Determination of mechanical properties of the egg by the sessile drop method

Visco- and poroelastic contributions of the zona pellucida to the mechanical response of oocytes

Probing mechanical properties of cells has been identified as a means to infer information on their current state, e.g. with respect to diseases or differentiation. Oocytes have gained particular interest, since mechanical parameters are considered potential indicators of the success of in vitro fertilisation procedures. Established tests provide the structural response of the oocyte resulting from the material properties of the cell's components and their disposition. Based on dedicated experiments and numerical simulations, we here provide novel insights on the origin of this response. In particular, polarised light microscopy is used to characterise the anisotropy of the zona pellucida, the outermost layer of the oocyte composed of glycoproteins. This information is combined with data on volumetric changes and the force measured in relaxation/cyclic, compression/indentation experiments to calibrate a multi-phasic hyper-viscoelastic model through inverse finite element analysis. These simulations capture the oocyte's overall force response, the distinct volume changes observed in the zona pellucida, and the structural alterations interpreted as a realignment of the glycoproteins with applied load. The analysis reveals the presence of two distinct timescales, roughly separated by three orders of magnitude, and associated with a rapid outflow of fluid across the external boundaries and a long-term, progressive relaxation of the glycoproteins, respectively. The new results allow breaking the overall response down into the contributions from fluid transport and the mechanical properties of the zona pellucida and ooplasm. In addition to the gain in fundamental knowledge, the outcome of this study may therefore serve an improved interpretation of the data obtained with current methods for mechanical oocyte characterisation.

Centrifuge microscope as a tool in the study of cell motility

The centrifuge microscope (CM) is composed of a centrifuge and a microscope optical system designed to observe minute objects, especially living cells, during the application of centrifugal acceleration. Structures and characteristics of various types of CM designed and constructed up to the present and studies done with the CM on cell biology, especially cell motility, are reviewed. These studies include observations of the behavior of cells and cell components in a centrifugal field, determination of the mechanical properties of the cell surface and cytoplasm, microsurgical operations on cells with centrifugal force, and determination of the magnitude and the site of generation of motive force for cell motility.