Biomedical Engineering Reference
In-Depth Information
FIGURE 6.4
(a) Scanning electron micrograph of the surface of an adult rabbit bone matrix, showing how the
collagen fibrils branch and interconnect in an intricate, woven pattern (×4800). (Adapted from Tiffit, J.T. 1980.
Fundamental and Clinical Bone Physiology
, p. 51, JB Lippincott Co., Philadelphia, PA.) (b) Transmission electron
micrographs of (×24,000) parallel collagen fibrils in tendon. (c) Transmission electron micrographs of (×24,000)
mesh work of fibrils in skin. ((b) and (c) With kind permission from Springer Science+Business Media:
Biomechanics,
Mechanical Properties of Living Tissues
, 2nd ed., 1993, p. 255, New York, Fung, Y.C.)
organization of collagen molecules in a fibril is tissue-specific (Katz and Li, 1972, 1973b). The 2-D struc-
ture (the projection of a 3-D structure onto a 2-D plane) of a type I collagen fibril has been unequivocally
defined both by an analysis of small-angle x-ray diffraction pattern along the meridian of a collagenous
tissue (Bear, 1952) and by examination of the transmission electron micrographs of tissues stained
with negative or positive stains (Hodge and Petruska, 1963). In this structure (Figure 6.2d), the col-
lagen molecules are staggered with respect to one another by a distance of
D
(64-67 nm) or multiple of
D
, where
D
is the fundamental repeat distance seen in the small-angle x-ray diffraction pattern, or the
repeating distance seen in the electron micrographs. Since a collagen molecule has a length of about
4.4
D
, this staggering of collagen molecules creates overlap regions of about 0.4
D
and hole or defect
regions of about 0.6
D
.
One interesting and important structural aspect of collagen is its approximate equal number
of acidic (
aspartic
and
glutamic acids
) and basic (lysines and
arginines
) side groups. Since these
groups are charged under physiological conditions, the collagen is essentially electrically neutral
(Li and Katz, 1976). The packing of collagen molecules with a
D
staggering results in clusters of
regions where the charged groups are located (Hofmann and Kuhn, 1981). These groups therefore
are in close proximity to form intra- and intermolecular hydrogen-bonded
salt-linkages
of the form
