Endochondral Ossification
Most bones are formed by the transformation of
cartilage "bone models", a process called endochondral
ossification. A periosteal bud invades the
cartilage model and allows osteoprogenitor cells to enter the cartilage. At
these sites, the cartilage is in a state of hypertrophy (very large lacunae and
chondrocytes) and partial calcification, which eventually leads to the death of
the chondrocytes.
Invading osteoprogenitor cells mature into osteoblasts, which use the framework
of calcified cartilage to deposit new bone. The bone deposited onto the
cartilage scaffold is lamellar bone. The initial site of bone deposition is
called a primary ossification centre. Secondary
ossification centres occur in the future epiphyses of the bone.
A thin sheet of bone, the periosteal collar, is
deposited around the shaft of the cartilage model. The periosteal collar
consists of woven bone.
Close to the zone of ossification, the cartilage
can usually be divided into a number of distinct zones :
- Reserve cartilage, furthest away from the zone of
ossification, looks like immature hyaline cartilage.
- A zone of chondrocyte proliferation contains
longitudinal columns of mitotically active chondrocytes, which grow in
size in
- the zone of cartilage maturation and hypertrophy.
- A zone of cartilage calcification forms the border
between cartilage and the zone of bone deposition.
Primary and secondary ossification centres do not
merge before adulthood. Between the diaphysis and the epiphyses a thin sheet of
cartilage, the epiphyseal plate, is maintained
until adulthood. By continuing cartilage production, the
epiphyseal plate provides the basis for rapid growth in the length of the bone.
Cartilage production gradually ceases in the epiphyseal plate as maturity is
approached. The epiphyseal plate is finally removed by the continued production
of bone from the diaphyseal side.
Bone formation and bone
resorption go hand in hand during the growth of bone. This first
deposited trabecular bone is removed as the zone of ossification moves in the
direction of the future epiphyses. This process creates the marrow cavity of
the bones. Simultaneously, bone is removed from the endosteal surface and
deposited on the periosteal surface of the compact bone which forms the
diaphysis. This results in a growth of the diameter of the bone.
sections of bones
during the early stages of their development - H&E,
van Gieson
Foetal vertebral column, human
- H&E
Hold the section against a light background. The cartilage models will stain
very light, and the outlines of the skeletal structures they will form can
often be identified. If an ossification centre is present, it will appear as a
darker area within the cartilage model. The zonation of the cartilage should be
visible in all ossification centres. How much bone is present depends on how
far ossification has proceeded. The newly formed bone trabeculae will often
consist of lamellar bone with a more or less extensive core of darkly staining
calcified cartilage. The lamellar organisation of the bone may not be visible.
How the ossification centre exactly will look also depends on the plane of the
section in relation to the ossification centre.
Suitable Slides
sections of the
epiphyseal disc of growing bones - H&E
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Growing bone, rabbit -
H&E
The cartilage model has almost entirely been transformed into bone. The only
remaining cartilage is found in the epiphyseal disk. Zones of cartilage
proliferation, hypertrophy and calcification are visible at high magnification,
but only on one side of the epiphyseal disk - towards the diaphysis, which
increases in length as the cartilage generated by the epiphyseal disc is
transformed into bone.
Osteoclasts may be found on the newly formed trabeculae or associated with
parts of the cartilage scaffold.
Draw the region of the epiphyseal disks. Identify in your
drawing the epiphyseal disk and the bone of the epiphysis and diaphysis.
Indicate the direction of cartilage proliferation in the epiphyseal disk and
the direction of bone growth.
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![thumbnail]( "Endochondral Ossification")
Title:
Endochondral Ossification
by:
om
at
2013-02-15T03:04:00+07:00
Rating: 4.8
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Endochondral Ossification