TRANSITIONAL STRUCTURES IN LAMELLAR BONE

Scanning electron micrographs of fractured surfaces of mineralized bon e show a lamellar structure with alternating smooth and rough regions. These have been interpreted as corresponding to two distinct collagen fibril and mineral crystal orientations in a rotated plywood structur e. However, in various bones, there are clear indications of transitio n zones between lamellae in which the fibrils, as well as the plate-li ke crystals, have intermediate orientations. Strong evidence for inter mediate collagen fibril orientations comes from vitrified cryo-section s of demineralized bone. These show zones of fibril segments graded in length between more homogenous regions of fibrils roughly parallel to the specimen section. Evidence for intermediate crystal orientations comes from transmission electron micrographs and electron diffraction patterns of crushed bone fragments. A tentative scheme is presented fo r an interlamellar transition zone, involving rotation about the colla gen fibril axis as well as tilting of this axis parallel to the plane of the interlamellar boundary. Although it may be convenient to think of the structure of lamellar bone as being composed of alternating thi ck and thin lamellae, it is probably more correct and biologically mor e relevant to consider one pair of lamellae as the product of a single depositional cycle of varyingly oriented collagen fibrils that subseq uently mineralize. (C) 1996 Wiley-Liss, Inc.