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that are important to appropriate repair of tissue
include ligament end apposition, nutritional
status, endocrine imbalances, severity
of injury, blood supply, and mechanical
stresses placed across the healing tissue.29,51 As
with muscle healing, allowing the ligament to
heal across a large gap results in excessive scar
tissue formation obstructing histologically
normal ligament frombridging the defect.29
Articular Cartilage Healing
Articular Cartilage Structure
and Function
Diarthrodial (synovial) joints consist of two or
more hyaline cartilage surfaces that are tightly
108 BASIC CONCEPTS OF VETERINARY MEDICINE
adherent to the underlying cortical end plates
of apposing bones. Hyaline cartilage is
designed to provide a smooth, low-friction,
gliding surface for jointmovement and to
transmit weight-bearing forces to the underlying
bone structure.11,53-56 Cartilage is primarily
avascular and without lymphatics. It is
approximately 70% to 80% water by weight
with the remainder composed of chondrocytes,
type II collagen (90% to 95% of the total
cartilage collagen), and proteoglycan aggregates.
11,53-57 Collagen provides tensile strength
to the jointsurface, which is divided into three
zones. The most superficial layer, the tangential
zone, has collagen fibers aligned parallel
to the joint surface creating primary resistance
to shear and tensile stresses placed on the
cartilage surface. Deeper within the cartilage,
the collagen fibers become oriented in a more
oblique fashion and form the intermediate
zone. The final, and largest, zone isthe radial
zone in which the collagen fibers are oriented
in a perpendicular columnar arrangement
and become embedded into the subchondral
bone.53-57
Chondrocytes comprise less than 10% of
the total cartilage volume.57 The chondrocytes
are oriented similar to collagen except for an
additional zone of calcification present deep
within the cartilage to provide transition to
the subchondralbone. Surrounding each
chondrocyte is the cartilage matrix separated
into three regions classified as pericellular,
territorial, and interterritorial. The function of
the chondrocyte early in life is primarily cell
proliferation and matrix synthesis. As maturation
proceeds, these processes slow, with cellular
density decreasing and interterritorial
matrix size increasing.53-57
Proteoglycanaggregates provide the compressive
strength to cartilage and progressively
increase in quantity from the intermediate to
the radial zones of the cartilage.54 The primary
constituents of the proteoglycan aggregate
are a series of glycosaminoglycans attached
to a core protein, which in turn are linked to
a backbone chain of hyaluronic acid. The
glycosaminoglycans are composed ofchondroitin-
6 sulfate, keratan sulfate, and some
chondroitin-4 sulfate. These highly sulfated
and carboxylated structures carry negative
charges that strongly repel each other, resulting
in stiffly extended aggregates. In addition, they
are highly hydrophilic. The resulting osmotic
swelling pressure, along with the fixed anionic
charges, produces dramatic compressive resistance.
These products aresynthesized by chondrocytes
and provide supportive structure to
the collagen fiber network.53-57
Acapsule surrounds the joint and is created
by an outer fibrous layer and an inner synovial
membrane. The outer fibrous layer,
continuous with the periosteum, is composed
primarily of collagen and helps to provide
mechanical support. The inner synovial membrane
is a thin layer of synovialcells closely
associated with a subsynovial vascular
plexus.11,53-55 The cellular component of the
synovial membrane is divided into type A
synoviocytes, which are primarily responsible
for phagocytic activity, and type B synoviocytes,
which are responsible for hyaluronic
acid production.53,54 Synovial fluid is a plasma
ultrafiltrate from the subsynovial plexus,
which allows molecules less...
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