Biomedical Engineering Reference
In-Depth Information
leucine-rich proteoglycans fibromodulin, decorin, biglycan, and lumican (which are
believed to modulate collagen fibrillogenesis) [
16
,
20
-
22
].
The AF is comprised primarily of sheets of aligned type I collagen, although
types II and III are also present but to a lesser extent than in the NP [
22
].
Additionally, 2% of the dry weight of the AF is comprised of elastin, which can
be found running parallel and perpendicular between lamellae and may prove
advantageous for returning the layers to their original orientation following
bending [
11
,
16
,
22
]. Proteoglycans are also found within the AF, but quantities
here are lower than those found within the NP. The presence of these molecules
may play a limited role in resisting compression in the AF; however, it is more
likely to contribute to the dissipation of shear between lamellae during bending
and torsion.
Maintenance of the distinct morphological structure of the IVD is a daunting
task for the cells of the IVD, especially considering that the IVD is the largest
avascular structure in the body, resulting in a relatively hypoxic and hypo-cellular
environment. Two phenotypically distinct cell types are found to inhabit the adult
IVD and account for between 0.25 and 1% of the total disc volume (Fig.
2
)[
14
,
20
].
Chondrocyte-like cells of an approximate density of 4
10
6
cells/cm
3
are found
within the human NP. These cells appear rounded or oval in shape, some of which
appear to reside within a lacunae similar to articular chondrocytes. Cells within the
AF appear more like fibroblasts and are found in greater numbers (9
10
6
cells/cm
3
)
extended along collagen fibrils within lamellae. The chondrocyte-like NP cells of
the IVD differ from those of normal articular cartilage not only in cell number, but
also in ECM production. These cells primarily produce aggrecan, which results in
an ECM containing a glycosaminoglycan to hydroxyproline ratio of 27:1, this is
considerably different from the 2:1 ratio found in normal articular cartilage [
23
].
AF cells have been shown to produce predominantly type I collagen and versican;
however, they have also been shown to produce minor amounts of aggrecan and
collagen type II [
24
]. A third population of cells can also be found in the IVD:
so-called notochordal cells (Fig.
2
), which are large in size and highly vacuolated.
Fig. 2 Representative histological images of (a) bovine annulus fibrosus (H&E staining),
(b) bovine nucleus pulposus (H&E staining), and (c) porcine notochordal cells (Alcian blue
staining for GAGs with fast red nuclei stain)
