MINERAL-PROTEIN COMPLEX STRUCTURE IN BONE

Wolfie Traub¹,Louise Zylberberg², Vivian de Buffrenil², Francoise Allizard², Talmon Arad¹ and Steve Weiner¹

¹DepartmentofStructuralBiology,WeizmannInstituteofScience,Rehovot, Israel
²Laboratoire d'Anatomie Comparee, CNRS URA 1137, Universite Paris 7-Denis Diderot, Paris, France, E-mail: cstraub@weizmann.weizmann.ac.il

Mineralized tissues are widespread in nature and have many different compositions and functions. Generally protein matrices are formed first and subsequently mediate the organized deposition of mineral crystals, forming composite materials with a wide range of mechanical properties.

In most bones the crystals are tiny plates of carbonated hydroxylapatite, which are arranged in layers through fibrils of type I collagen. In cortical bone the mineralized fibrils are stacked in lamellae, with alternating orientations, forming what has been described as a rotated plywood structure. The mechanisms of crystal nucleation and growth in bone continue to be the subject of extensive investigations.

Recently the structure has been elucidated of a hypermineralized bone from the rostrum of the toothed whale Mesoplodon densirostris, which has yielded the highest values for density (2.6 gm/cm3) and mineral content (87%) yet reported. Scanning and transmission electron microscopy show parallel rods of mineral oriented along the length of the rostrum. These consist of platey crystals of hydroxylapatite, which, judging from electron diffraction, are extremely well and coherently aligned. The collagen component of the rostral bone consists largely of very thin fibrils aligned in longitudinal register to form tubular networks. The collagen fibrils are also aligned with the lengths of the mineral rods, which are apparently accommodated in the tubular holes.

This novel ultrastructure does not include the mineralized fibrils and lamellar features commonly observed in vertebrate boney tissues, though histological examination indicates some vestiges of more "normal" bone in portions of the rostrum. However, this ultrastructure can explain the high density and fragility of the rostrum, and may have some relevance to the mechanical properties of dense bones in pathological conditions.