2.6. AGE-ASSOCIATED CHANGES IN THE DISC 39
for cultured articular chondrocytes. Collagen type II RNA was found, though at low levels relative
to control mandibular condyle cells. Kapila et al. [273] cultured leporine disc cells and assessed the
conditioned media for proteinases using polyacrylamide gels followed by western blots. They noted
the presence of several matrix metalloproteinases (MMPs), including gelatinase, procollagenase
and prostromelysin. They also isolated two proteinases inhibitors, most likely TIMP and TIMP-2,
using reverse zymography. These findings suggest disc cells contribute to ECM remodeling though
secretion of MMPs, and additionally, they express an MMP profile more reflective of a synovial
fibroblast than a chondrocyte.
In summary, there is no single description of the phenotype of a TMJ disc cell. Instead, the cells
should be viewed as a heterogeneous distribution expressing characteristics that fit somewhere along
the phenotypic spectrum between a fibroblast and a chondrocyte.Therefore,the term fibrochondrocyte
most accurately describes the heterogeneous population of TMJ disc cells [219].
2.6 AGE-ASSOCIATED CHANGES IN THE DISC
The mechanical strength and stiffness of many connective tissues, such as articular cartilage, skin,
and bone, increase into adulthood, then gradually decrease with advancing age [43, 274, 275].
However, several studies indicate that the mechanical integrity of the TMJ disc is sustained or
continues to increase past the point of skeletal maturation [45, 64, 126, 265, 276]. Tanaka et al. [64,
126, 276] examined the tensile, compressive, and dynamic viscoelastic properties of discs from
young-adult (3 year-old), adult (7 year-old), and mature-adult (10 year-old) cattle. Under creep
tension [276], mature-adult discs were around 10% stiffer than adult discs and also maintained
the least residual strain after unloading. For compressive stress relaxation [126], the instantaneous
(17 MPa) and relaxed moduli (4 MPa) were similar between adult and mature-adult groups. Under
dynamic (cyclical) compressive loads applied at 1 Hz [64], the storage modulus of young-adult discs
(0.69 MPa) was significantly smaller than that of adult (1.21 MPa) and mature-adult (1.44 MPa)
discs. In addition, the loss modulus for the mature-adult group (0.23 MPa) was significantly larger
than the loss moduli of the younger groups. In a study of discs from human donors,Tanaka et al. [45]
reported a significant correlation (p < 0.01) between aging (range 22 - 67) and tensile modulus (27.1
- 65.2 MPa). Finally, Lai et al. [265] found the shear modulus of human discs increased significantly
(p < 0.01) with increasing age (range 36 - 76).
Age-related changes in mechanical properties are accompanied by, and can be largely at-
tributed to, changes in ECM composition and organization. Nakano and Scott [244] quantified the
biochemical composition of bovine discs, separated into inner and peripheral regions, from prenatal
development through maturation. In both regions, collagen content increased rapidly during pre-
natal development, then plateaued thereafter, while water concentration decreased steadily, though
not significantly, from fetus to adult. Most notably, there was a dramatic increase in chondroitin
sulfate (13-fold) and keratan sulfate (1600-fold) concentration from immature to mature adult in
the inner tissue, though the outer tissue remained relatively constant in this regard. The authors
hypothesized that increased GAG concentration was an adaptive response to cyclic compressive

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