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Step 1.
The chondrocytes in the center of the cartilage
model become larger and the surrounding
cartilage matrix becomes calcified. These
chondrocytes undergo programmed death leaving
behind cavities within the cartilage
matrix. |
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Step 2.
The perichondrium of the cartilage model
becomes transformed into a periosteum as
an osteogenic layer forms. The periosteum
produces a thin collar of bone around the shaft
of the cartilage model. |
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Step 3. |
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Blood
vessels and osteoprogenitor cells from the periosteum
migrate into the cavities within the calcified
cartilage matrix. The calcified cartilage breaks down
but before doing so provides a surface on which
osteoblasts can produce spongy bone. This then
becomes a primary ossification center.
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Step 4.
As osteogenesis continues in the primary
ossification center, a cavity results as the
bone is remodeled. This central cavity is called
the marrow cavity. Further growth
involves an increase in both the length
and diameter of the bone. |
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Increasing the length of the developing bone |
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Before
describing how bones increase in length some terminology
needs to be introduced. The shaft of the long bone is
called the diaphysis and the two expanded ends
are called epiphyses (each is called an
epiphysis). The flared portion between the epiphysis
and the diaphysis is called the metaphysis.
Growth in the length of bone occurs in the epiphysis in
a piece of cartilage called the epiphyseal plate.
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Step 5.
Secondary ossification centers appear in
the epiphyses in a manner similar to that
described above for the primary ossification
center. |
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Step 6.
As ossification proceeds, the epiphyses become
filled with spongy bone and only a thin piece of
hyaline cartilage remains on the articular
surface as articular cartilage. A thin
plate of cartilage, the epiphyseal plate,
now separate the bone of the epiphyses from that
of the diaphysis. Growth in the length of the
bone occurs at the epiphyseal plate as is
evidenced by the appearance of zones within the
cartilage. Proceeding from the epiphyseal side
to the the diaphyseal side the zones are:
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Zone of
reserve or resting cartilage
The chondrocytes here exhibit no
division or active matrix
production. |
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Zone of
proliferation
The chondrocytes undergo cell
division and arrange
themselves in distinct columns
that are parallel to the
direction of growth.
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Zone of
hypertrophy
The chondrocytes enlarge
and compress the cartilage
matrix surrounding them.
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Zone of
calcification
The enlarged cells begin to
degenerate as the matrix becomes
calcified. |
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Zone of
resorption (a.k.a. ossification)
The cavities left by the dead
chondrocytes are invaded by
blood vessels that bring
osteoprogenitor cells with them.
Bone is produced on the surface
of the calcified cartilage as it
undergoes resorption.
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Step 7.
The bone increases in length as long as the rate
of cartilage production equals the rate by which
it is replaced by bone. As the bone matures the
rate of bone production "catches up" with the
rate of cartilage production and the cartilage
that was the epiphyseal plate is replaced by an
epiphyseal line.
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Increasing the diameter of the developing bone
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The diameter of bone increases by
appositional growth. Osteoblasts in the
cellular layer of the periosteum add success
layers of bone called circumferential
lamellae. At the same time, osteoclasts on
the interior surface resorb bone and widen the
marrow cavity. The final result is an increase
in both the diameter and marrow cavity of the
bone.
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