Author: Dr. Thomas Caceci
The evil that men do lives after them,
the good is oft interred with their bones.
—William Shakespeare (1564-1616)
Julius Caesar, Act III, Scene 2
Objectives for This Exercise
SLIDES USED IN THIS EXERCISE: 18, 18A, 17, VM05, 98, 80, VM01, VM10, 79, 81
Bone, like other connective tissues, has cells, fibers, and a matrix. In bone, however, the extracellular matrix is calcified, and the fibers (which are collagen) are very highly ordered. The cellular component is vital to bone function, as we'll see, but in terms of the total volume, it's a comparatively small proportion. Properties of Bone
Bone is a truly remarkable structural material, with properties that make it ideal for its functions of structural and physiological support. It's relatively light, but it has very high tensile and compressive strength, and a good deal of elasticity. There is a scene in the motion picture Never Cry Wolf in which the protagonist holds up a rib bone from an elk, then snaps it in two with his bare hands. Try this sometime; if you can't get hold of an elk rib, a beef rib will do. Superman would have trouble performing this trick, and the average person would have trouble doing it to a sheep rib, let alone one from an elk. It's obvious that the script writers were more concerned with drama than reality! One other misconception about bone is that it's a static tissue. Nothing could be further from the truth; it's about the most dynamic material in the body. Bone is a reservoir for calcium and phosphorus, and there is a constant flux of minerals in and out of it. Bone responds to injury by rapid healing. It can be made to grow in different ways by clever tricks of orthodontia and surgery (the braces you may once have worn were physically affixed to your teeth, but their purpose was really to remodel the bone of the jaw), and it's greatly affected by nutritional and metabolic changes. Bones are continually being modified, reshaped, remodeled, and overhauled. Fiber and Mineral Components
Bone has considerably more collagen content than does cartilage. The collagen fibers are much more orderly in their arrangement as well. Elastic fibers are minimal or absent. Perhaps as much as 25-30% of the total organic material in a bone is collagen. The collagen fibers give bones the ability to resist snapping and breaking. They act in some respects the way the cords in a tire sidewall do. The mineral component, which gives bone its hardness, is chiefly a form of calcium phosphate, called hydroxyapatite. The trigger for the crystallization of this material is not known, but it may involve the interaction of collagen fibers with the GAGs of the matrix material. Matured bone contains about 65% mineralized matter, the rest being collagen and matrix. The mineral gives bone its toughness and rigidity, the highly ordered fibrous component provides tensile strength and flexibility. Macroscopic Appearance of Anatomic Bones
In this exercise we'll be concerned mainly with mature bone, its structural variations, and the way in which it's produced. Macroscopically, anatomic bones are categorized as compact or spongy, based on their gross appearance. Consider first the macroscopic appearance of a typical long bone. The image at left is a typical example, a sketch of the knee joint. The articular ends of the long bones are covered with cartilage; each bone (including the patella) is composed of a hard "shell" of mature, fully developed bone: i.e., lamellar bone, so called because of the organization of its compact substance. Inside the hollow ends of each of the long bones is the trabecular network of spongy or cancellous bone. These flat, interlocking trabeculae are also lamellar, but arranged in somewhat of a different way than the denser compact bone of the shaft. The greater part of a long bone is the shaft, or diaphysis.The outer surface of the shaft is...