SPATIOTEMPORAL DYNAMICS OF INTRACELLULAR [CA2+](I) OSCILLATIONS DURING THE GROWTH AND MEIOTIC MATURATION OF MOUSE OOCYTES

Calcium oscillations occur during meiotic maturation of mouse oocytes, They also trigger activation at fertilization, We have monitored [Ca2 +](i) in oocytes at different stages of growth and maturation to exami ne how the calcium release mechanisms alter during oogenesis, Spontane ous calcium oscillations occur every 2-3 minutes in the majority of fu lly grown (but immature) mouse oocytes released from antral follicles and resuming meiosis, The oscillations last for 2-4 hours after releas e from the follicle and take the form of global synchronous [Ca2+](i) increases throughout the cell, Rapid image acquisition or cooling the bath temperature from 28 degrees C to 16 degrees C did not reveal any wave-like spatial heterogeneity in the [Ca2+](i) signal. Calcium appea rs to reach highest levels in the germinal vesicle but this apparent d ifference of [Ca2+] in nucleus and cytoplasm is an artifact of dye loa ding, Smaller, growing immature oocytes are less competent: about 40% are able to resume meiosis and a similar proportion of these oocytes s how spontaneous calcium oscillations, [Ca2+](i) transients are not see n in oocytes that do not resume meiosis spontaneously in vitro, Noneth eless, these oocytes are capable of [Ca2+](i) oscillations since they show them in response to the addition of carbachol or thimerosal, To e xamine how the properties of calcium release change during meiotic mat uration, a calcium-releasing factor from sperm was microinjected into fully grown immature and mature oocytes, The sperm-factor-induced osci llations were about two-fold larger and longer in mature oocytes compa red to immature oocytes, Calcium waves travelling at 40-60 mu m/second were generated in mature oocytes, but not in immature oocytes, In som e mature oocytes, successive calcium waves had different sites of orig in, The modifications in the size and spatial organization of calcium transients during oocyte maturation may be a necessary prerequisite fo r normal fertilization.