Ca2+ dynamics in oocytes from naturally-aged mice

The ability of human metaphase-II arrested eggs to activate following fertilisation declines with advancing maternal age. Egg activation is triggered by repetitive increases in intracellular Ca(2+) concentration ([Ca(2+)]i) in the ooplasm as a result of sperm-egg fusion. We therefore hypothesised that eggs from older females feature a reduced ability to mount appropriate Ca(2+) responses at fertilisation. To test this hypothesis we performed the first examination of Ca(2+) dynamics in eggs from young and naturally-aged mice. Strikingly, we find that Ca(2+) stores and resting [Ca(2+)]i are unchanged with age. Although eggs from aged mice feature a reduced ability to replenish intracellular Ca(2+) stores following depletion, this difference had no effect on the duration, number, or amplitude of Ca(2+) oscillations following intracytoplasmic sperm injection or expression of phospholipase C zeta. In contrast, we describe a substantial reduction in the frequency and duration of oscillations in aged eggs upon parthenogenetic activation with SrCl2. We conclude that the ability to mount and respond to an appropriate Ca(2+) signal at fertilisation is largely unchanged by advancing maternal age, but subtle changes in Ca(2+) handling occur that may have more substantial impacts upon commonly used means of parthenogenetic activation.