Anatomy and Physiology: The Parts of a Bone

The Parts of a Bone

Most people envision bone as being uniformly solid, but nothing could be farther from the truth. For one thing, as you will see later in this section, bones come in many different shapes—long, short, flat, irregular, wormian, and sesamoid—which have much in common, despite their differences. A typical bone can be broken down into multiple parts, each with a particular function:

• Epiphysis. This part is at the extreme ends of the bone (epi = above), where joints (articulations) form.
• Articular cartilage. A layer of hyaline cartilage, called articular cartilage, exists to reduce friction and absorb shock at synovial joints (see The Joints).
• Diaphysis. The shaft of a long bone, which is the direction at which the bone can withstand the most stress.
• Metaphysis. The metaphysis is the place where the diaphysis meets the epiphysis. This is where major bone growth occurs, as well as where blood enters the bone.
• Periosteum. A thin membrane that covers the outside of the bone, where tendons and ligaments attach to the bone. The outer fibrous layer is where blood vessels, nerves, and lymphatics connect to the bone, while the inner osteogenic layer has bone cells necessary for the growth and repair of bone.
• Medullary (or marrow) cavity. This hollow cavity, in the diaphysis, is for the storage of yellow marrow.
• Endosteum. This membrane lines the medullary cavity, and contains osteoprogenitor cells (unspecialized bone cells, as you will soon see).

Up, Down, and Middle

As you can see in Figure 5.1, the shaft of a long bone is called the diaphysis. The central, fat-storing marrow cavity is found inside the diaphysis. At each end of the bone, at the site of the synovial joint, is an area called epiphysis. At the juncture between the two is an area called the metaphysis.

Figure 5.1The many parts of a typical long bone. The example shown here is a femur. (©2003 www.clipart.com)

Remember that organs, including bones, need three connections: blood vessels (both arteries and veins), lymphatics, and nerves. These structures enter the bone through little holes called foramina. A hole specifically for blood vessels is called a nutrient foramen (the singular form of foramina). Any student can tell if a skeleton is real by simply looking for foramina around the metaphysis. Another clue is the weight: Real bones are lighter than solid models, due to the openings for red and yellow marrow.

Beyond the entering and exiting nerves and vessels, the metaphysis is also the location of the epiphyseal plates, which are the primary growth centers of a long bone. There are four zones in the epiphyseal plate. The zone of resting cartilage is not involved in growth, but it does anchor the plate to the rest of the bone. The zone of proliferating cartilage and zone of hypertrophic cartilage are both involved in producing chondrocytes (cartilage cells), but the latter zone is where maturation of the cells occurs. The last zone, where the bone actually forms, is known as the zone of calcified cartilage.

As we age, the epiphyseal plates, which are less dense than bone and show up darker on an X-ray, will ossify (turn to bone), at which point they will appear as a light line (called the epiphyseal line). This marks the end of a bone's ability to grow longer; this ossification is usually complete by the early to mid twenties (although the sternum doesn't finish until after 30). The facial bones, and often the hands and feet, however, do not stop growing, which explains why a young Jimmy Stewart looked very different than he did as an old man.

The Harder They Come

Compact bone is notable for the wide spacing of the cells within a hard crystal matrix (see Figure 5.2). You may remember that both wide spacing and a matrix were characteristics of connective tissue. The main feature of compact bone is its strength. It provides protection for places outside a soft structure, such as in the flat bones of the skull. Compact bone also supports the stress placed on it. In a long bone, the stress is best absorbed along the longitudinal axis of the diaphysis. This arrangement is great for a bone like the femur, which absorbs stress in that direction, but the same cannot be said for the clavicle, which can be easily fractured if it receives a downward blow perpendicular to the diaphysis.

Microscopically, compact (or dense) bone is distinguished by its arrangement of osteocytes (bone cells) in concentric circles of matrix. Just as people settle around sources of water, these rings, or concentric lamellae, are arranged around a central haversian canal, which holds blood vessels. The combination of the concentric lamellae and the haversian canal is called an osteon, or haversian system. In addition to the haversian canal, there are perpendicular ones called perforating canals that connect haversian canals, and help to provide blood not only to the deeper haversian systems, but also to the marrow cavity.

The osteocytes look a little like ants because of the arrangement of little canals called canaliculi around each cell; these canaliculi, whose name always makes me think of an Italian dessert, are where the interstitial fluid is found. Canaliculi extend outward in every direction from the lacuna, which is the space where the osteocyte is found.

Figure 5.2This is a diagram of haversian systems in compact bone. Note the organization of the bone is based on the location of blood vessels. (LifeART©1989-2001, Lippincott Williams & Wilkins)

Not Just For Mopping Up Spills

Spongy or cancellous bone is very different in appearance. Rather than rigid concentric systems, spongy bone looks, well, spongy. The appearance is due to an irregular collection of overlapping and interconnected spokes called trabeculae (refer to Figure 5.2). To understand the function of spongy bone, note that it appears most commonly in the epiphysis, just under a protective compact layer. The compact layer provides firm attachment for that articular cartilage, both of which help to protect from the friction found in every synovial joint.

So why the spongy part? In terms of stress at the joint, imagine jumping in the air and landing hard on your feet while keeping your legs straight; a great deal of stress will be felt not only in your knees, but also where your femur articulates with your pelvis, not to mention in your back. You can easily reduce the stress by bending your knees and ankles; such bending absorbs the stress of the impact. Now do you know the reason for spongy bone? That's right, to absorb some of the shock of impact at synovial joints.

The screwy multidirectional trabeculae make it possible to absorb stress from multiple directions. In addition, the spaces between the trabeculae make spongy bone much lighter, thus making the skeleton as a whole much lighter. These spaces serve another purpose; they are filled with red bone marrow, the site of hemopoiesis.

Excerpted from The Complete Idiot's Guide to Anatomy and Physiology © 2004 by Michael J. Vieira Lazaroff. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.

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