest that at least prior to mineralization the silk is in the form of a hydrated gel. This is supported by cryo-TEM structural observations of an artificial assembly of b-chitin with and without silk. This view of the nacreous organic matrix significantly changes previous models of the matrix structure, and hence hypotheses pertaining to the mechanisms by which mineral formation occurs.

Reference: J. Struct. Biol. 135, 8-17, 2001.

ESPI analysis of the deformation of a human tooth. Scale: microns

Teeth and bones have to perform demanding mechanical tasks during an animal’s lifetime. Their structures are therefore finely tuned to their mechanical functions. We use electron speckle phase interferometry (ESPI) to study the deformations of whole teeth, as well as small parts of teeth and bones. The deformations can be measured with tens of nanometers resolution and the changes can be mapped over irregular surfaces while under water – an essential requirement for the study of biological tissues. Methods have been devised to determine the compression elastic modulus with great accuracy and precision. A first study of root dentin shows an asymmetry in the modulus between opposing quadrants. The instrument can also be used to measure deformations in 3 dimensions, thus allowing us to investigate anisotropy in mechanical properties – a key parameter in trying to understand the relations between structure and mechanics.

3. Lamellar Bone: Structure-Function Relations

The term bone refers to a family of materials that have complex hierarchically organized structures. These structures are primarily adapted to the variety of mechanical functions that bone fulfils. Here we review the structure - mechanical relations of one bone structural type, lamellar bone. This is the most abundant type in many mammals, including humans. A lamellar unit is composed of 5 sub-layers. Each sub-layer is an array of aligned mineralized collagen fibrils. The orientations of these arrays differ in each sub-layer both with respect to collagen fibril axes and crystal layers, such that a complex rotated plywood-like structure is formed. Specific functions for lamellar bone, as opposed to the other bone types, could not be identified. It is therefore proposed that the lamellar structure is multi-functional - the "concrete" of the bone family of materials. Experimentally measured mechanical properties of lamellar bone demonstrate a clear-cut anisotropy with respect to the axis direction of long bones. A comparison of the elastic and ultimate properties of parallel arrays of lamellar units formed in primary bone, with cylindrically shaped osteonal structures in secondary formed bone, shows that most of the intrinsic mechanical properties are built into the lamellar structure. The major advantages of osteonal bone are its fracture properties.
Mathematical modelling of the elastic properties based on the lamellar structure and using a rule-of-mixtures approach, can closely simulate the measured mechanical properties, providing insight into the structure - mechanical relations of lamellar bone.

Reference: J. Structural Biology 126, 241-255, 1999.

4. Amorphous calcium carbonate is a transient precursor phase in the formation of biogenic crystalline calcite and aragonite

Y. Politi, S. Raz, I. Weiss, I. Sagi, L.Addadi and S. Weiner

It has now been demonstrated that the larvae of sea urchins and the larvae of molluscs form their skeletons via a transient precursor phase of amorphous calcium carbonate (ACC) (Beniash et al 1997, 199 and Weiss et al, 2002). We have now obtained evidence that a similar process occurs during the formation of the spine regeneration process of the adult sea urchin. We are also investigating the atomic short range order of the amorphous phase using EXAFS. We have shown that in contrast to the stable forms of ACC, the transient forms have little or no bound water. We suspect that the use of a transient ACC phase for forming crystalline mineralized skeletons is a widespread process in biomineralization.

5. Mollusk shell formation: mapping the distribution of organic matrix components underlying a single aragonitic tablet in nacre.

The surface of the organic matrix of mollusk shell nacre has been mapped using both histochemical stains, as well as antibodies against an aragonite nucleating fraction. In this way, 4 different zones have been revealed under each individual crystal tablet. The center of the tablet, where nucleation can be assumed to occur, has a central core which is rich in carboxylate groups, surrounded by a ring of sulfate groups.

It is suggested that a nucleation site of this type, may function along the lines proposed by Addadi et al (Addadi, L., Moradian, J., Shai, E., Maroudas, N. and Weiner, S., 1987.) A chemical model for the cooperation of sulfates and carboxylates in calcite crystal nucleation. Relevance to biomineralization. Proc. Natl. Acad. Sci. U.S.A. 84, 2732-2736). This is schematically shown in the figure below.

Nacreous organic matrix surface stained with aminoacridone. Note the structure of the nucleation centers. Schematic structure of the nucleation site composed of a structured carboxylate-rich core and surrounded by sulfate groups that are proposed to concentrate the calcium ions from solution.

Reference: Nudelman, F. Gotliv,B., Addadi, L. and Weiner, S. (2006).
Mollusk shell formation: mapping the distribution of organic matrix components underlying a single aragonitic tablet in nacre. J. Structural Biol. (in press).


Nacreous organic matrix surface stained with aminoacridone. Note the structure of the nucleation centers.	Schematic structure of the nucleation site composed of a structured carboxylate-rich core and surrounded by sulfate groups that are proposed to concentrate the calcium ions from solution.