Wolfie Traub

Bone structure: from angstroms to microns.

Bone has a complex hierarchical structure, which despite much investigation, is still not well, understood. Here we bring together pieces of this complicated puzzle, albeit from different sources, to present a tentative overview of bone structure. The basic building blocks are the extremely small plate‐shaped crystals of carbonate apatite, just hundreds of ångstroms long and wide and some 20–30 Å thick. They are arranged in parallel layers within the collagenous framework. At the next hierarchical level these mineral‐filled collagen fibrils are ordered into arrays in which the fibril axes and the crystal layers are all organized into a 3‐dimensional structure that makes up a single layer or lamella of bone a few microns thick. The orientations of the collagen fibrils and the crystal layers in alternating lamellae of rat bone differ such that in the thinner lamellae, the fibrils and the crystal layers are parallel to the lamellar boundaries. In the thicker lamellae the fibrils are parallel to the boundary, but the crystal layers are rotated out of the plane of the boundary. In many bones these alternating lamellae are organized into even larger ordered structures to produce what is truly a remarkably ordered material, all the way from the molecular scale to the macroscopic product.—Weiner, S.; Traub, W. Bone structure: from ångstroms to microns. FASEB J. 6: 879‐885; 1992.

Organization of hydroxyapatite crystals within collagen fibrils

Transmission electron micrographs of individual mineralized collagen fibrils show that hydroxyapatite crystals are located mainly within the fibrils at the level of the gap regions. The plate‐shaped crystals are observed to be more or less uniformly stacked across the fibril diameter. We therefore suggest that the crystals are primarily located in ‘grooves’ created by contiguous adjacent gaps. The proposal is consistent with the observed crystal distribution in the fibril and with their average widths, which are almost 10‐times greater than an individual gap diameter.

X‐ray diffraction study of the insoluble organic matrix of mollusk shells

Mollusk shells are generally composed of calcium carbonate in an organic matrix. The organic matrix is observed to form prior to mineralization [ 1,2], and it has been suggested that some of the matrix protein components may serve as a template for mineral deposition [3-61. The calcium carbonate of the shell can be dissolved in EDTA leaving an insoluble fraction which comprises the bulk of the organic matrix [5,7,8] and consists of a heterogeneous mixture of proteins and carbohydrate [9-l 21. X-ray diffraction patterns reported for various organic matrices [13-l 71 are quite poor in detail and, apart from suggestions that they may contain (Yand /3-keratin-type features [ 16,171, have not led to structural interpretations. We describe here a new X-ray fiber diffraction study of insoluble organic matrices of shells representing the three major classes of mollusks. This enabled us to establish that the ordered protein components in the insoluble fraction adopt an antiparallel P-sheet conformation and that, where recognizable, the polysaccharide phase is chitin. We have determined the structural parameters of these phases, and have also investigated the relative orientations of the protein, mineral crystals and, in some cases, the chitin for several shell layers.

Rotated plywood structure of primary lamellar bone in the rat: Orientations of the collagen fibril arrays

A basic structural motif of lamellar bone is the arrays of parallel collagen fibrils, with successive arrays having different orientations to form a plywood-like structure. Measurements of the angles between adjacent arrays from cryomicrotomed and vitrified thin sections of demineralized rat bone, cut approximately parallel to the lamellar boundary plane, show that most angles are around 30 degrees, although a subset are around 70 degrees. A structural model for collagen organization based on these measurements is proposed in which an individual lamellar unit (thick and thin lamellae together with transition zones) is composed of five arrays of parallel collagen fibrils, each offset by 30 degrees.

Polymers of tripeptides as collagen models: IV. Structure analysis of poly(l-prolyl-glycyl-l-proline)

Abstract Poly( l -prolyl-glycyl- l -proline) closely resembles collagen in its X-ray diffraction pattern, which indicates that it has a triple-helical structure with the same helical parameters as the protein. A detailed conformational analysis has been made of the polytripeptide. A computer was used systematically to generate structures having the observed helical parameters, as well as standard bond lengths and angles, and to test these structures in terms of acceptable intramolecular and intermolecular van der Waals distances and agreement between calculated and observed intensities. These computations narrowed down the possibilities to a unique conformation which resembles the collagen II model in its mode of interchain hydrogen bonding. Conformations of the collagen I type were found to make short intermolecular contacts, and those in which the glycyl NH is hydrogen bonded in the manner of the two-bonded model proved incompatible with the observed intensities of the equatorial reflections. It is suggested that the conformation found for the polytripeptide may be representative of the structure of collagen as a whole.

An X-ray study of the interaction of DNA with spermine

Abstract The tetramine spermine forms a complex with DNA, and thereby serves to protect it against denaturation and breakage. X-ray fibre photographs of the DNA-spermine complex were taken at various relative humidities. The most detailed pattern was obtained at 92% relative humidity. It shows the complex to have the same helical parameters as the naturally occurring B form of DNA, and presumably a closely similar conformation. However, at 76% and lower relative humidities, the complex conforms to the DNA C configuration. The intermolecular distance was found to vary with the degree of hydration of the specimen, but to have a definite upper limit. This and the range of intermolecular distance observed are consistent with spermine cross-links between DNA molecules. Studies of molecular models have shown that it is also possible for all four basic groups of a spermine molecule to make hydrogen bonds with phosphate groups on one molecule of DNA. The spermine molecules would lie across the small groove of the DNA and serve to bind its two strands together. Calculation of intensities for various models for the DNA-spermine complex indicates that no one model can account for the observed X-ray pattern. However, the assumption of spermine in the small groove of DNA as well as in intermolecular cross-links, or of an even greater variety of modes of attachment between DNA and spermine, does lead to satisfactory agreement between calculated and observed intensities.

Electron diffraction of mollusc shell organic matrices and their relationship to the mineral phase

Abstract Electron diffraction patterns showing orientation of the chitin and protein constituents of the insoluble organic matrix of mollusc shell nacreous layers have been obtained, using low dose conditions and samples cooled to −100°C. Diffraction patterns of the aragonite crystals were also observed. In a gastropod and a bivalve the spatial relationship between the organic matrix constituents and the aragonite crystallographic axes were shown to be the same as was previously observed for a cephalopod using X-ray diffraction, supporting the notion that mineral crystal growth occurs epitaxially upon a matrix template.

Crystal organization in rat bone lamellae

The plate‐shaped crystals of rat bone are arranged in parallel layers that form coherent structures up to the level of individual lamellae. The crystal layers of the thin lamellae are parallel to the lamellar boundary, whereas those of the thicker lamellae are oblique to the boundary. The basic structure of rat bone can be described as ‘rotated plywood’; a structure hitherto unrecognized in either biologic or synthetic materials.

Organic Matrix in Calcified Exoskeletons

A speculative model of organic matrix structure and function, based primarily on aspects of mollusk shell matrix biochemistry, is presented. A brief review of the biochemistry of organic matrices from various other phyla, suggests that at least the essential elements of the mollusk matrix model are more generally applicable viz. the widespread presence of soluble acidic proteins, as well as insoluble “framework” macromolecules. The latter show considerable variation between the various phylogenetic groups. The observed common matrix properties provide an encouraging basis for more effectively utilizing the comparative biochemical approach for studying organic matrix functions in biomineralization.

Crystal Size and Organization in Bone

Size measurements of dispersed rat bone crystals show that with age a greater proportion exceeds 400 A in length. The surface fractures of more heavily mineralized bones tend to be smooth and stepped, whereas those of less mineralized bones are fibrous. These observations combined with information reported elsewhere on the crystal-collagen relations in adjacent fibrils in turkey tendon, suggest that some crystals grow out of the confines of the collagen gap regions to form extended aggregates of large flat crystals with well developed fracture planes.