The mechanical properties of cross-linked microtubule bundles were mea
sured from outer pillar cells isolated from the mammalian inner ear. M
easurements were made using a three-point bending test and were incorp
orated into a mathematical model designed to distinguish between the s
tiffness contributions from microtubules and their cross-linking prote
ins. Outer pillar cells were composed of 1000-3000 parallel bundled mi
crotubules in a square array that was interdigitated and cross-linked
with actin filaments. The average midpoint bending stiffness of intact
cells was 7 x 10(-4) N/m. After removal of both the actin filaments a
nd cross-links with detergent in the presence of DNase I, the square a
rray was disrupted and the stiffness decreased by a factor of 4, to 1.
7 x 10(-4) N/m. The bending modulus for individual microtubules was ca
lculated to be 7 x 10(-23) Nm(2), and the Young's modulus for these 15
protofilament microtubules was 2 x 10(9) Pa. The shear modulus betwee
n microtubules in intact cells was calculated to be 10(3) Pa. It was c
oncluded that cross-linking proteins provided shear resistance between
microtubules, which resulted in a fourfold increase in stiffness. The
model can be used to estimate the mechanical properties of cross-link
ed microtubule bundles in cells from which direct measurements are not
available.