Step 1 Fetus
A long bone begins in the fetus as a hyaline cartilage model of the bone.
At the appropriate time, the cartilage model is invaded by a mass of material
that begins to destroy the cartilage and replace it with bone tissue.
This invading mass and the subsequently developed bone structure are called
the primary center of ossification, or diaphysis.
Step 2 Time of Birth
At about the time of birth or thereafter, a secondary center of ossification, or
epiphysis, develops at each end of the developing long bone.
Step 3 Early Years of Life
A plate of cartilage, called the epiphyseal plate, remains between the
diaphysis and each epiphysis. In the early years of life, the cartilage grows faster than
the diaphysis can tear it down. This results in gradual lengthening of the long bone.
Step 4 Between Puberty and Adulthood
At the proper time, between puberty and adulthood, the bone development
overtakes completely destroys the cartilage. After this, the diaphysis and the epiphysis
are solidly fused to one another. The dense bony line of fusion between the diaphysis
and epiphysis is called the epiphyseal line. The epiphyseal line is easily visible in the
radiographs ("x-rays") of young adults.
While the bone has been growing in length, it also grows in width. The
osteogenic layer of the periosteum gradually adds bony tissue to the outside surface of
the bone. At the same time, osteoclastic activity removes bone material from the wall of
the marrow cavity.
Step 5 Throughout Life
Throughout the entire life of the individual, the continuous tearing down
(osteoclastic activity) and rebuilding (osteoblastic activity) remodel the bony substance.
These processes occur in response to the forces or stresses applied to the body.
Step 6 Medical Conditions Related to Bone Growth
Many factors are involved in the process of bone growth. One of the primary
factors is a hormone of the anterior pituitary gland known as somatotropin.
Overproduction of somatotropin in a young person (before fusion of the ossification
centers) results in gigantism. Overproduction of somatotropin in adults (after fusion of
the ossification centers) results in a condition called acromegaly. Acromegaly involves
excessive growth of the jaw, hands, and feet.