Increased birefringence in the meiotic spindle provides a new marker for the onset of activation in living oocytes

The newly developed Pol-Scope allows imaging of spindle retardance, which i s an optical property of organized macromolecular structures that can be ob served in living cells without fixation or staining. Experiments were under taken to examine changes in meiotic spindles during the initial stages of a ctivation of living mouse oocytes using the Pol-Scope, Parthenogenetic acti vation of oocytes treated with calcium ionophore evoked a dynamic increase in meiotic spindle retardance, particularly of the midregion, before spindl e rotation and second polar body extrusion. The pronounced increase in spin dle retardance, which could, for the first time to our knowledge, be quanti fied in living oocytes, was maintained during polar body extrusion. Spindle retardance of newly in vivo fertilized oocytes was significantly higher th an that of ovulated, metaphase II oocytes, Pol-Scope imaging of fertilized oocytes did not affect subsequent development. These results establish that increased spindle retardance precedes polar body extrusion and pronuclear formation. The increased birefringence in the spindle provides an early ind icator of oocyte activation. Thus, noninvasive, quantitative imaging of the onset of activation in living oocytes might improve the efficiency of assi sted fertilization and other embryo technologies.