The cartilage matrix immediately surrounding each chondrocytes is rich in glycosaminoglycans and poor in collagen. This peripheral zone, called the territorial, or capsular,matrix, histochemically exhibits an intense basophilia, and metachromasia than does the matrix located between the capsules, the interterritorial matrix.
Except in the articular cartilage of joints, all hyaline cartilage is covered by a layer o dense connective tissue, the perichondrium, which is essential for the growth and maintenance of cartilage. It is composed of an outer fibrous layer, containing collagen type I fibers, fibroblasts and blood vessels and an inner cellular (chondrogenic) layer composed of chondrogenic cells. Chondrogenic cells in the inner layer of the perichondrium differentiate into chondroblasts, the cells that produce cartilage.
Cartilage derives from the mesenchyme. The cells formed by the direct differentiation of mesenchymal cells, are called chondroblasts. Chondroblasts have a ribosome-rich basophilic cytoplasm. Synthesis and deposition of the matrix then begin to separate the chondroblasts from one another. The differentiation of cartilage takes place from the center outward; therefore, the more central cells have characteristics of chondrocytes while the peripheral cells are typical chondroblasts.
Chondroblasts differentiate in the mature cell type – chondrocytes. At the periphery of hyaline cartilage, young chondrocytes have an elliptic shape, with the long axis parallel to the surface. Farther in, they are round, and may appear in groups of up to 8 cells originating from mitotic divisions of a single chondrocyte. These groups are designated isogenous groups.
Chondrocytes. The electron microscope reveals irregular chondrocytes surfaces with some small processes. Mature chondrocytes have organelles typical of protein secretory cells – an elaborate rER and well-developed Golgi complex. They synthesize type II collagen, proteoglycans, and chondronectin.
The growth of cartilage is attributable to 2 processes: interstitial growth, resulting from the mitotic division of preexisting chondrocytes; and appositional growth, resulting from the differentiation of perichondrial cells.
· Interstitial growth (Fig.22) occurs only during the early phases of cartilage formation, when it increases tissue mass by expanding the cartilage matrix from within. Interstitial growth also occurs in the epiphyseal plates of long bones and within articular cartilage.
· Appositional growth.In cartilage found elsewhere in the body, interstitial growth becomes less pronounced as the matrix becomes increasingly rigid from the cross-linking of matrix components. Cartilage then grows in girth only by apposition. Chondroblasts of the perichondrium proliferate and become chondrocytes once they have surrounded themselves with matrix and are incorporated into the existing cartilage.
Fig.22. Histogenesis of hyaline cartilage. A:The mesenchyme, the precursor tissue of all types of cartilage. B: Mitotic proliferation of mesenchymal cells gives rise to a highly cellular tissue. C: Chondroblasts are separated from one another by the formation of a great amount of matrix. D: Multiplication of cartilage cells gives rise to isogenous groups, each surrounded by a condensation of territorial (capsule) matrix.
In contrast to other tissues, hyaline cartilage is subjected to the degenerative
processes that increase with age. The most common is calcification of the matrix. Although calcification is a regressive alteration, it occurs normally in certain cartilages, providing a model for bone development.
Except in young children, damaged cartilage regenerates with difficulty and often incompletely. Regeneration occurs because of the activity of the perichondrium. When cartilage fractures, chondroblasts from the perichondrium invade the fractured area and generate new cartilage. In extensively damaged areas, the perichondrium, instead of forming new cartilage, generates a scar of dense connective tissue.
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