lower portion of the figure shows considerably heavier elastic and collagen fibers. Also note that many of the elastic fibers appear as short rectangular profiles. These profiles simply represent fibers traveling through the thickness of the section at an oblique angle to the path of the knife. Careful examination will also reveal a few fibers that appear as dot-like profiles. They represent cross-sectioned elastic fibers. Overall, the elastic fibers of the dermis have a three-dimensional interlacing configuration, thus the variety of forms.
The empty-appearing spaces between elastic layers contain collagen fibers and smooth muscle cells, but they remain essentially unstained. In the muscular layer of blood vessel, both elastin and collagen are secreted by the smooth muscle cells.
Tissues of the body containing large amounts of elastic material are limited in distribution to the walls of elastic arteries and some ligaments that are associated with the spinal column.
OVERVIEW OF CARTILAGE / 198 HYALINE CARTILAGE / 199 ELASTIC CARTILAGE / 204 FIBROCARTILAGE / 204 CHONDROGENESIS AND CARTILAGE GROWTH / 206 REPAIR OF HYALINE CARTILAGE / 207 Folder 7.1 Clinical Correlation: Osteoarthritis / 199 Folder 7.2 Clinical Correlation: Malignant Tumors of the Cartilage; Chondrosarcomas / 208
Cartilage is a form of connective tissue composed of cells called chondrocytes and a highly specialized extracellular matrix.
Cartilage is an avascular tissue that consists of chondrocytes and an extensive extracellular matrix. More than 95% of cartilage volume consists of extracellular matrix, which is a functional element of this tissue. The chondrocytes are sparse but essential participants in producing and maintaining the matrix (Fig. 7.1).
The extracellular matrix in cartilage is solid and firm but also somewhat pliable, which accounts for its resilience. Because there is no vascular network within cartilage, the composition of the extracellular matrix is crucial to the survival of the chondrocytes. The large ratio of glycosaminoglycans (GAGs) to type II collagen fibers in the cartilage matrix permits diffusion of substances between blood vessels in the surrounding connective tissue and the chondrocytes dispersed within the matrix, thus maintaining the viability of the tissue. Close interactions are seen between two classes of structural molecules that possess contrasting biophysical characteristics: the meshwork of tension-resisting collagen fibrils and the large amounts of heavily hydrated proteoglycan aggregates, the latter being extremely weak in shear, make the cartilage well adapted to bear weight, especially at points of movement, as in synovial joints. Because it maintains this property even while growing, cartilage is a key tissue in the development of the fetal skeleton and in most growing bones.
FIGURE 7.1 • General structure of hyaline cartilage. This photomicrograph of a routine H&E preparation of hyaline cartilage shows its general features. Note the extensive extracellular matrix that separates a sparse population of chondrocytes. 450.
Three types of cartilage that differ in appearance and mechanical properties are distinguished on the basis of characteristics of their matrix:  Hyaline cartilage is characterized by matrix containing type II collagen fibers, GAGs, proteoglycans, and multiad hesive glycoproteins.  Elastic cartilage is characterized by elastic fibers and elas tic lamellae in addition to the matrix material of hyaline cartilage.  Fibrocartilage is characterized by abundant type I colla gen fibers as well as the matrix material of hyaline cartilage.
Table 7.1 lists the locations, functions, and features of each type of cartilage.
Hyaline cartilage is distinguished by a homogeneous, amorphous matrix.