  In this work, we reevaluated long standing conjectures as to the source of
the exceptionally large compliance of the bladder wall. Whereas, these
conjectures were based on indirect measures of loading mechanisms, in this work
we take advantage of advances in bioimaging to directly assess collagen fibers
and wall architecture during loading. A custom biaxial mechanical testing
system compatible with multiphoton microscopy (MPM) was used to directly
measure the layer dependent collagen fiber recruitment in bladder tissue from 9
male Fischer rats (4 adult and 5 aged). As for other soft tissues, the bladder
loading curve was exponential in shape and could be divided into toe,
transition and high stress regimes. The relationship between collagen
recruitment and loading curves were evaluated in the context of the inner
bladder wall (lamina propria) and outer detrusor smooth muscle layer. The large
extensibility of the bladder was found to be possible due to folds in the wall
(rugae) that provide a mechanism for low resistance flattening without any
discernible recruitment of collagen fibers throughout the toe regime. For
elastic bladders, as the loading extended into the transition regime, a gradual
coordinated recruitment of collagen fibers between the lamina propria and
detrusor smooth muscle layers was found. A second important finding is that
wall extensibility could be lost by premature recruitment of collagen in the
outer wall that cut short the toe region. This work provides, for the first
time, a mechanistic understanding of the role of collagen recruitment in
determining bladder capacitance.
