Biomedical Engineering Reference
In-Depth Information
There are two different methods of the ossification process: intramembranous
ossification is bone formation from an organic matrix membrane, whereas en-
dochondral ossification occurs within a cartilaginous model. However, there is
only one mechanism of bone formation: the laying down of the osteoid matrix by
osteoblasts, followed by the deposition of crystalline apatite [65].
In embryogenesis, the skeletal system is derived from the mesoderm germ layer.
Chondrification (or chondrogenesis) is the process by which cartilage is formed
from the condensed mesenchymal tissue, which differentiates into chondrocytes
and begins secreting the molecules that form the ECM. Early in the fetal develop-
ment, the greater part of the skeleton is cartilaginous. It is the
temporary
cartilage
that is gradually replaced by the bone (endochondral ossification), a process that
ends at puberty. The cartilage in the joints is
permanent
- it remains unossified
during the whole of life.
During the fetal stage of development the bone can be formed by two processes:
intramembranous or endochondral ossification. Intramembranous ossification
mainly occurs during the formation of the flat bones of the skull; the bone is then
formed from the mesenchymal tissue.
Endochondral (intracartilaginous) ossification occurs in long bones. In this
process, the bone is formed from cartilage, which is gradually replaced by the
bone as the embryo grows. The steps of endochondral ossification are visible in
Figure 1.6.
Adult hyaline articular cartilage is progressively mineralized at the junction
between cartilage and bone. It is termed
articular calcified cartilage
.Aminer-
alization front advances through the base of the hyaline articular cartilage at
a rate dependent on cartilage load and shear stress. Adult articular calcified
cartilage is penetrated by vascular buds, and the new bone produced in the
vascular space in a process similar to endochondral ossification at the physis.
A cement line separates the articular calcified cartilage from the subchondral
bone.
Both the bone and the cartilage are classified as supportive CT.
•
Bone (osseous tissue) makes up the skeleton in adult vertebrates.
• Cartilage makes up the skeleton in chondrichthyes (known also as
cartilaginous
fishes
). In most other adult vertebrates, the cartilage is primarily found in joints,
where it provides bearing and cushioning.
Bone-forming cells called
osteoblasts
deposit a matrix of collagen, but they also
release calcium, magnesium, and phosphate ions, which chemically combine and
harden within the matrix into the mineral hydroxyapatite. The combination of hard
mineral and flexible collagen makes the bone harder than cartilage without being
brittle.
Bone marrow can be found in almost any bone that holds cancellous tissue. In
newborns, all such bones are filled with red marrow only, but as the child ages it is
mostly replaced by yellow, or fatty, marrow. In adults, red marrow is mostly found
in the flat bones of the skull, the ribs, the vertebrae, and pelvic bones, cf. [67-73].
