Did you know your bones are used for more than just standing upright? This lesson looks at the fascinating contributions made by the diaphysis in long bones towards your body’s overall health. At the end of the lesson, you’ll find a short quiz.

What Is a Diaphysis?

Did you ever watch the beginning of the movie, 2001: A Space Odyssey? In one scene, a pre-human ape picks up a long bone and uses it to violently break apart the skeleton from which it originated. Structurally, long bones are very strong, as seen in the movie. They’re long because of the shaft in the middle called the diaphysis.

Parts of the Diaphysis

Compared to the other parts that make up a long bone, the diaphysis is quite different. It has a hollow inside with an open space called the medullary cavity. The cavity is filled with yellow bone marrow, along with blood vessels that supply the living bone tissue. The light color of the marrow is due to the high concentration of fat cells, as opposed to the red marrow where blood cells are made.

As a child, all of your bone marrow was red, but as you age into adulthood, much of it becomes yellow. Red marrow is found at the ends of long bones called epiphyses, which are covered with articular cartilage used to connect, or articulate, with other bones. Epiphyses are composed of spongy bone, which look like a sponge you use in the kitchen. Its tiny chambers are filled with red marrow, a very different setup from the diaphysis. The shaft is made of compact bone, a matrix of minerals, which makes it strong and useful for supporting a good deal of weight.

Bone Remodeling

The surface of the diaphysis is often slightly irregular, rather than completely smooth, and is covered in a thin layer of connective tissue called the periosteum, which provide areas where tendons and ligaments can attach, along with adhering nerves and blood vessels to the bone surface so they can enter the diaphysis. The periosteum also houses the live cells that create and remove bone material, a process called remodeling, which constantly reshapes bones. Osteoblasts create bone material, while osteoclasts break down bone material (the prefix ‘osteo’ means ‘bone’). If a long bone becomes broken, these cells are used in the healing process to help mend the fracture site.

Although bones constantly reshape themselves, they can only grow in length during childhood and adolescence. There is also an inherent problem with bones that grow: you still need to use them. It would be easy to just add bone material to either end, but the epiphyses are being used to connect with other bones. So how does the diaphysis become longer?

There is an area between the diaphysis and the epiphysis called the metaphysis; the prefix ‘meta-‘ refers to being ‘in the middle.’ In this area, we find the epiphyseal or growth plate. Other types of bone cells, called chondrocytes, produce cartilage on the joint side of the plate, while the other side becomes calcified. In other words, minerals are added which lengthens the diaphysis. By the time bones reach their full length in adulthood, the epiphyseal plate has become completely calcified. This creates a thin, detectable area called the epiphyseal line.

Functions

The diaphysis of long bones serves several purposes, including ones you may not expect.

Strength, Support and Locomotion

Because compact bone is composed of dense mineral layers, the diaphysis can be as hard as rock. Therefore, it is no coincidence that the longest bones in the body are the femurs. Both support most of your body’s weight and are used as part of a lever system to get you from one place to another. Consequently, they must support and move simultaneously. The humerus in your upper arm is similar and uses a lever system that allows you to throw a baseball or rock. Regardless of the limb, the elongated diaphysis is what helps make the actions possible.

Mineral Storage

Diaphyses have high concentrations of calcium and phosphorous. They serve as storage areas your body can access to ensure that mineral levels are always balanced in the blood and organs. For example, when blood calcium is too high or too low, hormones and vitamin D regulate the activity of osteoblasts and osteoclasts, which remove or replace calcium in the diaphyses. Because blood goes almost everywhere in your body, this process distributes calcium to where it is needed, such as the kidneys, nerves, muscles or even the blood itself. However, things can go wrong, such as the body’s low absorption of vitamin D or osteoporosis. These disorders can lead to the overall loss of calcium in bones, which make the diaphyses brittle and break more easily.

Fat Storage

During times of famine, fat stores in the body release energy to virtually every cell in your body. These include fat from the yellow marrow inside the diaphyses, which wouldn’t be available if all your marrow had remained red. Access to yellow marrow could be the critical difference between living and dying under extreme conditions.

Lesson Summary

The shaft in the middle of the bone is called the diaphysis. It has a hollow inside with an open space called the medullary cavity. The cavity is filled with yellow bone marrow, along with blood vessels that supply the living bone tissue. The yellow part is due to the high concentration of fat cells, and the red marrow is where blood cells are made. The surface of the diaphysis is covered in a thin layer of connective tissue, called the periosteum, which helps give other tissues purchase. It also houses osteoblasts that create bone material and osteoclasts that break down bone material. Minerals are added to lengthen the diaphysis. Diaphyses are useful in strength, support and locomotion, as well as in mineral and fat storage.