Fibrocartilage is intermediate in character between hyaline cartilage and dense connective tissue.
1. Composition and organization. Fibrocartilage is characterized by abundant type I collagen fibers; at low magnification, it closely resembles dense connective tissue. The ground sub stance contains equal amounts of dermatan sulfate and chondroitin sulfate. The matrix immediately surrounding the chondrocytes resembles that of hyaline cartilage and contains some type II collagen. The chondrocytes are distributed in columnar isogenous groups between the densely packed type I collagen bundles. There is no distinguishable perichondrium.
2. Histogenesis and growth. At sites where strong mechanical stresses occur, fibrocartilage develops from dense regular connective tissue through the transformation of fibroblasts or fibroblast like precursors into chondrocytes. Fibrocartilage growth has not been closely examined.
3. Function and location. Fibrocartilage is always associated with dense connective tissue, and the border between the two is usually indistinct. Its combination of cartilaginous ground substance and dense collagen bundles allows fibrocartilage to resist deformation under great stress; it is important in attaching bone to bone and providing restricted mobility. Sites in humans include the annulus fibrosus of the intervertebral disks, the symphysis pubis, and certain bone-ligament junctions.
Fibrocartilage
B. Elastic Cartilage:
ØElastic cartilage is yellowish when fresh. It is more Rexible than hyaline cartilage .
Ø1. Composition and organization. Elastic cartilage is structurally identical to hyaline cartilage except that it contains, in addition to type II collagen fibers, a dense network of branching and anastomosing elastic fibers. This network is densest at the core of the cartilage mass and when stained with elastic stains tee, Verhoeff's or Weigert's), may obscure the organization of the tissue. The chondrocytes characteristically occur in isogenous groups. A perichondrium surrounds the elastic cartilage mass.
Ø2. Histogenesis and growth, Elastic cartilage develops from a primitive connective tissue containing wavy bundles of fibrils that differ in protein composition from both elastin and collagen. Fibroblasts eventually secrete elastin, and the fiber bundles are transformed into branching elastic fibers by an unknown mechanism. The development of chondrocytes and production of the other matrix materials is the same as in hyaline cartilage. Further growth resembles that of hyaline cartilage. 3. Function and location. Elastic cartilage provides flexible support. It occurs alone and with hyaline cartilage; the two may grade into each other in a single cartilage mass. In humans, elastic cartilage is found in the auricle of the external ear, the walls of the external auditory canals and auditory tubes, the epiglonis, and the corniculate and cuneiform cartilages of the larynx.
Hyaline Cartilage
5. Repair. Repair of cartilage fractures involves invasion of the breach by mesenchymal stem cells from the perichondrium, which then differentiate into chondrocytes. If the gap is large. a dense connective tissue scar may form.
6. Function and location. Its ability to grow rapidly while maintaining its rigidity makes hyaline cartilage an ideal fetal skeletal tissue. As fetal cartilage is replaced by bone, hyaline cartilage remains in the epiphyseal plates at the ends of lone: bones, allowing these bones to lengthen between birth and adulthood. At all ages, hyaline cartilage without a perichondrium (articular cartilage) covers the articular surfaces of bone, where its resistance to compression and its smooth texture make it a good cushion and low-friction surface. Hyaline cartilage is the most abundant and widely distributed cartilage type in the body. The costal (rib) cartilages, most of the laryngeal cartilages, the cartilaginous rings supporting the trachea, and the irregular cartilage plates in the walls of the bronchi are hyaline cartilage.
Histogenesis - Hyaline Cartilage
3. Histogenesis, All cartilage derives from embryonic mesenchyme. During the development of hyaline cartilage, mesenchymal cells retract their cytoplasmic extensions and assume a rounded shape, becoming chondroblasts; at the same time, they become more tightly packed, forming a mesenchymal condensation, or precartilage condensation. The increased cell-to-cell contact stimulates cartilage differentiation, which progresses from the center outward. Chondroblasts at the core of the condensation are the first to secrete cartilaginous matrix materials, which separate the cells again. When it is completely surrounded by cartilage matrix, a chondroblast is termed a chondrocyte. Peripheral mesenchyme condenses around the developing cartilage mass to form the fibroblast-containing. dense, regular connective tissue of the perichondrium.
4. Growth. Cartilage grows by 2 distinct processes. Both involve mitosis and the deposition of additional matrix. Matrix synthesis is enhanced by growth hormone, thyroxine, and testesterone and is inhibited by estradiol and excess cortisone.
a. Interstitial growth involves the division of existing chondrocytes and gives rise to the isogenous groups, It is important in the formation of the fetal skeleton and continues in the epiphyseal plates and articular cartilages.
b. Appositional growth involves the differentiation into chondrocytes by chondroblasts and stem cells on the inner surface of the perichondrium. It is responsible for continued increase in the girth of the cartilage masses.
Organization of Hyaline Cartilage
Organization. The consistency of hyaline cartilage results from extensive cross-linking among its components. Link protein attaches the core proteins of proteoglycans to long chains of hyaluronic acid to form proteoglycan aggregates. The GAG side chains of the proteoglycans associate with type iI collagen fibrils. The chondrocytes are embedded in the matrix either singly or in isogeuous groups of 2-8 cells derived from one parent cell. The potential space occupied by each chondrocyte, called a lacuna, is visible only after the cell's death or after shrinkage during tissue processing. The chondrocytes at the core of a tissue mass are usually spheric; those at the periphery are flattened or elliptic. The matrix immediately surrounding the chondrocytes, called the capsular (territorial) matrix, is more intensely basophilic and PAS-positive than the intercapsular (interterritorial) matrix owing to the higher concentration of sulfated GAGs and lower concentration of collagen. Except for articular (joint) cartilage, all hyaline cartilage is surrounded and nourished by perichondrium. Articular cartilage is nourished by the synovial ffuid in the joint cavity.
Cartilage types: a)Hyaline Cartilage
Fibers, Hyaline cartilage matrix contains thin fibrils of type II collagen. Their small size and their refractive index (close to that of the ground substance) make them difficult to distinguish with the light microscope. Type II collagen contains a higher proportion of hydroxylysine than does type I. b. Ground substance, the predominant tissue component, comprises the following: (1) GAGs, mostly chondroitin sulfates and hyaluronic acid, with smaller amounts of keratan sulfate and heparan sulfate
(2) Proteoglycans, core proteins with GAG side chains;
(3) Proteoglycan aggregates proteoglycans covalently linked to long chains of hyaluronic acid by link protein;
(4) Glycoproteins, which attach various matrix components to one another and cells to the matrix, including link protein, fibronectin, chondronectin; and (5) Tissue fluid, an ultrafiltrate of blood plasma.
THERE ARE 3 TYPES OF CARTILAGE
Hyaline cartilage, elastic cartilage, and fibrocartilage differ in appearance and mechanical proper ties, owing to differences in the composition of their extracellular matrix