Biomedical Engineering Reference
In-Depth Information
hydroxyapatite. Through the incorporation of these secondary components, these crystals
become very stiff but remain quite brittle. Similar to ceramic materials, these crystalline
compounds can withstand intense compressive forces but cannot withstand torsion, bend-
ing, or forces in tension.
Roughly 33% of bone mass is accounted for by collagen fibers. Different types of colla-
gen can be found in bone matrix and each collagen type forms into a different three-
dimensional structure. For instance, collagen type I forms into fibers, whereas collagen
type IV forms a mesh-like structure. Most collagen fibers are long and un-branched pro-
teins and are composed of individual protein sub-units that are wound together. Collagen
fibers are very strong, especially when subjected to forces in tension. Structurally, collagen
resembles a rope and as such does not withstand any forces in compression. Collagen can
withstand some pure torsional and bending stresses, but if there is any compressive load-
ing on the collagen fiber, it will tend to fail.
Ground substance fills the spaces between the extracellular proteins, collagen, and the
cells within bone tissue. Ground substance is typically clear and viscous due to the pres-
ence of proteoglycans. Ground substance is so viscous that most cells have a difficult time
migrating through the fluid. Therefore, it is uncommon to see a significant amount of
transport occurring within this portion of bone. However, if a foreign cell or a foreign par-
ticle enters the ground substance, then the highly viscous fluid aids in the inflammatory
response by increasing the amount of time that the foreign cell/particle is visible to white
blood cells. Also, the amount of time needed to enter the vascular system is increased, and
therefore, the likelihood for spreading to other regions of the body is diminished. The dan-
ger of inflammation arises when foreign cells/particles overwhelm the body's defenses in
the bone. New inflammatory cells can slowly migrate into the ground substance to combat
the infection, which may have become very strong and stable.
The remaining 2% of bone mass is accounted for by the cells within the bone matrix.
Bone cells are divided into four cell types, which are classified based on their function and
appearance. The first type of bone cells are the osteocytes. These are mature cells that
make up the vast majority of all of the bone cells. Osteocytes can be found within void
spaces throughout the bone matrix. These void spaces are termed lacunae. Each lacuna is
occupied by one osteocyte that remains in this location during its entire life span (unless
the bone is broken down to such an extent that the lacunae open to the surrounding inter-
stitial space). Lacunae are connected to each other via narrow channels that infiltrate the
entire bone matrix. These channels are termed canaliculi, and their primary function is to
provide nutrients to osteocytes as well as to allow osteocytes to communicate with each
other. In fact, osteocyte cellular extensions fill the canaliculi, and these cellular extensions
directly connect to neighboring osteocytes via gap junctions. Similar to endothelial cell gap
junctions, osteocyte gap junctions allow for the transfer of ions and other signaling mole-
cules to facilitate cell-cell communications.
Osteocytes perform two major functions for bone tissue. The first function is to maintain
the mineral content and protein composition of the bone matrix. Bone formation is a pro-
cess that is continually occurring, depending on the particular loading conditions that the
bone is subjected to. This is interesting because if a person starts to run daily as exercise,
this will be accompanied by an increased bone density. After bed rest or space travel
(reduced gravity) the bone density decreases. Therefore, bone is a very adaptive tissue.
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