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Osteogenesis
Bone
development
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Processes
of bone development
Intramembranous
ossification
Endochondral
ossification
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Intramembranous ossification
1. Osteoblasts differentiate directly from mesenchyme and begin secreting osteoid
2. Woven bone is produced first and is soon replaced by stronger lamellar bone
3. Areas of woven bone, bone resorption, and lamellar bone exist contiguous to one another during growth
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Intramembranous
ossification
Occurs within condensed sheets ("membranes") of embryonic mesenchymal tissue
Most bones of the skull, jaws, scapula and
clavicle
are formed this way
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Intramembranous
ossification process
1.
Osteoprogenitor
cells arise, proliferate, and form incomplete layers of
osteoblasts
around a network of developing capillaries
2.
Osteoid
secreted by osteoblasts calcifies, forming small irregular areas of
woven bone
3. Continued
matrix
secretion and calcification
enlarges
these areas and leads to the fusion of neighboring ossification centers
4.
Mesenchymal
regions that do not undergo ossification give rise to the endosteum and the
periosteum
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Cranial
flat bones formed by
intramembranous
ossification
Internal
and external plates of
compact
bone arise, while the central portion (diploë) maintains its cancellous nature
Fontanelles or "soft spots" on newborn heads are areas where the membranous tissue is
not
yet ossified
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Endochondral ossification
1. Ossification occurs within hyaline cartilage shaped as a small version of the bone to be formed
2. Osteoblasts in the perichondrium (transitioning to periosteum) produce a bone collar around the cartilage model diaphysis
3. Chondrocytes in the cartilage model hypertrophy, compress the matrix, and initiate calcification
4. Chondrocytes die, creating empty spaces
5. Blood vessels and osteoprogenitor cells from the periosteum penetrate the bone collar, producing woven bone
6. Primary ossification center forms in the diaphysis, and secondary ossification centers later appear at the epiphyses
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Endochondral ossification
Produces cavities filled with
bone marrow
and
cancellous bone trabeculae
as the primary and secondary ossification centers expand and remodel
Articular cartilage within
joints
and epiphyseal cartilage (
growth plate
) remain after ossification
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Epiphyseal cartilage (growth plate)
1. Allows longitudinal bone growth
2. Disappears upon
completion
of bone development (epiphyseal closure) by around age
20
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Zones of the epiphyseal growth plate
Zone of
reserve
(resting)
cartilage
Proliferative
zone
Zone of
hypertrophy
Zone of
calcified
cartilage
Zone of
ossification
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Longitudinal growth of a bone occurs by
cell proliferation
in the
epiphyseal
plate cartilage
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Chondrocytes
in the diaphysis side of the epiphyseal plate undergo hypertrophy, their matrix becomes calcified, and the cells
die
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Osteoblasts
lay down a layer of new bone on the calcified
cartilage
matrix
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The epiphyseal plate does not change thickness, but is instead displaced away from the center of the
diaphysis
as the
length
of the bone increases
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Appositional
growth of long bones
1.
Osteoblasts
developing from
osteoprogenitor
cells in the periosteum form the bone collar on the cartilaginous diaphysis
2. Osteoclasts in the endosteum enlarge the
central marrow
cavity as the bone circumference
increases
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