|Ph.D Student||Marom Yulia|
|Subject||Biomechanical and Biomolecular Processes in Stretched Fetal|
|Department||Department of Mechanical Engineering||Supervisors||Professor Doron Shilo|
|Professor Emeritus Eliezer Shalev|
|Full Thesis text|
The preterm labor rate is particularly high in pregnancies associated with uterine over-distention and particularly in multiple pregnancies. This indicates on a possible relation between preterm labor and mechanical stresses that are applied on the human fetal membrane (FM). It is therefore imperative to study biomechanical and biomolecular processes that occur in the FM under mechanical stress.
We perform bulge tests on ex-vivo FM tissues where mechanical conditions similar to those prior to uterine muscle contractions were applied. Experimental results reveal an irreversible mechanical behavior which is unique only for the first loading cycle and results in a residual strain that does not recover upon unloading.
We introduce a new model for the collagen tissues mechanical response, which accounts for the irreversible deformation and provides predictions in agreement with our experimental results. The model basic assumption is that the constitutive stress-strain relationship of individual elements that compose the collagen fibers has a plateau segment during which an irreversible deformation occurs. Fittings of calculated and measured stress-strain curves reveal a collagenous tissues well-defined single-value property, which is related to the threshold strain of for irreversible transformation.
Second harmonic generation (SHG) images of the FM under different loads values reveal an unexpected collagen rearrangement in the compact layer. The observed collagen fibers bundling and alignment indicate a deviation from the expected equibiaxial stress state. The fiber orientations statistical analysis provides information on two driving forces for collagen alignment: microscale flaws and macroscale deviation from the equibiaxial strain state. A model that explains these observations and relates them to the material properties is presented.
Disconnection and reconnection of collagen interfibrillar molecular bonds is the fundamental process that governs collagen fibrils bundling and alignment in the FM. Therefore, we study the activation enthalpy of interfibrillar bonds in ex-vivo FM and analyze strain-rate and temperature dependence of this irreversible deformations. The activation enthalpy is found to be 1.6 ± 0.8 eV, implying that these bonds are mainly ionic. The FM is rich in calcium and magnesium ions that are known to play an important role in the uterus activity regulation associated with uterine contractions at term and preterm births. Our results imply that manipulations of these ions concentration may allow regulation of the strength and structural integrity of the FM.
Furthermore, we explore whether the stress-induced biomolecular changes eventually lead to contractions and labor initiation. We focus on proteins which are known to be associated with preterm contractions such as different isoforms of Progesterone Receptors (PR-B) and PR-A, Matrix Metalloproteinases (MMPs) and Heat Shock proteins A and B. It was found that with the rising pressure the levels of PR-B RNA are significantly reduced, whereas the levels of PR-A RNA significantly increased. No significant changes were observed in the expression of MMP2 and Heat Shock proteins A and B RNA.
The results of this study indicate that a temporal increase in intrauterine pressure or uterine cervix dilatation causes irreversible changes in collagen molecular connections that may lead to biological changes, such as the initiation of premature labor.