Figure 79

Photomicrograph of fibrocartilage from an intervertebral disc. The collagen fibers are stained green in this Gomori trichrome preparation. The tissue has a fibrous appearance and contains a relatively small number of fibroblasts with elongated nuclei (arrows) as well as more numerous chondrocytes with dark round nuclei. The chondrocytes exhibit close spatial groupings and are arranged either in rows among the collagen fibers or in isogenous groups. xl60. Inset. High magnification of an isogenous group. Chondrocytes are contained within lacunae. Typically, there is little cartilage matrix surrounding the chondrocytes. x700.

and certain places where tendons attach to bones. The presence of fibrocartilage in these sites indicates that resistance to both compression and shearing forces is required of the tissue. The cartilage acts much like a shock absorber. The degree to which such forces occur is reflected in the amount of cartilage matrix material present.

o histogenesis, growth, and repair of hyaline cartilage

Most cartilage arises from mesenchyme

The process of cartilage development begins when mesenchymal cells aggregate and form a mass of rounded, closely apposed cells. In the head, most of the cartilage arises from aggregates of ectomesenchyme derived from neural crest cells. Known as a blastema of precartilage (protochondral tissue), an aggregate of mesenchymal or ectomesenchymal cells marks the site of hyaline cartilage formation. The blastema cells begin to secrete cartilage matrix and at this point are called chondroblasts. They progressively move apart as they deposit matrix. When they are completely surrounded by matrix material, the cells are called chondrocytes. The mesenchymal tissue immediately surrounding the chondrogenic blastema gives rise to the perichondrium.

Cartilage is capable of two kinds of growth, appositional and interstitial

With the onset of matrix secretion, cartilage growth continues by a combination of two processes:

• Appositional growth, the process that forms new cartilage at the surface of an existing cartilage

• Interstitial growth, the process that forms new cartilage within an existing cartilage

New cartilage cells produced during appositional growth are derived from the inner portion of the surrounding perichondrium. The cells resemble fibroblasts in form and function, producing the collagen component of the perichondrium (type I collagen). When cartilage growth is initiated, however, the cells undergo a change: the cytoplasmic processes disappear, the nucleus becomes rounded, and the cytoplasm increases in amount and becomes more prominent. These changes result in the cell becoming a chondroblast. Chondroblasts function in cartilage matrix production including secretion of type II collagen. The new matrix increases the cartilage mass; at the same time, new fibroblasts are produced to maintain the cell population of the perichondrium.

New cartilage cells produced during interstitial growth arise from the division of chondrocytes within their lacunae (see Fig. 7.2). This is possible only because the chondrocytes retain the ability to divide and the surrounding matrix is distensible, thus permitting further secretory activity. Initially, the daughter cells of the dividing chondrocytes occupy the same lacuna. As new matrix is secreted, a partition is formed between the daughter cells; at this point each cell occupies its own lacuna. With continued secretion of matrix, the cells move even further apart. The overall growth of cartilage thus results from both the interstitial secretion of new matrix material by chondrocytes and the appositional secretion of matrix material by newly differentiated chondroblasts.

Cartilage has limited ability for repair

Cartilage can tolerate considerable amounts of intense and repetitive stress. However, when damaged, cartilage manifests a striking inability to heal, even in the most mi nor injuries. This lack of response to injury is due to cartilage's avascularity, the immobility of the chondrocytes, and the limited ability of mature chondrocytes to proliferate. Some repair can occur, but only if the defect involves the perichondrium. In these injuries, repair results from activity of the pluripotential progenitor cells located in the perichondrium. Even in this case, however, few cartilage cells, if any, are produced. Repair mostly involves the production of dense connective tissue.

At the molecular level, cartilage repair is a tentative balance between deposition of type I collagen in the form of scar tissue and repair by expression of the cartilage-specific collagens. However, in adults, new blood vessels commonly develop at the site of the healing wound, which stimulate the growth of bone rather than actual cartilage repair. The limited ability of cartilage to repair itself can cause significant problems in cardiothoracic surgery, such as coronary artery bypass surgery, when costal cartilage must be cut to enter the chest cavity. A variety of treatments may improve the healing of articular cartilage, including perichondral grafts, cell transplantation, insertion of artificial matrices, and application of growth factors.

When hyaline cartilage calcifies, it is replaced by bone

Hyaline cartilage is prone to calcification, a process in which calcium phosphate crystals become embedded in the cartilage matrix. The matrix of hyaline cartilage undergoes calcification as a regular occurrence in three well-defined situations:

• The portion of articular cartilage that is in contact with bone tissue in growing and adult bones, but not the surface portion, is calcified.

• Calcification always occurs in cartilage that is about to be replaced by bone (endochondral ossification) during the growth period of an individual.

• Hyaline cartilage in the adult calcifies with time as part of the aging process.

In most of these situations, given sufficient time, cartilage that calcifies will be replaced by bone. For example, in older individuals, it is not uncommon to find portions of the cartilage rings in the trachea replaced by bone tissue (Fig. 7.10). Chondrocytes normally derive all of their nutrients and dispose of wastes by diffusion of materials through the matrix. When the matrix becomes heavily calcified, diffusion is impeded and the chondrocytes swell and die. The ultimate consequence of this event is removal of the calcified matrix and its replacement by bone.

Some investigators have described a cell type, the cbon-droclust, that resembles an osteoclast (page 190) in both morphology and function. This cell is thought to play a role in the digestion of calcified cartilage that is to be replaced by bone. These cells appear to enter the cartilage along with newly sprouting blood vessels and may, in fact, be derived from perivascular or bone marrow stem cells. Prechondro-clasts resemble fibroblasts when seen with the TEM. Most studies of chondroclast structure and function have been carried out on the developing mandible, in which true endochondral ossification does not take place. It is still unclear whether chondroclasts are cells found where bone is replacing cartilage or whether they are limited to cartilages and bones that are derived from the ectomesenchyme that originates from neural crest cells.

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