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Increased Birefringence in the Meiotic Spindle Provides a New Marker for the Onset of Activation in Living Oocytes¹

^a Department of Ob/Gyn, Women & Infants Hospital, Brown University, Providence, Rhode Island 02905 ^b Laboratory for Reproductive Medicine, Marine Biological Laboratory, Woods Hole, Massachusetts 02543

ABSTRACT

The newly developed Pol-Scope allows imaging of spindle retardance, which is an optical property of organized macromolecular structures that can be observed in living cells without fixation or staining. Experiments were undertaken to examine changes in meiotic spindles during the initial stages of activation of living mouse oocytes using the Pol-Scope. Parthenogenetic activation of oocytes treated with calcium ionophore evoked a dynamic increase in meiotic spindle retardance, particularly of the midregion, before spindle rotation and second polar body extrusion. The pronounced increase in spindle retardance, which could, for the first time to our knowledge, be quantified in living oocytes, was maintained during polar body extrusion. Spindle retardance of newly in vivo fertilized oocytes was significantly higher than 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 indicator of oocyte activation. Thus, noninvasive, quantitative imaging of the onset of activation in living oocytes might improve the efficiency of assisted fertilization and other embryo technologies.

FOOTNOTES

First decision: 21 December 1999.

¹ Supported in part by the National Institute of Health (NIH K081099) and Women & Infants Hospital/Brown Faculty Research Fund.

² Correspondence: David Keefe, Department of Ob/Gyn, Women & Infants Hospital, 101 Dudley St., Providence, RI 02905. FAX: 401 453 7500; dkeefe{at}smtp.wihri.org

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