Abstract
Tibia trabeculae and vertebrae of rats as well as human femur were investigated by high-resolution TEM at the atomic scale in order to reveal snapshots of the morphogenetic processes of local bone ultrastructure formation. By taking into account reflections of hydroxyapatite for Fourier filtering the appearance of individual alpha–chains within the triple–helix clearly shows that bone bears the feature of an intergrowth composite structure extending from the atomic to the nanoscale, thus representing a molecular composite of collagen and apatite. Careful Fourier analysis reveals that the non–collagenous protein osteocalcin is present directly combined with octacalcium phosphate. Besides single spherical specimen of about 2 nm in diameter, osteocalcin is spread between and over collagen fibrils and is often observed as pearl necklace strings. In high-resolution TEM, the three binding sites of the γ-carboxylated glutamic acid groups of the mineralized osteocalcin were successfully imaged, which provide the chemical binding to octacalcium phosphate. Osteocalcin is attached to the collagen structure and interacts with the Ca–sites on the (100) dominated hydroxyapatite platelets with Ca-Ca distances of about 9.5 Å. Thus, osteocalcin takes on the functions of Ca–ion transport and suppression of hydroxyapatite expansion.
Highlights
Since the first report on bone structure by Havers[1] the structural hierarchy described for bone comprised mostly five levels and over the years became more refined and was extended to seven[2] and nowadays to even nine levels[3]
The presence of the twisted plywood pattern is clearly demonstrated by collagen fibrils lying in plane and running along the view direction as shown in Fig. S6f 6,38–40
Further analysis of the TEM micrographs reveals that the non–collagenous protein osteocalcin is present at/on the calcified collagen together with octacalcium phosphate
Summary
Since the first report on bone structure by Havers[1] the structural hierarchy described for bone comprised mostly five levels and over the years became more refined and was extended to seven[2] and nowadays to even nine levels[3]. About 25% of the NCPs are calcium binding serum proteins such as albumin with a molecular weight of 69 kilo Dalton (kD), and fetuin precursor α2-HS glycoprotein 2q11-13. This group encompasses only two different proteins. Besides type I collagen (85–90 wt.%), OC is the most abundant protein within the organic matrix of mature human bone and plays a major role as structure directing molecule[10,11] and in the regulation of osteoblast and osteoclast activity[12]. Our intention was to reveal the nanostructural relationships between the collagen fibrillar structure and osteocalcin as well as the nucleation and deposition mechanisms for octacalcium phosphate and HAP growth. Because of the complex structure of bone, characterized by various levels of hierarchy, the investigations were restricted to the basic composite units on the fibril level from the atomic to the nanoscale
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