This figure shows a developing long bone of the finger and its articulation with the distal and proximal bones. Before the stage shown here, each bone consisted entirely of a hyaline cartilaginous structure similar to the cartilages seen in figure above but shaped like the long bones into which they would develop. Here, only the ends, or epiphyses, of the bone remain as cartilage, the epiphyseal cartilage (C). The shaft, or diaphysis, has become a cylinder of bone tissue (B) surrounding the marrow cavity (MC). The dark region at the ends of the marrow cavity is calcified cartilage (arrowhead) that is being replaced by bone. The bone at the ends of the marrow cavity constitutes the metaphysis. With this staining method, the calcified cartilage appears dark brown. The newly formed metaphyseal bone, which is admixed with this degenerating calcified cartilage and is difficult to define at this low magnification, has the same yellow-brown color as the diaphyseal bone. By the continued proliferation of cartilage, the bone grows in length. Later, the cartilage becomes calcified; bone is then produced and occupies the site of the resorbed cartilage. With the cessation of cartilage proliferation and its replacement by bone, growth of the bone stops, and only the cartilage at the articular surface remains. The details of this process are explained under endochondral bone formation (Plates 13 and 14).
Elastic cartilage has a matrix containing elastic fibers and elastic lamellae in addition to type II collagen. It is found in the auricle of the exter-nal ear, in the auditory tube, in the epiglottis, and in part of the larynx. The elastic material imparts properties of elasticity, as distinguished from resiliency, which are not shared by hyaline cartilage. Elastic cartilage is surrounded by perichondrium, and it, too, increases in size by both ap-positional and interstitial growth. Unlike hyaline cartilage, however, elastic cartilage does not normally calcify.
Elastic cartilage, epiglottis, human, H&E and orcein stains ×80.
This section of the epiglottis contains elastic cartilage (EC) as the centrally located structure. The essential components of the cartilage, namely, the matrix that stains deep blue and the light, unstained lacunae surrounded by matrix, are evident in this low-magnification micrograph. The perimeter of the cartilage is covered
PLATE 9 • E LASTIC CARTI LAG E KEY AT, adipose tissue E, elastic fiber EC, elastic cartilage MG, mucous gland PC, perichondrium SE, stratified squamous epithelium
Elastic cartilage, epiglottis, human, H&E and orcein stains ×250; inset ×400.
This shows an area of the elastic cartilage at higher magnification. The elastic fibers appear as the blue, elongate profiles within the matrix. They are most evident at the edges of the cartilage, but they are obscured in some deeper parts of the matrix, where they blend with the elastic material that forms a honeycomb about the lacunae. Elastic fibers (E) are also apparent in the adipose tissue (AT), between the adipocytes.
Some of the lacunae in the cartilage are arranged in pairs separated by a thin plate of matrix. The plate of matrix appears as a bar between the adjacent lacunae. This is a reflection of interstitial growth by the cartilage, in that the adjacent cartilage cells are derived from the same parent cell. They have moved away from each other and secreted a plate of cartilage matrix by perichondrium (PC); its fibrous character is just barely visible in this figure. Also note the adipose tissue (AT) within the boundaries of the elastic cartilage.