A typical long bone, which is still young remains under the process of ossification for many years. It ossifies in three parts; the two ends and one shaft. The ends ossify from secondary centers of ossification while the shaft ossifies from primary center of ossification. Primary center of ossification is the center where the process of ossification starts first of all. Secondary centers of ossification are the centers where the process of ossification starts after the primary center.
In a young bone in which the process of ossification is not complete and the following important parts can be distinguished from the others;
- Epiphyseal plate of cartilage
The ends and the tips of a long bone that ossify from secondary centers are known as epiphyses. Epiphyses are of 4 types and they can be better understood by explaining their types;
- Pressure epiphyses: These epiphyses are articular that is they take part in joint formation. These epiphyses are the weight transmitting epiphyses. Examples are head of the humerus, lower end of the radius etc.
- Traction epiphyses: These are non-articular and do not take part in the formation of joints. They also do not take part in transmission of weight. The actual job of traction epiphyses is to provide attachment to tendons of muscles. The traction epiphyses ossify later than the pressure epiphyses. Examples of these epiphyses are tubercles of humerus (greater tubercle and lesser tubercle) and trochanters of femur (greater trochanter and lesser trochanter)
- Atavistic epiphyses: These epiphyses are phylogenetically independent but they become fused in man. Examples are coracoid process of scapula and os trigonum.
- Aberrant epiphyses: As the name indicates (aberrant = not usual) these epiphyses are not always present. Examples are epiphyses at the head of the first metacarpal bone and at the base of other metacarpal bones.
Diaphysis is the term used for elongated shaft of the long bone. It ossifies from a primary center.
The ends of diaphysis near the epiphyses are known as metaphyses. Since a long bone has two ends so there are two metaphyses. Each metaphysis is the zone of active growth of a long bone. Before the fusion of diaphysis and epiphyses, the metaphyses are richly supplied with blood through end arteries forming hair pin bends. This is the common site of osteomyelitis in children.
Epiphyseal plate of cartilage:
This is a plate of cartilage separating epiphyses from metaphyses. The cells of this cartilage plate proliferate and are responsible for the length wise growth of a long bone. When the age of maturity arrives, this cartilage plate ossifies and then the bone can no longer grow in length. This plate of cartilage is nourished by both epiphyseal and metaphyseal arteries.
For detailed study of human bones, please visit: Basic Anatomy of Bone