Hyaline cartilage, the most common type in both fetus and adult, is white and translucent when fresh, with a firm, gel-like consistency. 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. Ground substance, the predominant tissue component, comprises the following:
1. GAGs, mostly chondroitin sulfates and hyaluronic acid, with smaller amounts of keratan sulfate and heparin 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
5. Tissue fluid, an ultrafiltrate of blood plasma
The consistency of hyaline cartilage results from extensive cross-linking among its components. The chondrocytes are embedded in the matrix either singly or in isogenous 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 cells 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 Periodic Acid Shiff(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.
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.
Cartilage grows by 2 distinct processes. Both involve mitosis and the deposition of additional matrix. Matrix synthesis is enhanced by growth hormone, thyroxine, and testosterone and is inhibited by estradiol and excess cortisone.
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.
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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.
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.
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 long 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.
