Published online by Cambridge University Press: 26 September 2008
The ability of mouse oocytes to become activated after exposure too the calcium ionophore A23187 has been investigated at different stages of meiotic maturation. The potential to respond to ionophore has been studied in relation to the time since resumption of meiotic maturation, the chromosomal conformation of the DNA within each cell and the protein synthetic profile of the maturing oocyte. Our studies demonstrate that when maturing oocytes from an MF1 strain of mice were treated with A23187 activation occured only in oocytes which had reached second meiotic metaphase (MII). However, development of the ability to respond to ionophore was not dependent on an orderly progression through normal chromosomal rearrangements such as separation at metaphase I (MI) and subsequent polar body extrusion, since there process could be prevented and the capacity to be activated became apparent in such oocytes at a time when control cells had reached MII. These data suggest that the ability to respond to ionophore depends on the development of a cytoplasmic 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 they had been prevented from undergoing normal chromosomal rearrangements, showed them to be synthesising a group of proteins known as the 35 kDa complex.