Cartilaginous Tissue

Cartilaginous tissue belongs to the group of solid connective tissues and differs from bone tissue macroscopically by its semi-transparent appearance, as well as significant elasticity and flexibility. Microscopically, it consists of cells and intercellular, basic substance. Based on the structure of the intercellular substance, three main types of cartilage are distinguished: hyaline, elastic, and fibrous; in recent years, cellular cartilage has also been added to these. 1. Hyaline, or glass-like cartilage (figs. 1 and 2) is the most common type; it forms the skeleton of lower vertebrates (selachians, cartilaginous ganoids), in higher vertebrates the embryonic skeleton, which is then replaced by bone, while cartilage remains on the articular surfaces of bones and ribs; furthermore, hyaline cartilage consists of the cartilages of the nose, larynx (thyroid and cricoid), trachea, and bronchi. Among invertebrates, well-developed hyaline cartilage is found in cephalopods mollusks. In cross-sections of cartilage, cells are visible, separated from each other by large amounts of basic substance, which is uniform and transparent like glass. The cells have various shapes: round, oval, spindle-shaped, often in the shape of a semicircle or crescent; cells with processes are rarely encountered (calf patella, holothurians). The cells contain a nucleus, mitochondria, the Golgi apparatus, often glycogen granules and fat droplets. Usually, the cells are arranged in groups - two or several together, which indicates their common origin (isogenous groups). Around the cell, a border that strongly refracts light, the so-called capsule, can be seen; when the cell is fixed, it often shrinks and detaches from the capsule, revealing the so-called cartilage cavities. The basic substance of cartilage, which appears uniform, actually consists of interwoven thin connective tissue fibrils immersed in a substance with the same refractive index, which determines its uniformity. The fibrils become clearly visible when the section is treated with potassium permanganate, lime water, or barium water; the same happens in old cartilages during their calcification (asbestos cartilage). From the surface, the cartilage is covered with the perichondrium, consisting mainly of bundles of fibrous connective tissue running parallel to the surface; this fibrous mass merges without sharp boundaries into the uniform basic substance. By staining the cartilage sequentially with basic and dyes (e.g., methyl violet and tropeolin according to Mögling), differentiation can be detected in the intercellular substance, which varies in cartilages of different ages and from different locations. Usually, under the perichondrium, the basic substance is uniformly acidophilic, and in the depth of the cartilage, around cell groups, and less often around individual cells, fields of varying width are stained with basic dyes (chondrinous clumps or balls, Schaffer's territories); between them remain bars that stain with acidic dyes (interterritorial spaces) (fig. 1). Sometimes between these two zones there is a transitional zone where, conversely, the cell fields are acidophilic and the bars are basophilic (fig. 2); in young cartilages, no differentiation is observed. The different coloring finds its explanation in the chemical composition of the 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 determined by the distribution of fibrils in the basic substance. The functional unit of cartilage is the so-called chondron, corresponding to the cell group surrounded by a chondrinous ball: here the fibrils are arranged concentrically, forming a spherical vesicle with a considerable degree of turgor. The totality of chondrons determines the firmness of the cartilage, its ability to counteract pressure (Benninghoff, Schaffer). The bars between them are formed by fibers coming from the perichondrium; they can cross the cartilage throughout its entire thickness or, after describing an arc, return back; their arrangement is trajectory-like, i.e., connected with the lines of tension and bending of the tissue. Depending on the functional features of a particular cartilage, the distribution of chondrons and trajectories, as well as their relative size, differ. Cartilage in the normal state is devoid of blood vessels; they appear only in old age when calcification has begun. The nutrition of cartilage comes from the blood vessels of the perichondrium, from where substances reach the cells by diffusion, along the interstices between the fibrils. Special juice canals, which appear in the basic substance with certain treatments, are now considered artifacts. The development of hyaline cartilage goes from mesenchyme: at the site of the future cartilage, a collection of round cells (chondroblasts) forms, densely pressed against each other and outlining the shape of the cartilage; fibroblasts located at the periphery of the rudiment form the perichondrium. The histogenesis of cartilage begins with the appearance of thin layers of basophilic basic substance, which pushes the cells apart. Further growth proceeds in two ways: 1) by apposition from the perichondrium, the fibers of which, impregnated with chondromucoid, give the basic substance, while the flat cells lying between them turn into cartilage cells, and 2) by internal growth, or intussusception: cartilage cells multiply and secrete intercellular substance, part of which adjacent to the cell forms the capsule; the method of fibril formation in this case is not entirely clear. Some cells die during development. Regeneration of cartilaginous tissue when it is lost occurs from the perichondrium and is possible only if it is preserved.

Fig. 1. Hyaline cartilage of the frog: 1-capsule; 2-chondrinous ball; 3-crossbar.

;

Figure 2. Cartilage of the human nasal septum: 1-acidophilic crossbars; 2-intermediate layer with basophilic basic substance; 3-acidophilic layer; 4-perichondrium; 5-capsule; 6-chondrinous ball.

them. The different coloring finds its explanation in the chemical composition of the 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 determined by the distribution of fibrils in the basic substance. The functional unit of cartilage is the so-called chondron, corresponding to the cell group surrounded by a chondrinous ball: here the fibrils are arranged concentrically, forming a spherical vesicle with a considerable degree of turgor. The totality of chondrons determines the firmness of the cartilage, its ability to counteract pressure (Benninghoff, Schaffer). The bars between them are formed by fibers coming from the perichondrium; they can cross the cartilage throughout its entire thickness or, after describing an arc, return back; their arrangement is trajectory-like, i.e., connected with the lines of tension and bending of the tissue. Depending on the functional features of a particular cartilage, the distribution of chondrons and trajectories, as well as their relative size, differ. Cartilage in the normal state is devoid of blood vessels; they appear only in old age when calcification has begun. The nutrition of cartilage comes from the blood vessels of the perichondrium, from where substances reach the cells by diffusion, along the interstices between the fibrils. Special juice canals, which appear in the basic substance with certain treatments, are now considered artifacts. The development of hyaline cartilage goes from mesenchyme: at the site of the future cartilage, a collection of round cells (chondroblasts) forms, densely pressed against each other and outlining the shape of the cartilage; fibroblasts located at the periphery of the rudiment form the perichondrium. The histogenesis of cartilage begins with the appearance of thin layers of basophilic basic substance, which pushes the cells apart. Further growth proceeds in two ways: 1) by apposition from the perichondrium, the fibers of which, impregnated with chondromucoid, give the basic substance, while the flat cells lying between them turn into cartilage cells, and 2) by internal growth, or intussusception: cartilage cells multiply and secrete intercellular substance, part of which adjacent to the cell forms the capsule; the method of fibril formation in this case is not entirely clear. Some cells die during development. Regeneration of cartilaginous tissue when it is lost occurs from the perichondrium and is possible only if it is preserved. 2. Elastic, reticular, or flexible cartilage (fig. 3) differs by its yellowish color, greater elasticity and flexibility; it consists of the epiglottis cartilage, Wrisberg's and Santorini's cartilages of the larynx, the cartilages of the auricle and external auditory canal. It is built in basic features the same as hyaline cartilage, only in the basic substance dense networks of branching elastic fibers are embedded. The cells are surrounded by wide capsules; very often two cells lie in one capsule; near the cells, a dense interweaving of thin elastic fibers is noticed. The layer of cartilage adjacent to the perichondrium resembles the corresponding section of hyaline cartilage and contains a rare network of thin elastic fibers. Elastic cartilage is laid down and grows like hyaline cartilage; elastic fibers develop in it later. 3. Fibrous, or connective tissue cartilage (fig. 4) is essentially a dense connective tissue of tendon character, into which islands of hyaline cartilage of varying sizes are embedded; sometimes it differs from connective tissue only by the presence of capsules and small fields around individual cells. It is found in certain areas of intervertebral cartilages, in symphyses and intra-articular cartilages. 4. Cellular, vesicular, or parenchymatous cartilage consists mainly of cells of round shape, vesicular character, and a small amount of basic substance in the form of thin layers between cells. This includes the sesamoid node of the Achilles tendon of the frog, the cartilage of the larva of the lamprey (sand-lantern), which has an obviously embryonic character, and some cartilages of invertebrates. It is difficult to draw a boundary between cellular cartilage and vesicular connective tissue, to which these formations are often also attributed. Pathology of cartilaginous tissue is very diverse. Mainly atrophic and degenerative-necrobiotic changes are noted, e.g., in arthropathies, in chronic arthritis. Mucous degeneration of cartilage is especially common. Regeneration of cartilaginous tissue is very limited and occurs at the expense of the perichondrium. In the process of pathological regeneration, tumors of cartilaginous tissue often develop (chondromas, chondrosarcomas). A special form of pathology of cartilage growth and development is so-called achondroplasia (see).

Figure 3. Cartilage of the human ear: 1-cell; 3-capsule; 3-elastic network around the capsule.

Figure 4. Fibrous cartilage of the human symphysis: capsules and basophilic fields around cells.

V. Karpov.