ROSTRUM OF A TOOTHED WHALE - ULTRASTRUCTURAL-STUDY OF A VERY DENSE BONE

The rostral bones of the toothed whale, Mesoplodon densirostris, consi st mainly of hypermineralized secondary osteons and have yielded among the highest values for density (2.6 g/cm(3)) and mineral content (86. 7%) yet reported for any bone. Scanning and transmission electron micr oscopy show parallel rods of mineral oriented along the length of the rostrum, These consist of platey crystals of carbonated hydroxyapatite , which, judging from electron diffraction, are extremely well and coh erently aligned, The collagen component of the rostral bone consists l argely of very thin fibrils aligned in longitudinal register to form t ubular networks, The collagen fibrils are also aligned with the length s of the mineral rods, which are apparently accomodated in the tubular spaces of the collagenous network. This peculiar ultrastructure clear ly differs from the densely packed mineralized fibrils commonly observ ed in vertebrate lamellar osseous tissues, although histological exami nation has indicated some vestiges of ''normal'' primary bone surround ing the secondary osteons, Thus, the bone tissue in the rostrum is cha racterized by a remarkably sparse collagenous component. This ultrastr ucture can explain the high density, stiffness, and brittleness of the rostrum that have been observed. It also raises interesting questions about possible modes of crystal growth during ongoing mineralization in normal bone, and may have some relevance in the mechanical behavior of dense bones in pathological conditions. (Bone 23: 241-247; 1998) ( C) 1998 by Elsevier Science Inc. All rights reserved.