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110 6. REVIEW OF THE ESSENTIAL ROLES OF SMCS IN ATAA BIOMECHANICS

FIG. 6.10 A vicious circle: under
some stimuli (orange box), the homeo-
static state is endangered and the cell
(violet circle) sets up complex chain
reactions. Several regulation loops
cross each other (blue, gray, green)
and thus the equilibrium is difficult
to reach again, especially as the elastin
degradation leads to a permanent loss
of elasticity and this effect is amplified
with the duration of the pathological
sate. Hence, although the phenotypic
switching may be reversible, the per-
manent alteration of the ECM may
prevent the cells from going back to
homeostatic conditions that enhance
the pathology.

However, the quantification of levels of SMC contractility that results in one type of evolution or another is still an open
issue. There is still a pressing need to characterize the basal tone of SMCs in healthy aortas and ATAAs at the cellular
scale.

6.5 SUMMARY AND FUTURE DIRECTIONS

The mechanobiology and physiopathology of the aorta have received much attention so far, but there is still a press-
ing need to characterize the roles of SMCs at the cellular scale. Despite the difficulties of characterizing cells having
complex dynamics and fragility, techniques such as AFM or traction force microscopy could permit important pro-
gress to be made about SMC nanomechanics and provide relevant information on how SMC biomechanics are related
to the irreversible alteration of stress distribution in ATAAs. This will also imply the development of new biomechan-
ical models of the aortic wall, taking into account the contractility of SMCs.

Acknowledgments

The authors are grateful to the European Research Council for grant ERC-2014-CoG BIOLOCHANICS. They are also thankful to E. Planus and T.C.
Gasser for the courtesy of the corresponding figures.

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