2.5. CELL TYPES 37
relative to the mediolateral direction across the range of frequencies.Tanaka et al.  repeated the
same procedure in another study but this time varied the compressive strain (5% - 15%) and shear
strain (0.5% - 1.5%). Dynamic shear moduli increased with increasing compressive strain, which
the authors attributed to a decrease in porosity with increased loading. This result would seem to
corroborate the strain dependence of compressive properties [254, 256]. Interestingly, shear moduli
decreased with increasing shear strain. The explanation for this behavior is less clear. The authors
speculated that water and proteoglycans in the disc may display non-Newtonian shear-softening
behavior similar to synovial ﬂuid.
Overall, the frictional and shear properties of the disc remain relatively unknown. It is reason-
able to assume that unphysiological shear loading may have a degradative effect, similar to what is
seen in other cartilages [262, 263, 267], necessitating the execution of more characterization work. It
is likely that disc traction forces increase with the onset of osteoarthrosis as a result of synovial ﬂuid
degradation and increased surface roughness. These increased forces could then cause greater shear-
ing in the disc, possibly leading to derangement and disease . An understanding of shear and
frictional forces in joint disease processes will no doubt be necessary to avoid premature deterioration
of a tissue engineered TMJ disc.
2.5 CELL TYPES
The TMJ disc contains a heterogeneous collection of morphologically variable cells . Some cells
are ﬂattened and spindle-shaped much like the tenocytes found in tendons, while others appear
rounded, surrounded by a distinct pericellular matrix similar to what is seen in hyaline articular
cartilage (Figure 2.8). When taken as a whole population, the TMJ disc cells may be appropriately
referred to as ﬁbrochondrocytes.
Detamore et al.  studied the regional distribution of cells in the porcine disc using
histology and transmission electron microscopy, yielding an overall density of 681 ± 197 cells/mm
70% ± 11% of which appeared ﬁbroblast-like based on morphology.The intermediate and posterior
bands were signiﬁcantly more cellularized than the anterior band, and the central region of the
intermediate zone had approximately 10% fewer cells than the lateral and medial regions. The
anterior and posterior bands contained a higher percentage of ﬁbroblasts than the intermediate
zone, and the superior and middle layers had higher levels of ﬁbroblast-like cells than the inferior
surface. In contrast to these ﬁndings, Milam et al.  found mostly rounded, chondrocyte-like
cells surrounded by lacunae in primate TMJ discs. Mills et al.  also reported the presence of
rounded, chondrocyte-like cells in the interstices between collagen bundles in primate TMJ discs.
Cells were typically found in groups of three to six cells, and were generally smaller and less rounded
at the surfaces and close to the peripheral attachments.
Berkovitz and Pacy [270, 271] examined TMJ disc cell anatomy in two studies. The ﬁrst
study  explored age-related differences in rats and marmosets, and reported the presence of
a microﬁlamentous pericellular matrix surrounding the cells. This matrix was structurally different
from the pericellular matrix of hyaline articular chondrocytes, as it was lacking a pericellular capsule