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.