Dr. Moran opens this program on Cartilage with a lucid description of its three major categories: Hyaline Cartilage, Elastic Cartilage, and Fibrocartilage. The toughness of cartilage and its impact resistance is dramatically demonstrated by a film of the super-sized Denver Broncos slamming into each other in Fall Practice. The concomitant surgical repair of hyaline Cartilage and fibrocartilage are shown in actual footage filmed during arthroscopic knee surgery.
Dr. Moran goes on to explain that cartilage is a tissue in which there is lots of extracellular material (the “Matrix”) inhabited by relatively few cells (the “chondrocytes”). It is the special properties of the matrix (Latin for “of the mother”) that lend cartilage its unique morphology. In general, cartilage matrix consists of collagen fibers and ground substance. The ground substance is rich in proteoglycans — large macromolecules that resemble test-tube brushes with a protein core (the wire in the center of the brush) and many bristles sticking out from it (the proteoglycans). These proteoglycans, rich in sulfur, are highly anionic, and are therefore hydrophilic (attract lots of water). In hyaline cartilage — which is avascular — the matrix is 80% water. That allows nutrients to diffuse from the capillaries outside the perichondrium to the chondrocytes embedded deep in the matrix itself.
Going to the microscope, each type of cartilage is investigated in order. First, hyaline cartilage is illustrated at low, intermediate, and high magnification with standard H&E stained paraffin sections of the trachea. The chondroblasts are seen in the perichondrium. In the body of the cartilage, chondrocytes are seen in their lacunae, spaces surrounded by the purple-staining matrix. Then, the trachea is investigated in thin, H&E-stained plastic (methacrylate) sections. Here, the cytoplasm of the chondrocytes is better preserved, and details of structure more evident.
Next, elastic cartilage is demonstrated by a Weigert-stained section through the ear. Elastic fibers are not evident in standard H&E preparations. Special silver stains are used to make them visible. In this elegant preparation, the elastic fibers — seen at low, medium, and high magnification — appear as an anastomosing network of spider-webs that give the cartilage of the ear its special elastic “memory,” allowing it to bounce back in place when deflected.
Fibrocartilage, which contains more collagen fibers than ground substance, is illustrated by a magnificent Mallory’s Trichrome preparation of the intervertebral disk. Here, the wavy collagenous fibers appear blue, the cells are red, and the material of the nucleus pulposes appears bright orange. (This is one of our favorite images on the screen-saver available free on this very website!)
Finally, to put it all together, Dr. Moran shows us a series of stunning low-magnification electron micrographs of all kinds of cartilage that reveal the ultrastructure of the chondroblasts, chondrocytes, and fibrous components of the matrix.
This program — like all others in the Series — is rounded out by The Practical Exam, in which Dr. Moran shows you images of cartilage, quizzes you on them, gives to time to answer, and goes on to identify the structure in question and how to recognize it.
