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.

Off-center spindle positioning in mammalian oocytes enables asymmetric divisions in size, important for subsequent embryogenesis. The migration of the meiosis I spindle from the oocyte center to its cortex is mediated by F-actin. Specifically, an F-actin-cage surrounds the microtubule spindle and applies forces to it. To better understand how F-actin transmits forces to the spindle, we studied a potential direct link between F-actin and microtubules. For this, we tested the implication of Myosin-X, a known F-actin and microtubule binder involved in spindle morphogenesis and/or positioning in somatic cells, amphibian oocytes and embryos. Using a mouse strain conditionally invalidated for Myosin-X in oocytes and by live cell imaging, we show that Myosin-X is not localized on the spindle and dispensable for spindle and F-actin assembly. It is not required for force transmission since spindle migration and chromosome alignment occur normally. More broadly, Myosin-X is dispensable for oocyte developmental potential and female fertility. We therefore exclude a role for Myosin-X in transmitting F-actin-mediated forces to the spindle, opening new perspectives regarding this mechanism in mouse oocytes, which differs from most mitotic cells.

Meiotic maturation is a crucial step of oocyte formation allowing its potential fertilization and embryo development. Elucidating this process is important both for fundamental research and assisted reproductive technology. Few computational tools based on non-invasive measurements are however available to characterize oocyte meiotic maturation. Here, we develop a computational framework to phenotype oocytes based on images acquired in transmitted light. We trained neural networks to segment the contour of oocytes and their zona pellucida using oocytes from diverse species. We defined a comprehensive set of morphological features to describe an oocyte. These steps are implemented in an open-source Fiji plugin. We present a feature based machine learning pipeline to recognize oocyte populations and determine their morphological differences. We first demonstrate its potential to screen oocyte from different strains and automatically identify their morphological characteristics. Its second application is to predict and characterize the maturation potential of oocytes. We identify the texture of the zona pellucida and the cytoplasmic particles size as features to assess mouse oocyte maturation potential and tested whether these features were applicable to human oocyte's developmental potential.