The epiphysis and diaphysis are different parts of a long bone, or a bone found in a limb. Knobby ends of a long bone are referred to as the epiphyses, and the diaphysis is the shaft or middle section of the long bone. The epiphysis and diaphysis are different in their size, structure, and function. Separating them is a part of the bone known as the metaphysis.
One of the main differences between the epiphysis and diaphysis is their shape. The swollen rounded ends of the long bone are the epiphyses. These structures look somewhat similar to a clenched fist. Tubular and centrally positioned, the diaphysis makes up the main shaft of the long bone. On a long bone, the diaphysis is much larger than the epiphysis.
Structurally, the epiphysis and diaphysis are also distinct. The epiphysis is composed of compact or cortical bone on the outside and spongy, or trabecular, bone on the inside. Compact bone is fairly dense, providing strength. In the epiphysis, the spongy bone is slightly porous, leaving some space for red bone marrow and blood vessels. The porous nature of spongy bone also lightens the weight of the bone.
The diaphysis is composed of compact bone surrounding a marrow cavity filled with a fairly porous yellow marrow. Both red and yellow marrow are packed with blood vessels and capillaries to feed the bone. Red marrow contains white and red blood cells and platelets. Yellow marrow contains primarily fat cells, with some white blood cells.
Functionally, the epiphysis and diaphysis are very different. The epiphysis is the site of bone growth and is also often the place where a tendon anchors onto the long bone. In children, an epiphyseal plate is located between the epiphysis and the metaphysis. Long bones elongate when new cartilage, produced in the epiphyseal plate, is pushed to the edge of the epiphysis while older cartilage, located at the diaphysis side of the epiphysis plate, gets converted into bone.
When the body has finished growing, the epiphyseal plate ceases to manufacture new cartilage. Gradually, the cartilage in the plate area is transformed into bone. Once this occurs, the epiphyseal plate disappears and all that is left behind is a small line in the epiphysis area that is henceforth referred to as the epiphyseal line.
In contrast, the function of the diaphysis is to structurally support the body. With a thick outer shell of compact bone encompassing a porous inner cavity, this bone is perfectly constructed for this task. The outer margin of the diaphysis can bear the weight, while the inner core can house the marrow and minimize the weight of the bone.