56 Canine Sports Medicine and Rehabilitation
promotes differentiation of regenerating
myoblasts. Low‐grade mechanical strains are
stimulatory for early myogenic differentiation
and muscle regeneration. However, loss of the
endomyseal and perimyseal ECM leaves heal
ing muscle fragile and susceptible to reinjury.
Aggressive early stretching of injured muscles,
or rapid return to high‐intensity activity after
muscle injury commonly leads to re‐injury
and re‐initiation of an acute inflammatory
response. Recurrent or sustained monocytic
infiltration of injured or diseased muscle ulti
mately triggers fibroblast activation, collagen
deposition, and excessive fibrosis, leading to
permanent loss of muscle function.
Examination of exercising sled dogs before
an endurance race, 5 days into racing, and at
the finish showed there were no elevations in
the any of the cytokines (IL‐6, TNFα, IL‐8, or
IL‐15) other than monocyte chemoattractant
protein 1 (MCP‐1), which was elevated at the
mid‐point and finish of the race. The lack of
cytokine response was thought to be due to the
often transient nature of the inflammation of
exercise as an early event as this response had
never been examined in ultra‐endurance
exercise such as endurance sled dog racing
(Yazwinski et al., 2013). More recent examina
tion of this response in exercising sled dogs 2
days into an endurance event suggested mild
elevations in IL‐6 and IL‐10, thought to be part
of the M1 and M2 responses to exercise, but
these were far lower than the IL‐6 response
seen after prolonged daily exercise in people,
suggesting that this metabolic response may
be dampened in these unique canine athletes
(Von Pfeil et al., 2015)
Bone
Structural and functional organization
of bone
Bone is a complex and highly dynamic tissue
that serves many functions throughout the
body. The bony skeleton provides a mechanical
frame that supports locomotion by organizing
and directing the forces of muscular contrac
tion. In the adult, hematopoiesis takes place
primarily within a highly specialized bone
marrow niche, and is regulated by resident
bone cells. Bone is also a storehouse for calcium
and other minerals as well as fat, and plays
important roles in mineral homeostasis
and energetics.
Cortical bone has a dense structure, and is
made up of overlapping Haversian units or
osteons. The basic Haversian unit has a cylindri
cal organization with a central vascular channel
(central, or Volkmann’s canal) surrounded by
several layers of concentrically organized bone
matrix (circumferential lamellae) populated by
osteocytes (Figure 3.9). Trabecular, or cancellous
bone is present only within the medullary cav
ity and consists of an interconnected system of
thin bony shelves and struts called trabeculae.
Trabecular bone has an extensive surface area
and is organized into a vast number of micro
compartments that contain fat and islands of
hematopoietic cells (Figure 3.10). The external
and medullary surfaces of a bone are covered by
thin periosteal and endosteal membranes,
respectively. These membranes are highly vas
cular, richly innervated and populated by osteo
blasts and smaller numbers of osteoprogenitor
cells. The ECM of the periosteal and endosteal
membranes contains primarily type I collagen,
large amounts of elastin, and a variety of proteo
glycans. Bone can also be classified according to
the organization of the type I collagen fibrils
within the matrix. Woven bone is an intermedi
ate form of bone with random orientation of col
lagen fibrils that occurs during the initial phase
of endochondral ossification or fracture healing.
Lamellar bone is a mature form of bone with
greater density in which the collagen fibrils are
deposited in a highly ordered, multidirectional,
layered pattern. Mature cortical bone is a form
of lamellar bone.
The chemical composition and molecular
organization of bone is highly conserved.
Living bone contains approximately 70% min
eral, up to 20% organic matrix, and 5–10%
water. Bone mineral is primarily calcium
hydroxyapatite—Ca 10 (PO 4 ) 6 (OH) 2. Other min
erals such as magnesium, carbonate, and phos
phate are present in small amounts. The organic
matrix is made up of 90% type I collagen. The
remainder is composed of other collagen types
(types III and V), a variety of proteoglycans,
several bone‐specific proteins such as bone
sialoprotein and osteocalcin, and a variety of
growth factors elaborated during osteogenesis