CAPACITY OF MOUSE OOCYTES TO BECOME ACTIVATED DEPENDS ON COMPLETION OF CYTOPLASMIC BUT NOT NUCLEAR MEIOTIC MATURATION

The ability of mouse oocytes to become activated after exposure to the calcium ionophore A23187 has been investigated at different stages of meiotic maturation. The potential to respond to ionophore has been st udied in relation to the time since resumption of meiotic maturation, the chromosomal conformation of the DNA within each cell and the prote in synthetic profile of the maturing oocyte. Our studies demonstrate t hat when maturing oocytes from an MF1 strain of mice were treated with A23187 activation occurred only in oocytes which had reached second m eiotic metaphase (MII). However, development of the ability to respond to ionophore was not dependent on an orderly progression through norm al chromosomal rearrangements such as separation at metaphase I (MI) a nd subsequent polar body extrusion, since these processes could be pre vented and the capacity to be activated became apparent in such oocyte s at a time when control cells had reached MII. These data suggest tha t the ability to respond to ionophore depends on the development of a cytoplasmic component or complex capable of monitoring the time since initiation of germinal vesicle breakdown. Metabolic radiolabelling of oocytes which were able to respond to calcium ionophore, even though t hey had been prevented from undergoing normal chromosomal rearrangemen ts, showed them to be synthesising a group of proteins known as the 35 kDa complex.