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The early steps that lead to the rise in calcium and egg activation at fertilization are unknown but of great interest--particularly with the advent of in vitro fertilization techniques for treating male infertility and whole-animal cloning by nuclear transfer. This calcium rise is required for egg activation and the subsequent events of development in eggs of all species. Injection of intact sperm or sperm extracts can activate eggs, suggesting that sperm-derived factors may be involved. Here we show that nitric oxide synthase is present at high concentration and active in sperm after activation by the acrosome reaction. An increase in nitrosation within eggs is evident seconds after insemination and precedes the calcium pulse of fertilization. Microinjection of nitric oxide donors or recombinant nitric oxide synthase recapitulates events of egg activation, whereas prior injection of oxyhaemoglobin, a physiological nitric oxide scavenger, prevents egg activation after fertilization. We conclude that nitric oxide synthase and nitric-oxide-related bioactivity satisfy the primary criteria of an egg activator: they are present in an appropriate place, active at an appropriate time, and are necessary and sufficient for successful fertilization.

During animal fertilizations, each oocyte or egg must produce a proper intracellular calcium signal for development to proceed normally. As a supplement to recent synopses of fertilization-induced calcium responses in mammals, this paper reviews the spatiotemporal properties of calcium signaling during fertilization and egg activation in marine invertebrates and compares these patterns with what has been reported for other animals. Based on the current database, fertilization causes most oocytes or eggs to generate multiple wavelike calcium oscillations that arise at least in part from the release of internal calcium stores sensitive to inositol 1,4,5-trisphosphate (IP3). Such calcium waves are modulated by upstream pathways involving oolemmal receptors and/or soluble sperm factors and in turn regulate calcium-sensitive targets required for subsequent development. Both "protostome" animals (e.g., mollusks, annelids, and arthropods) and "deuterostomes" (e.g., echinoderms and chordates) display fertilization-induced calcium waves, IP3-mediated calcium signaling, and the ability to use a combination of external calcium influx and internal calcium release. Such findings fail to support the dichotomy in calcium signaling modes that had previously been proposed for protostomes vs deuterostomes and instead suggest that various features of fertilization-induced calcium signals are widely shared throughout the animal kingdom., (Copyright 1999 Academic Press.)

To monitor calcium and endoplasmic reticulum (ER) dynamics during oocyte maturation and fertilization, oocytes of the marine worm Cerebratulus lacteus were injected with the calcium-sensitive indicator calcium green dextran and/or the ER-specific probe "DiI." Based on time-lapse confocal imaging of such specimens, prophase-arrested immature oocytes failed to develop normally after insemination and typically produced non-wave-like calcium transients that were lower in amplitude and less persistent than the wave-like oscillations observed during fertilizations of mature oocytes. Accordingly, the ER of DiI-loaded immature oocytes lacked an obvious substructure, whereas ER clusters, or "microdomains," began to form in maturing specimens at about the time that these oocytes became competent to undergo normal fertilization-induced calcium dynamics and cleavage. The ER microdomains of mature oocytes typically reached widths of 1-8 micrometer and disappeared approximately 1 h after fertilization, which in turn coincided with the termination of the calcium oscillations. Collectively, these findings indicate: (i) changes in ER structure are temporally correlated with the onset and cessation of the calcium oscillations required for subsequent cleavage, and (ii) such ER reorganizations may play an important role in early development by enabling mature oocytes to generate a normal calcium response., (Copyright 1998 Academic Press.)

To analyze fertilization-induced calcium dynamics in a protostome worm, unfertilized oocytes of the nemertean Cerebratulus lacteus were co-injected with calcium green (CG) and rhodamine (Rh) dextrans for dual-channel confocal imaging of early development. Based on CG/Rh ratioed images collected every 800 msec, fertilization elicits a "cortical flash" of elevated free calcium that spreads rapidly around the oocyte without propagating as a point-source wave. A similar calcium transient occurs in unfertilized oocytes treated with KCl to depolarize the oolemma, and the fertilization-induced cortical flash is eliminated if cobalt is used to block calcium channels, collectively indicating that fertilization initially triggers an influx of calcium ions through voltage-gated calcium channels in the oolemma. However, within minutes after producing a cortical flash, C. lacteus oocytes begin to display a series of point-source, oscillating waves of elevated free calcium that are propagated at about 15 micron/sec. The first two calcium waves arise at the site of sperm fusion and typically fail to reach the antipode, but after sperm incorporation, the waves spread globally throughout the ooplasm and typically shift their origin to a pacemaker region in the vegetal cortex. About 10 oscillations with an average duration of 3.3 +/- 1.2 min are generated for approximately 60-100 min postfertilization as meiotic maturation is completed, and such waves continue to occur in cobalt-containing seawater or calcium-free seawater. Thus, wavelike calcium oscillations induced by fertilization are apparently dependent upon internal calcium stores, which in turn may contain IP3-insensitive and/or IP3-sensitive receptors based on experiments using ryanodine, caged IP3, and heparin. Unfertilized oocytes also display repetitive calcium waves following intracytoplasmic injections of whole sperm, and such oscillations are eliminated if the sperm suspensions are boiled prior to injection, suggesting the possible presence of a heat-labile sperm component that can elicit wavelike oscillations during fertilization.

Intracellular free calcium levels in starfish oocytes have been monitored during meiotic maturation and fertilization using calcium-sensitive fluorescent dyes combined with confocal laser scanning microscopy or fura ratioing techniques. In time-lapse analyses of prophase-arrested and maturing oocytes, calcium transients were elicited by inositol 1,4,5-trisphosphate (IP3), ryanodine, or caffeine, indicating that both the IP3-sensitive and IP3-insensitive receptors of the oocyte's calcium release channels could be stimulated to mobilize calcium ions. Fertilization also triggered a global calcium wave that appeared to travel faster around the cortex than through the center of the oocyte, and maturing oocytes developed normally after their fertilization-induced calcium waves had been imaged. Prophase-arrested specimens, on the other hand, did not undergo germinal vesicle breakdown or cleavage after displaying a fertilization-induced calcium transient throughout their cytoplasm and nucleus, confirming previous observations that calcium spikes are not sufficient to induce development in immature oocytes. In addition, although the calcium spikes triggered by sperm or caffeine reached similar normalized peak heights, fertilization-induced calcium waves in maturing oocytes tended to be more prolonged than the fertilization waves observed in prophase-arrested oocytes or the caffeine-triggered spikes elicited at any stage of maturation. Collectively, such findings suggest that the total amount of releasable calcium does not vary appreciably during maturation, but the patterns of the calcium transients can differ depending on the stage of maturation and/or the type of calcium-releasing agent. Possible artifacts affecting these findings are assessed, and the results are discussed relative to the functioning of calcium release pathways during starfish oocyte maturation and fertilization.

Although confocal microscopy has typically been utilized in studies of fixed specimens, its potential for exploring dynamic processes in living cells is rapidly being realized. In this report, confocal laser scanning microscopy is used to analyze the calcium wave that occurs following fertilization in living sea urchin eggs microinjected with the calcium-sensitive fluorescent probes fluo-3 or calcium green. Time-lapse recordings of optical sections depicting calcium dynamics within the eggs are also subjected to volumetric reconstructions. Such analyses indicate that (1) cytoplasmic free calcium levels become elevated throughout the fertilized egg, (2) fertilization also causes the egg nucleus to undergo a transient increase in free calcium, and (3) normal cleavage can be obtained following time-lapse imaging of the calcium waves.