Cartilage Tissue

CARTILAGE TISSUE belongs to the group of hard connective tissue and differs from bone macroscopically by its semi-translucent appearance, as well as significant elasticity and flexibility. Microscopically, it consists of cells and intercellular, or ground, substance. According to the structure of the intercellular substance, three main types of cartilage are distinguished: hyaline, elastic, and fibrous; cellular cartilage has also been added to them recently. 1. Hyaline, or glassy cartilage (Figs. 1 and 2)—the most common type; it forms the skeleton of lower vertebrates (selachians, cartilaginous ganoids), and the embryonic skeleton in higher vertebrates, which is subsequently displaced by bone, with cartilage remaining on the articular surfaces of bones and ribs; further, the nose, laryngeal (thyroid and cricoid), tracheal, and bronchial cartilages consist of hyaline cartilage. Among invertebrates, developed hyaline cartilage is present in cephalopod mollusks. On cartilage sections, cells are visible, separated from each other by a large amount of ground substance that is homogeneous and transparent like glass. The cells have various shapes: round, oval, spindle-shaped, often the shape of a semicircle or crescent; cells with processes are encountered rarely (patella of a calf, cephalopods). The cells contain a nucleus, mitochondria, the Golgi apparatus, and frequently glycogen blocks and fat droplets. Usually, the cells are located in groups—two or several together, which indicates the commonality of their origin (isogenic groups). Around the cell, a strongly light-refracting rim can be seen, the so-called capsule; upon fixation, cells frequently shrink and detach from the capsule, revealing the so-called cartilage cavities. The ground substance of cartilage, which appears homogeneous, actually consists of a tangle of thin connective tissue fibrils immersed in a substance with the same refractive index as

Fig. 1. Hyaline cartilage of a frog: 1—capsule; 2—chondrin ball; 3—trabecula.

; 4

Figure 2. Cartilage of the human nasal septum: 1—acidophilic trabeculae; 2—intermediate layer with basophilic ground substance; 3—acidophilic layer; 4—perichondrium; 5—capsule; 6—chondrin ball. them, which causes its homogeneity. Fibrils stand out clearly upon treatment of the section with potassium permanganate, lime water, or baryta water; the same occurs in old cartilages upon their calcification (asbestos cartilage). From the surface, the cartilage is covered by the perichondrium, consisting in the main mass of bundles of fibrous connective tissue running parallel to the surface; this fibrous mass passes without sharp boundaries into the homogeneous ground substance. By staining cartilage sequentially with basic and acidic stains (e.g., methyl violet and tropaeolin according to Möllendorff), one can discover differentiation in the intercellular substance, which is different in cartilages of different ages and from different locations. Usually beneath the perichondrium, the ground substance is entirely acidophilic, whereas deep within the cartilage, around cell groups, or more rarely individual cells, broad or narrow fields are stained by basic stains (chondrin clumps or balls, Schaffer territories); trabeculae stained by acidic stains (interterritorial spaces) remain between them (Fig. 1). Sometimes between these two zones there is a transitional one where, conversely, cell fields are acidophilic and trabeculae are basophilic (Fig. 2); in young cartilages, differentiation is not observed. The different coloration finds its explanation in the chemical composition of substances impregnating the fibrils and possessing basophilia (chondromucoid and chondroitin sulfuric acid) and acidophilia (chondroalbuminoid). The functional-mechanical features of cartilage, its resistance to pressure, elasticity, and flexibility are conditioned by the distribution of fibrils in the ground substance. The functional unit of cartilage is the so-called chondron, corresponding to a cell group surrounded by a chondrin ball: here the fibrils are arranged concentrically, forming a spherical vesicle possessing a significant degree of turgor. The totality of chondrons determines the hardness of cartilage and its ability to counteract pressure (Benninghoff, Schaffer). The trabeculae located between them are formed by fibers running from the perichondrium; they can cross the cartilage across its entire thickness or, having described an arc, return back; their arrangement is trajectorial, i.e., associated with the lines of stretching and bending of the tissue. Depending on the functional features of one or another cartilage, the distribution of chondrons and trajectories, as well as their relative size, varies. Cartilage in the norm lacks vessels; they appear only in old age with the onset of calcification. The nutrition of cartilage comes from the vessels of the perichondrium, whence substances reach the cells via diffusion, through interstices between the fibrils. Special juice canals, appearing in the ground substance upon certain treatments, are currently recognized as artifacts. The development of hyaline cartilage proceeds from mesenchyme: at the site of the future cartilage, an accumulation of round cells (chondroblasts) is formed, tightly pressed against each other and outlining the shape of the cartilage; fibroblasts located along the periphery of the primordium form the perichondrium. Histogenesis of cartilage begins with the appearance of thin interlayers of basophilic ground substance, which pushes the cells apart. Further growth proceeds in a twofold manner: 1) by apposition from the perichondrium, whose fibers, becoming impregnated with chondromucoid, produce ground substance, with the flat cells lying between them transforming into cartilage cells, and 2) by internal growth, or intussusception: cartilage cells multiply and secrete intercellular substance, a part of which, adjoining the cell, forms the capsule; the method of fibril formation in this case is not entirely clear. A portion of the cells perishes during development. Regeneration of cartilage tissue upon its losses proceeds from the perichondrium and is possible only if it is preserved. 2. Elastic, reticular, or resilient cartilage (Fig. 3) is distinguished by its yellowish color, greater elasticity and flexibility; the cartilage of the epiglottis, the Wrisberg and Santorini cartilages of the larynx, the cartilages of the auricle and the external acoustic meatus consist of it. It is structured in main features the same as hyaline, only dense networks of branching elastic fibers are embedded in the ground substance. The cells are surrounded by wide capsules; very frequently two cells lie in one capsule; in the vicinity of the cells, a dense tangle of thin elastic

Figure 3. Human ear cartilage: 1—cell; 2—capsule; 3—elastic network around the capsule.

fibers is noticed. The layer of cartilage adjoining the perichondrium resembles the corresponding section of hyaline cartilage and contains a sparse network of thin elastic fibers. Elastic cartilage is laid down and grows like Figure 4. Fibrous cartilage of the human symphysis: capsules and basophilic fields around cells. hyaline; elastic fibers develop in it later. 3. Fibrous, or connective-tissue cartilage (Fig. 4) is essentially dense connective tissue of a tendinous character, into which islands of hyaline cartilage of greater or lesser magnitude are embedded; sometimes it differs from connective tissue only by the presence of capsules and small fields around individual cells. It is encountered in certain sections of intervertebral cartilages, in symphyses, and intra-articular cartilages. 4. Cellular, vesicular, or parenchymatous cartilage consists in its main mass of round-shaped cells of a vesicular character and a small amount of ground substance in the form of thin interlayers between the cells. Included here are the sesamoid nodule of the Achilles tendon of the frog, the cartilage of the larva of the lamprey (ammocoetes), which bears a clearly embryonic character, and certain cartilages of invertebrates. It is difficult to draw a boundary between cellular cartilage and vesicular connective tissue, to which these formations are frequently attributed. The pathology of cartilage tissue is quite diverse. Main note is taken of atrophic and degenerative-necrobiotic changes, e.g., in arthropathies, in chronic arthritis. Mucous degeneration of cartilage is encountered particularly often. Regeneration of cartilage tissue is very limited and proceeds at the expense of the perichondrium. In the process of pathological regeneration, tumors of cartilage tissue frequently develop (chondromas, chondrosarcomas). A peculiar form of pathology of growth and development of cartilage is represented by so-called achondroplasia (see).

V. Karpov.