Evidence that calcium-dependent cellular processes are involved in the stiffening response of holothurian dermis and that dermal cells contain an organic stiffening factor

Although previous investigations have shown that experimental increases and decreases of the concentration of extracellular Ca2+ produce correlated changes in the stiffness of holothurian dermis, they have failed to determine whether the Ca2+-correlated changes were due to Ca2+-dependent cellular events or to direct effects of Ca2+ on the viscosity of the extracellular matrix. We have addressed this question by testing two explicit predictions of the latter hypothesis: that dermal stiffness should be correlated with the Ca2+ concentration in the absence of viable cells; and that, in the presence of a normal extracellular Ca2+ concentration, drugs that inhibit cellular pathways dependent on Ca2+ should not affect dermal stiffness. Our results are inconsistent with the hypothesis and support the alternative hypothesis that Ca2+ is important only in the cellular regulation of dermal stiffness. In addition, we have extracted from dermal cells an organic factor that stiffens the extracellular matrix.

Pretwisted Beams and Columns

A theory is developed for the behavior of pretwisted structural members of thin-walled section with slight initial bending. The stresses are at first determined along and perpendicular to the fibers and are then transformed to stresses in the cross section and along the axis. Although the development is perfectly general the integrations are only indicated for doubly symmetric sections. The buckling of doubly symmetric columns which are initially straight but are pretwisted at a constant rate is treated in detail. The results show that columns of decidedly unequal principal moments of inertia can be strengthened up to 90 per cent, but columns of equal moments of inertia are weakened by initial twist. In analogy to the Euler load of the buckling theory for straight, untwisted columns, a reduced Euler load is defined. The buckling load is the product of this reduced Euler load and a stiffening factor.