Biomedical Engineering Reference
In-Depth Information
that take place in condensation and differentiation. For a more in depth discussion, the reader should
consult topics and articles on development, such as Hall's excellent and thorough volume [
120
].
Many
in vitro
studies have manipulated the molecular players highlighted above to promote
the accumulation of cartilage-specific matrix, both for developmental studies and for tissue engineer-
ing. As TGF-
β
1,
β
2, and
β
3 have been observed during differentiation
in vitro
[
121
], their efficacy
has been examined in enhancing matrix production. The same is true for the BMPs. TGF-
β
has
been shown as effective on cells that have not yet condensed while BMP-2 achieves similar effects
after the cells have condensed or differentiated [
121
]. A more thorough treatment of select factors
related to development and used in tissue engineering (e.g., TGF family, shh) will be presented in
the next chapter, in Section 3.4.
2.1.2 HYPERTROPHY ANDOSSIFICATION
Developmentally, cartilage can serve as a transition tissue to bone, and understanding this process
may allow for its manipulation in tissue engineering. Osteogenesis can occur via intramembranous
ossification, which is the direct conversion of mesenchymal tissue into bone, or via endochondral
ossification, which is through the calcification of cartilage tissue [
122
]. Endochondral ossification
occurs in both somatic and lateral plate cartilages to form, for example, the vertebrae and limbs,
respectively, while stopping just short of facet joint and the articulating cartilages of the limbs. From
the formation of cartilage tissue, the chondrocytes stop dividing and can undergo hypertrophy,
during which the cells increase their volume [
123
]. The cartilage matrix is altered with the addition
of collagen type X and increased fibronectin content. Collagen type X allows the tissue to become
calcified, while VEGF, which transforms mesodermal mesenchyme cells to blood vessels, is secreted
by the hypertrophic chondrocytes [
124
,
125
]. From here the blood vessels then infiltrate the cartilage,
the hypertrophic chondrocytes die, and the cells that surrounded the cartilage become osteoblasts
to make bone [
126
] (the last two steps of Figure 2.1). Our interest here lies just before this last step,
on how the hypertrophy is regulated. That is, what are the factors that initiate it and, more pertinent
to tissue engineered constructs, which factors prevent it from occurring.
It has been shown that hypertrophy follows chondrocytes switching from aerobic to anaerobic
respiration. Evidence for this is provided by examining creatine kinase, an enzyme that catalyzes
the formation of ATP in tissues under oxygen stress. Creatine kinase activity is related to both
chondrocyte maturation and hypertrophy, as the activity of this enzyme increases to prepare for a
hypoxic state [
127
]. Growth factors that have been shown to affect hypertrophy include the BMPs
and TGF-
β
. BMP-2, -4, -6, and -7 have all been implicated in chondrocyte hypertrophy. Of these,
BMP 6 and 7 are expressed in hypertrophic chondrocytes [
128
,
129
], and the exogenous addition
of BMP-2 and -4 results in increases in chondrocyte hypertrophy [
130
,
131
]. Other factors include
Rac1 and Cdc42, as overexpression of these small GTPases results in acceleration of hypertrophic
differentiation [
132
]. Particularly interesting are the results Wu and associates [
133
] obtained with
cyclic matrix loading. Collagen type X was shown to be up-regulated by stretch-induced matrix
deformation, hinting that mechanical stimulation may also play a role in hypertrophy.
