Biomedical Engineering Reference
In-Depth Information
where ossifi cation began. Furthermore, the entire region of calcifi ed spicules
becomes surrounded by compact mesenchymal cells that form the periosteum.
The cells on the inner surface of the periosteum also become osteoblasts and
deposit matrix parallel to the existing spicules. The mechanism of intramembra-
nous ossifi cation involves bone morphogenetic proteins (BMPs) and the activation
of a transcription factor called Runx2.
Endochondral ossifi cation involves the formation of cartilage tissue from aggre-
gated mesenchymal cells and the subsequent replacement of cartilage tissue by
bone [9]. This is the type of bone formation characteristic of the vertebrae, ribs,
and limbs. The process of endochondral ossifi cation can be divided into fi ve stages,
as shown in Figure 14.2. First, the mesenchymal cells commit to becoming carti-
lage cells. This commitment is caused by paracrine factors that induce the nearby
mesodermal cells to express two transcription factors, which will then activate
a)
b)
c)
d)
Hypertrophi
chondrocyte
Osteoblasts
(bone)
Mesenchyme
Cartilage
e)
f )
g)
h)
Proliferating
chondrocytes
Epiphyseal
cartilage
Blood
vessel
}
Growth plate
Bone marrow
Bone
}
Growth plate
Secondary ossification center
Figure 14.2
Schematic diagram of endochondral ossifi cation.
