- Discussion:
-
hyaline cartilage caps ends of bones that form synovial joints;
- in other hyaline cartilage structures, surrounding perichondrium
contains both capillaries for nutrition and the
cells that become involved in
appositional
growth;
- thickness of articular cartilage varies from joint to joint, and
in humans it is thickest over ends of femur &
tibia, ranging from 2-4 mm;
- cartilage contains predominantly
type II collagens
w/ lesser amounts of type IX and type XI;
- function:
- it provides both a
cushion & slick surface for movement; (see
lubrication)
- consistency of the
extracellular matrix allows the tissue to bear mechanical stresses without
permanent distortion;
- shock-absorbing because
it is resilient;
- smooth surface allows sliding against it
- synovial
fluid
- nourishment is supplied synovial fluid that bathes cartilage;
- withdrawal of synovial fluid
often leads to rapid deterioration of cartilage;
- ref:
Synovial fluid
depletion: successful arthrodesis without operative cartilage removal
- perichondrium:
- articular cartilage has no more than a peripheral rim of
perichondrium on its free surface, and calcified
cartilage abutting bone limits
diffusion from blood vessels supplying subchondral bone;
- free surfaces of most hyaline
cartilage (but not articular cartilage) are covered by a layer of fibrous
connective tissue, perichondrium;
- deep portion of perichondrium
is composed of chondroblasts;
- external portion is less
cellular and more densely fibrous;
- extracellular matrix
- filler
material of cartilage is composed of
proteoglycan aggregates w/
chondroitin sulfate &
keratan
sulfate as chief
glycosaminoglycans;
- approx 10% of wet
weight of cartilage is collagen;
- approx 75% of matrix is
water;
- remainder is a
nonfibrous filler material;
- these entities together
form stiff sol;
- containes fibers, ground substance
- collagen, hyaluronic
acid, proteoglycans, glycoproteins, elastic (in elastic cartilage)
- macromolecules, water, fibers bind together and producing function
properties and flexibility;
- no blood, nerve supply
- low metabolic rate;
- collagen alignment and function:
-
type II collagen
is the primary type of collagen in articular cartilage;
- leaf-like arcades
of collagen fibers within which the chondrocytes are oriented
perpendicular to the
articular surface in the deep zone
- arch through
the middle zone
- become horizontal and
parallel to the articular surface in the superficial tangential zone;
- functions of
collagen fibers w/ in
cartilage:
- provides tensile strength to tissue and resist movement of interstitial water
&
proteoglycans from
the cartilage, esp. while it sustains compressional
loading;
- to anchor ground substance of articular cartilage to subchondral bone;
- references:
-
Quantitative structural organization of normal adult human articular cartilage.
-
Three-dimensional collagen architecture in bovine articular cartilage.
- The
ultrastructure of mouse articular cartilage: collagen orientation and
implications for
tissue functionality. A polarised light and electron
microscope study.
- Histology:
-
chondrocytes, occupy lacunae generously distributed through the matrix;
- young chondrocytes & chondroblasts have rounded nuclei (or double
nuclei);
- cytoplasm contains elongated mitochondria, well-developed Golgi
apparatus, varying amounts of glycogen, &
lipid droplets;
- each
peripheral lacuna typically houses a single chondrocyte;
- deeper
lacunae may contain two or more chondrocytes;
- surrounding
each cell is a territorial matrix w/ a higher concentration of
proteoglycans;
- references:
-
Histology of Cartilage and Bone
- 4 Zones of artiuclar cartilage:
- superficial layer
(tangential zone);
- makes up 10% of cartilage;
- consists of 2 sub-zones:
- fibrilar sheet / lamina splendens is the more superficial layer;
- clear film consisting of a sheet of small fibrils with little polysaccharide
and no cells;
- cellular layer w/ flattened
chondrocytes;
- flat chondrocytes and collagen fibers are arranged tangentially to the
articular surface;
- thinnest layer, with the
highest content of
collagen and the lowest concentration of
proteoglycans;
- collagen (type IX) is arranged at right angles to adjacent bundles and
parallel to the
articular surface;
- subsequently has greatest
ability to resist shear stresses and serves as a gluiding surface for joint;
- may also function to limit
passage of large molecules between
synovial fluid
and cartilage;
- superficial zone is the
first to show changes of osteoarthritis;
- transitional layer
- this zone is involves
transition between the shearing forces of surface layer to compression forces in
the cartilage layers;
- composed almost entirely of
proteoglycans
- spherical chondrocytes
- less strongly bound;
- deep radial layer
- largest part of the
articular cartilage
- it distributes loads and
resists compression;
-
collagen fibers and
chondrocytes are
perpendicular to the subchondral plate;
- calcified cartilage layer
- contains the tidemark layer;
- tidemark is basophilic line which stradles the boundry between calcified and
uncalcified cartilage;
- separates
hyaline
cartilage from subchondral bone;
- type X collagen is present
mainly in the calcified cartilage layer and in
hypertrophic
zone of the growth plate;
- much of hyaline cartilage in non
articular areas of the body ultimately calcifies w/ maturation;
- when calcification occurs,
chondrocytes die &
matrix disintegrates;
- articular cartilage is unique in that its more superficial zones do not
calcify, except in pathologic states
such as
pseudogout