Biomedical Engineering Reference
In-Depth Information
12 CHAPTER 1. HYALINE ARTICULAR CARTILAGE
with superficial zone cells being smaller than middle/deep zone cells [
83
]. The morphology for cells
near the surface of the tissue is flattened and discoidal, whereas in the middle zone the cells are more
rounded and, in the deep zone, the cells are ellipsoidal and organized in columns perpendicular to the
surface [
42
]. In general, middle/deep zone cells possess greater synthetic capabilities for the major
molecular constituents of cartilage than superficial zone cells when cultured
in vitro
[
83
,
86
,
87
].
Chondrocytes from these two zonal populations also have different mechanical properties. The
Young's modulus of superficial zone chondrocytes is approximately twice that of middle/deep zone
chondrocytes (
E
Y
= 460 vs. 260 Pa, respectively) [
85
]. A similar relationship is seen for other elastic
and viscoelastic properties as well. These variations are likely caused by the different strain levels
that cells experience within the zones of cartilage. Tissue near the surface is compressed more than
that in the bulk of the cartilage [
13
,
49
], and hence, those cells might need to be stiffer to survive
the high strains.
Chondrocytes are known to lose their phenotypic markers
in vitro
, as evidenced by a tempo-
ral loss of morphologic characteristics and changes in metabolic activities of cells when cultured in
monolayers [
90
,
96
]. However, chondrocytes cultured in agarose retain morphological and proteogly-
can synthesis characteristics [
83
,
87
,
89
,
97
]. This is likely due to the constrained three-dimensional
environment, which forces a rounded morphology on the cells. Alternatively, chondrocytes cultured
in monolayer flatten over the course of days and begin to proliferate, rapidly losing their character-
istic expressions [
96
]. Three-dimensional culture in a hydrogel or similarly constraining material is
hypothesized to facilitate the synthesis of cartilage-specific molecules. Additionally, the application
of mechanical stimuli such as stress, strain, and pressurization can affect their phenotypic expressions
through a phenomenon termed mechanotransduction [
98
] (Figure 1.7).
1.3 CHAPTER CONCEPTS
Hyaline articular cartilage is a glass-like tissue that is avascular, aneural, and alymphatic.
Articular cartilage is composed largely of water, collagen, proteoglycans, and cells. These
components are arranged into zones and vary accordingly.
Articular cartilage is composed of 70-80% water (per ww), 50-75% collagen (per dw), and
15-30% proteoglycans (per dw).
Collagen type II is most abundant in hyaline articular cartilage, with collagen types V, VI, IX,
and XI also being present.
Charged proteoglycans cause the tissue to imbibe and retain water. Physiological compressive
forces are borne and dissipated as the water is forced out of the tissue.
The synovial fluid reduces friction and plays an important role in transporting both nutrients
and waste within the tissue.
