Laser Ablation of Microtubule-Chromosome Attachment in Mouse Oocytes

Mechanical Characterization of Murine Oocytes by Atomic Force Microscopy

The quality of murine and human oocytes correlates to their mechanical properties, which are tightly regulated to reach the blastocyst stage after fertilization. Oocytes are nonadherent spherical cells with a diameter over 80 μm. Their mechanical properties have been studied in our lab and others using the micropipette aspiration technique, particularly to obtain the oocyte cortical tension. Micropipette aspiration is affordable but has a low throughput and induces cell-scale deformation. Here we present a step-by-step protocol to characterize the mechanical properties of oocytes using atomic force microscopy (AFM), which is minimally invasive and has a much higher throughput. We used electron microscopy grids to immobilize oocytes. This allowed us to obtain local and reproducible measurements of the cortical tension of murine oocytes during their meiotic divisions. Cortical tension values obtained by AFM are in agreement with the ones previously obtained by micropipette aspiration. Our protocol could help characterize the biophysical properties of oocytes or other types of large nonadherent samples in fundamental and medical research.

Artificially decreasing cortical tension generates aneuploidy in mouse oocytes

Human and mouse oocytes’ developmental potential can be predicted by their mechanical properties. Their development into blastocysts requires a specific stiffness window. In this study, we combine live-cell and computational imaging, laser ablation, and biophysical measurements to investigate how deregulation of cortex tension in the oocyte contributes to early developmental failure. We focus on extra-soft cells, the most common defect in a natural population. Using two independent tools to artificially decrease cortical tension, we show that chromosome alignment is impaired in extra-soft mouse oocytes, despite normal spindle morphogenesis and dynamics, inducing aneuploidy. The main cause is a cytoplasmic increase in myosin-II activity that could sterically hinder chromosome capture. We describe here an original mode of generation of aneuploidies that could be very common in oocytes and could contribute to the high aneuploidy rate observed during female meiosis, a leading cause of infertility and congenital disorders.

The developmental potential of human and murine oocytes is predicted by their mechanical properties. Here the authors show that artificial reduction of cortex tension produces aneuploid mouse oocytes and speculate that this may contribute to the high aneuploidy rate typical of female meiosis.