Front Matter

46 Canine Sports Medicine and Rehabilitation

of elongation and elastic recoil such as joint
capsules or the nuchal ligament.
Elastin is a high‐molecular weight insoluble
protein biopolymer made up of numerous tro­
poelastin subunits (Mithieux & Weiss, 2005).
Tropoelastin is a 65 kDa hydrophobic protein.
Biosynthesis of elastin involves transcription
and translation of a single tropoelastin gene.
Tropoelastin is secreted from the cell as a
complex with specific elastin‐binding proteins.
Within the extracellular environment, tropoelas­
tin undergoes temperature‐dependent confor­
mational changes that expose hydrophobic
residues within the molecule that allow its
incorporation into a developing elastin macro­
molecule through hydrophobic interactions
(Indik et al., 1989; Vrhovski & Weiss, 1998). This
process is facilitated by many ECM components,
most notably the glycoproteins fibrillin and
fibronectin. Following assembly, elastin poly­
mers are stabilized by covalent intermolecular
crosslinks, the formation of which is catalyzed
by various copper‐dependent lysyl oxidases.
Groups of elastin polymers are further assem­
bled in hierarchical fashion into higher order
fibers that consist of a central core of crosslinked
elastin surrounded by an organized coat of
microfibrillar glycoproteins rich in fibrillin.

Elastic fibers are highly extensible and can

withstand up to 200% increases in length from

the resting state. Elastic fibers are also extremely

durable, and are capable of virtually unlimited

numbers of cycles of elongation and elastic

recoil without loss of strength (Keeley et  al.,

2002). Production of elastin (elastogenesis) is

prominent during growth, as well as during the

repair and remodeling phases of tissue healing.

However, once formed, elastic fibers are

extraordinarily stable and may persist through­

out adulthood without turnover.

Proteoglycans

Proteoglycans (PGs) are glycosylated proteins

that are prominent components of the extracel­

lular matrix of all connective tissues. The basic

structure of a PG consists of a core protein to

which a variable number of glycosaminoglycan

(GAG) side chains are covalently attached

(Figure 3.2). Core proteins vary greatly in length

and amino acid sequence. The GAG side chains

are long, linear polysaccharide polymers

composed of repeating disaccharide units.

The disaccharide units are composed of two

six‐ carbon sugars, the first of which is either

a  hexose or a hexuronic acid, and the second

Aggrecan

Chondroitin sulfate

Keratan sulfate

Core protein

Link protein

Hyaluronic acid backbone

Figure 3.2 Aggrecan structure. Multiple proteoglycan monomers are bound by specific link proteins to a backbone of
hyaluronic acid to form macromolecular aggregating proteoglycans.