Confocal fluorescence imaging of QD-BRET (QD)-labeled cumulus-oocyte complexes (COCs). COCs were labeled with 0 nM (QD0) or with 1 nM plasminogen-conjugated (QD1+) or non-conjugated (QD1−) QD-BRET nanoparticles. Non-labeled mature or immature COCs were used to set up the imaging conditions (a, b). Micrographs in the upper panel (Blue frame) show QD-BRET fluorescence signals detected in COCs labeled before (Immature; c, d) and after (Mature; e, f, g, h) in vitro maturation. The lower panel (Black frame) shows corresponding overlaid visible and fluorescence light images. Nuclei are counterstained in blue with DAPI. The white and red arrows indicate the cumulus cells and oocytes, respectively.