Cartilage

Cartilage and bone are specialized forms of connective tissue. They contain cells which produce fibers and ground substance. Together, the fibers and ground substance comprise the organic matrix. The cells of cartilage and bone, called chondrocytes or osteocytes respectively, live in holes or lacunae within the organic matrix, all of which is contained within an outer, fibrous membrane of connective tissue, the perichondrium or periosteum..

All cartilage develops from mesenchyme. At the site of chondrogenesis, mesenchymal cells round out and proliferate, and differentiate into chondroblasts. Subsequently the chondroblasts begin to synthesize and secrete matrix into the extracellular space, entrapping themselves within lacunae. They become further separated by the formation of additional matrix, and the cells are now called chondrocytes. At the same time, the mesenchymal cells of the periphery condense to form a fibrous sheath around the newly formed cartilage. This is the perichondrium.

When the chondrocytes secrete their extracellular products (fibers and ground substance), they become trapped within the matrix. (In a histological preparation the chondrocytes may shrink, thereby revealing the small spaces in which they reside). These spaces are called lacunae. Chondrocytes possess an extensive rough ER and well developed Golgi to support their protein synthesizing activity.

Cartilage matrix has an affinity for basic dyes (basophilia). The principal constituents of ground substance are proteoglycans, which consist of protein combined with complex carbohydrates such as chondroitin sulfate and keratan sulfate. The acidic sulfate groups account for the basophilia of the matrix. The proteoglycans are themselves attached to hyaluronic acid to form giant molecules which form a hard gel because their strong negative charges repel the chains. They tie up almost all of the water molecules in cartilage and are attached as gigantic molecular complexes to collagen fibrils. Typically the concentrations of proteoglycans are higher right around the cells, giving bluer rings called territorial matrix. The paler matrix away from the cells is called inter-territorial matrix.

Embedded in the matrix are either collagen (Type II) or elastic fibers. It is the extracellular matrix, with its embedded fibers which gives cartilage its resiliency, ability to bear weight, and tensile strength.

Cartilage is enclosed by a layer of dense connective tissue called the perichondrium, except on the articulating surfaces in joint cavities. The perichondrium has two layers-- a fibrous outer layer and an inner layer of cells with chondrogenic (cartilage-forming) potential. The chondrogenic cells undergo division and differentiate into chondroblasts.

Growth of cartilage occurs by two mechanisms: appositional and interstitial. In appositional growth, chondroblasts located in the chondrogenic layer of the perichondrium differentiate into chondrocytes. The newly formed cells produce fibers and ground substance that add to the matrix of the existing cartilage and increase its mass. In interstitial growth, chondrocytes within the cartilage divide by mitosis and elaborate new matrix. Daughter cells from these divisions tend to remain in clusters (nests) which represent the descendants of a single chondrocyte.

Cartilage has neither vasculature nor innervation of its own. Chondrocytes obtain all of their nourishment from blood vessels in in the perichondrium and surrounding connective tissues, by diffusion of oxygen and nutrients through the matrix. It is because of this lack of a direct blood supply that cartilage heals so poorly. With age, cartilage tends to calcify. This retards diffusion of nutrients through the matrix and may result in chondrocyte death and degeneration of the cartilage.

Three varieties of cartilage are distinguished: hyaline cartilage, elastic cartilage and fibrocartilage.

Hyaline cartilage.

This is the most widespread form of cartilage and is called hyaline cartilage because of its glassy, whitish-blue appearance in the fresh state. In adults it is found in the nasal septum, larynx, trachea and bronchi of the respiratory tract, and on the articular surfaces of synovial joints. It is also found on the ventral ends of the ribs where they attach to the sternum. Hyaline cartilage forms most of the temporary skeleton in mammalian embryos.

About 40% of the organic matrix of hyaline cartilage is collagen (type II). The rest is ground substance--mostly proteoglycans.

The function of hyaline cartilage is to provide flexible support. It has great tensile strength (due to the collagen) and is highly resistant to pressure (due to the ground substance). Hyaline cartilage provides a framework for the developing embryo prior to the appearance of bone. Later, it supplies the mechanism by which bones grow in length.

Articular cartilage is merely hyaline cartilage that covers the articular surfaces of bones. No perichondrium is present presumably since this would provide a fibrous articulating layer rather than a smooth surface with the necessary resilience (ability to resist deformation). Because no perichondrium is present, growth must be purely interstitial. The arrangement of collagen fibers in this cartilage is such as to provide an ideal structure for absorbing the constantly changing stresses on the joint surface.

Elastic cartilage.

Elastic cartilage is found supporting the external ear, the auditory and eustachian tubes, the epiglottis and the larynx. In the fresh state it appears yellow.

The organic matrix, which contains less ground substance than hyaline cartilage, is filled with large numbers of elastic fibers which branch to form a dense network, and also type II collagen fibers. The chondrocytes in elastic cartilage are more abundant and more closely packed than is the case in hyaline cartilage.

Fibrocartilage.

Fibrocartilage is a tissue intermediate in structure between hyaline cartilage and dense regular connective tissue, which it resembles. It is found in intervertebral discs, at the insertion of tendons and ligaments, the semilunar cartilage of the knee, and in healing fractures. It lacks a perichondrium. Its extremely sparse ground substance is heavily infiltrated with dense collagen fibers (type I) arranged in parallel bundles. The cells are located between the collagen bundles, are in small lacunae and are aligned in rows.

Fibrocartilage combines the stress-bearing properties of cartilage and the tensile strength of tendons or ligaments, for a firm but not rigid support.