A diaphysis is the long cylindrical section of a long bone and is made up of several distinct features. Like the outside of any bone, its superficial surface is covered by periosteum. This is a layer of dense, irregular connective tissue with an underlying layer of cells that develop into osteoblasts. These osteoblasts are responsible for the outward growth of bone, which is called appositional growth.
Deep in the diaphysis is the cortical bone, which is also called compact bone and makes up the bulk of it. This cortical bone is much denser and stronger than spongy bone. The cortical bone plays a major role in the body as a storage site for certain minerals, particularly calcium and phosphorous. A body can deposit or withdraw these minerals from it as necessary. Although other bones also have cortical bone, the diaphysis of long bones is particularly crucial because it is so abundant.
The interior of the diaphysis is a hollow cavity called the medullary cavity, which is also called a marrow cavity. This is filled with yellow bone marrow, which consists of adipose tissue. It is a major storehouse of triglycerides, or fat, in the body and can be broken down for metabolic use. In newborns, the medullary cavity is used as a source of red bone marrow; this gradually changes into yellow bone marrow with age. The inner lining of the medullary cavity is covered with endosteum, which is a thin layer of connective tissue.
In the center are small holes called nutrient foramina. Arteries, veins, and nervous tissue enter into the bone and supply blood to the medullary cavity and the insides of cortical bone. This is necessary because bone is not simply made up of minerals. Like anywhere else in the body, a large number of cells live in the bone and they require the nutrients in blood to survive. In smaller bones, there is generally only one nutrient foramin, while in a larger bone, such as a femur bone, there may be several.
The diaphysis is separated from the ends of the bone, called the epiphysis, by a layer bone called the metaphysis. In growing bones, part of the metaphysis is a layer of cartilage called the epiphyseal growth plate. When bones are still growing in length — called interstitial growth — this cartilage is constantly being converted into bone to become a new part of it. In adulthood, the bone stops growing and the epiphyseal growth plate becomes a section of bone known as the epiphyseal line.
Generally, it should be noted that the diaphysis is a section of bone found only in long bones. Long bones are bones that are much longer than they are wide and function as levers, such as the femur or humerus. The structure does not usually articulate with other bones, although there are exceptions to this, such as the radius and ulna articulating via the interosseus membrane of the forearm.
Ewing’s sarcoma is a type of tumor that often appears in long bones, particularly the femur and humerus. Sometimes, however, it can be found in soft tissue as well. This type of tumor most often affects male children and teenagers.