The terms osteogenesis and ossification are often used synonymously
to indicate the process of bone formation. Parts of the skeleton
form during the first few weeks after conception. By the end
of the eighth week after conception, the skeletal pattern
is formed in cartilage and connective tissue membranes and
ossification begins.
Bone development continues throughout adulthood. Even after
adult stature is attained, bone development continues for
repair of fractures and for remodeling to meet changing lifestyles.
Osteoblasts, osteocytes and osteoclasts are the three cell
types involved in the development, growth and remodeling of
bones. Osteoblasts are bone-forming cells, osteocytes are
mature bone cells and osteoclasts break down and reabsorb
bone.
There are two types of ossification: intramembranous and
endochondral.
Intramembranous
Intramembranous ossification involves the replacement of
sheet-like connective tissue membranes with bony tissue. Bones
formed in this manner are called intramembranous bones. They
include certain flat bones of the skull and some of the irregular
bones. The future bones are first formed as connective tissue
membranes. Osteoblasts migrate to the membranes and deposit
bony matrix around themselves. When the osteoblasts are surrounded
by matrix they are called osteocytes.
Endochondral Ossification
Endochondral ossification involves the replacement of hyaline
cartilage with bony tissue. Most of the bones of the skeleton
are formed in this manner. These bones are called endochondral
bones. In this process, the future bones are first formed
as hyaline cartilage models. During the third month after
conception, the perichondrium
that surrounds the hyaline cartilage "models" becomes infiltrated
with blood vessels and osteoblasts and changes into a periosteum.
The osteoblasts form a collar of compact bone around the diaphysis.
At the same time, the cartilage in the center of the diaphysis
begins to disintegrate. Osteoblasts penetrate the disintegrating
cartilage and replace it with spongy bone. This forms a primary
ossification center. Ossification continues from this center
toward the ends of the bones. After spongy bone is formed
in the diaphysis, osteoclasts break down the newly formed
bone to open up the medullary cavity.
The cartilage in the epiphyses continues to grow so the developing
bone increases in length. Later, usually after birth, secondary
ossification centers form in the epiphyses. Ossification in
the epiphyses is similar to that in the diaphysis except that
the spongy bone is retained instead of being broken down to
form a medullary cavity. When secondary ossification is complete,
the hyaline cartilage is totally replaced by bone except in
two areas. A region of hyaline cartilage remains over the
surface of the epiphysis as the articular cartilage and another
area of cartilage remains between the epiphysis and diaphysis.
This is the epiphyseal plate or growth region.
Bone Growth
Bones grow in length at the epiphyseal plate by a process
that is similar to endochondral ossification. The cartilage
in the region of the epiphyseal plate next to the epiphysis
continues to grow by mitosis. The chondrocytes, in the region
next to the diaphysis, age and degenerate. Osteoblasts move
in and ossify the matrix to form bone. This process continues
throughout childhood and the adolescent years until the cartilage
growth slows and finally stops. When cartilage growth ceases,
usually in the early twenties, the epiphyseal plate completely
ossifies so that only a thin epiphyseal line remains and the
bones can no longer grow in length. Bone growth is under the
influence of growth hormone from the anterior pituitary gland
and sex hormones from the ovaries and testes.
Even though bones stop growing in length in early adulthood,
they can continue to increase in thickness or diameter throughout
life in response to stress from increased muscle activity
or to weight. The increase in diameter is called appositional
growth. Osteoblasts in the periosteum form compact bone around
the external bone surface. At the same time, osteoclasts in
the endosteum break down bone on the internal bone surface,
around the medullary cavity. These two processes together
increase the diameter of the bone and, at the same time, keep
the bone from becoming excessively heavy and bulky.
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