<i>In vivo</i> volumetric visualization and quantification of cumulus expansion in mice with intravital optical coherence tomography

Ovulation is preceded by a critical physiological process known as cumulus expansion, during which the cumulus cell layer surrounding the oocyte undergoes structural remodeling. Despite the recognized importance of this process for reproductive success, live quantitative imaging of cumulus expansion has not been previously achieved due to limitations of current imaging technologies for deeply located ovaries. In this study, we employed intravital optical coherence tomography for three-dimensional visualization of mouse follicles containing cumulus-oocyte complexes (COC) within the physiological context of the ovary, both <i>ex vivo</i> and <i>in vivo</i>. This method enabled time-lapse measurement of cumulus layer thickness and COC volume. Longitudinal imaging in live mice revealed the physiological spatiotemporal dynamics of cumulus matrix expansion preceding ovulation. These findings establish a novel in vivo platform for dynamic investigation of previously inaccessible preovulatory processes within a physiological context.