L. Liu et al., Increased birefringence in the meiotic spindle provides a new marker for the onset of activation in living oocytes, BIOL REPROD, 63(1), 2000, pp. 251-258
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.