Engineering Fibrous Tissues and Their Interfaces with Bone - Structural Interfaces and Attachments in Biology - page 315

compared to the 70-80% in ligaments [ 18 , 21 ]. Collagen type I is the predominant
fiber in tendon and ligaments, constituting 95 and 90%of the total collagen content in
each tissue, respectively [ 10 , 18 , 21 ]. In this molecule, three left-handed helices with
a base peptide unit of glycine-proline-hydroxyproline assemble to form fibers with a
right-handed superhelix in a coiled structure [ 13 ]. The superhelical strands then pack
next to each other to form collagen microfibrils [ 14 ], as previously described. This
structure accounts for the tissue's high tensile strength and relative inelasticity [ 21 ].
Other major collagens found in tendon and ligament include types III and V.
Collagen type III forms smaller and less organized fibrils than collagen type I,
which can decrease mechanical strength. It is present in the endotenon/
endoligament and the epitenon/epiligament and is synthesized during early phases
of remodeling and repair [ 24 ]. Collagen V is cross-linked to other collagen types
and regulates fibril growth [ 25 , 26 ]. Additionally, collagens II, VI, IX, X, XI, and
XII are all found in trace amounts [ 27 , 28 ].
15.2.3.2 Elastin
In tendons and ligaments, elastin accounts for 2-3% and up to 10-15% of the dry
weight of the tissue, respectively [ 10 , 21 ]. Elastin is an insoluble globular protein
that contains large hydrophobic domains and adopts a coiled arrangement in a
relaxed state [ 10 ]. Stretching causes the coils to unravel and exposes the hydropho-
bic domains to water, creating a thermodynamic driving force for the coils to return
to their original configuration. Elastin and microfibrillar proteins contribute to the
recovery of a crimped structure after an applied strain [ 10 ].
15.2.3.3 Proteoglycans
Proteoglycans contain protein cores with covalently attached glycosaminoglycans.
Glycosaminoglycans are complex carbohydrate molecules that provide proteoglycans
with unique properties [ 25 ]. Two groups of proteoglycans exist in fibrous tissues:
small leucine-rich proteoglycans and large modular proteoglycans. In fibrous tissues,
proteoglycans are found in relatively small amounts (1-5%dryweight) trappedbetween
collagen fibers and fibrils [ 14 ]. Their negative charge plays a large role in attracting
water in the tendon and ligament tissue and creates repulsion forces in proteoglycans
that provide the tissue with the ability to resist high tensile and compressive loads [ 25 ].
Proteoglycans also can create space between collagen fibers, thus enabling diffusion
of water-soluble molecules, the migration of cells, as well as allowing fibers to slide
past each other in response to compressive or tensile load [ 16 , 25 ].
Decorin is the most abundant proteoglycan found in tendons. Its main function is to
maintain and regulate collagen fibril structure. Decorin is considered a key regulator in
matrix organization and is associated with remodeling in a tendon under tensile forces
as evidenced by the observation that the skin of decorin-deficient mice was not able to
withstand tensile strain [ 29 , 30 ]. Fibromodulin, similar to decorin, binds to collagen