The experiments compare intracellular changes in porcine eggs induced by electrical activation with those induced by sperm penetration. Adequate electrostimulation induces changes in both cortical granule exocytosis and protein synthesis similar to those induced by sperm during fertilization. However, ionic changes induced by electrostimulation differ markedly from those initiated at fertilization. Thus, dynamic video imaging using Fura-2 as a Ca2+ probe provides evidence that parthenogenetic activation induced by electrostimulation is initiated by a single sharp rise in the concentration of intracellular free calcium ([Ca2+]i) in the egg. The intracellular Ca2+ transient increase is triggered by an influx of extracellular Ca2+ immediately after elec-trostimulation. The amplitude of the intracellular Ca2+ transient increase is a function both of the extracellular Ca2+ concentration and of electric field parameters (field strength and pulse duration). Imaging demonstrates further that a single electrical pulse can only induce a single Ca2+ transient which usually lasts three to five minutes; no further Ca2+ transients are observed unless additional electrical stimuli are applied. By contrast, sperm-induced activation is characterised by a series of Ca2+ spikes which continue for at least 3 hours after sperm-egg fusion. The pattern of Ca2+ spiking after fertilization is not consistent during this period but changes both in frequency and amplitude.

Overall, the results demonstrate that, although electrostimulation induces both cortical granule exocytosis and protein reprogramming in porcine eggs, it does not reproduce the pattern of [Ca2+]i changes induced by sperm entry at fertilization.

At fertilization the sperm not only delivers its DNA to the egg to restore diploidy, but also activates the egg and launches it on a path leading to DNA synthesis and cleavage (Whitaker and Patel, 1990; Miyazaki, 1990). In mammalian species, sperm activate eggs by causing a series of repetitive and transient rises in Ca2+ that persist for more than an hour after sperm penetration (Cuthbertson and Cob- bold, 1985; Miyazaki, 1990; Kline and Kline, 1992). The means by which sperm generate Ca2+ transients in eggs remains unknown. Nevertheless, there is substantial evidence to implicate inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) as the second messenger for stimulating the increase in [Ca2+]i at fertilization (see review articles by Swann and Whitaker, 1990; Miyazaki, 1990).

Calcium is recognized as the primary intracellular signal responsible for initiating the activation process in mammalian eggs. This concept is supported by two sets of experiments. First, an increase in [Ca2+]i occurs in eggs during fertilization; suppressing the natural rise in [Ca2+]i prevents the initiation of egg activation events (Kline and Kline, 1992). Second, artifically raising intracellular Ca2+ by a variety of physical and chemical approaches usually initiates egg activation (Whittingham, 1980; Kaufman, 1983; Ducibella et al., 1988; Colonna et al., 1989; Marcus, 1990). As to the biological role of Ca2+ transient rises on mammalian embryo development, Ozil (1990) has demonstrated for the first time that some developmental events such as compaction and blastocyst formation can be determined by the process of egg activation two or four days earlier.

In recent years, electrofusion has become the most widely used method of transplanting nuclei from embryonic cells into enucleated eggs. Since the electrical parameters required for cell fusion are also those able to induce parthenogenetic development, it has become standard practice to activate the cytoplast and transplant the nucleus using the same electrostimulus (Willadsen, 1986; Prather et al., 1987; Sun, 1989; Collas and Robl, 1991). However, it is unclear whether electrical activation mimics in all respects the range of intracellular events induced by sperm penetration. It is apparent that if electrostimulation can induce only partial but not complete egg activation, the true developmental capacity of the nuclear-transplanted eggs obtained by electrofusion may have not been appropriately tested, because the eggs may in the first instance have not been activated normally.

This paper has as its purpose the comparison of the two systems of activation as a first step in determining the true developmental capacity of embryonic and somatic nuclei by nuclear transplantation. More specifically, experiments have been carried out to define the changes that occur in ultrastructure, protein synthesis and ionic balances in porcine eggs following fertilization and electroactivation.

Preparation of porcine oocytes

Oocyte collection

Pig ovaries were obtained from a local abattoir. Intact, non-atretic follicles (>4 mm in diameter) were dissected from the ovaries and then opened to remove the entire cumulus-oocyte complex. All these procedures were performed in PBS at room temperature.

Oocyte maturation in vitro

Maturation of groups of 20-30 cumulus-enclosed oocytes were carried out in 35 mm plastic culture dishes (Sterilin, UK) containing 2 ml of culture medium (TCM 199) supplemented with 10% fetal calf serum (FCS: Sera-Lab Ltd), and gonadotrophins (NIH-LH-022, 2.5 μg/ml; NIH-FSH-P2, 2.5 μg/ml). In all instances, two freshly prepared follicle shells were added to the medium in which the oocytes were cultured. The maturation was performed at 38.5 ° C in 5% CO2 in air.

Egg selection

After 45 hours of maturation culture, the oocytes were denuded of associated somatic cells. Those oocytes that had extruded their first polar body were identified, isolated and maintained at 38.5 ° C in TCM 199 + 0.25 mM Hepes supplemented with 10% FCS pending further treatment.

Electrical stimulation and egg culture in vitro

The electrofusion system used in this investigation was the same as that described by Sun and Moor (1989, 1991). The basic pulsing medium was 0.3 M mannitol (Sigma) supplemented with 100 μM Ca2+, 100 μM Mg2+ and 0.01% polyvinyl alcohol (PVA). The eggs, after three washes in a pulsing medium, were transferred to a fusion chamber (Sun and Moor, 1989) containing the same pulsing medium and stimulated thereafter by a DC pulse treatment using different fields and a variety of pulse durations. After pulse treatment, eggs were washed in TCM 199 supplemented with 0.25 mM Hepes and 10% FCS and cultured in TCM 199 + 10% FCS at 38.5 oC in 5% CO2 in air.

Cytological assessment

After 15 hours of culture, one group of electrically stimulated eggs were fixed for 48 hours in ethanol/acetic acid (3:1), stained with lacmoid and assessed for egg activation using phase contrast microscopy.

To assess cortical granule exocytosis following stimulation, a second group of treated eggs were fixed and prepared as described by Cran and Cheng (1986) and then examined by transmission electron microscopy.

In vitro fertilization procedures

Fertilization of porcine eggs in vitro was performed following the procedures of Cheng (1985). Briefly, sperm-rich fractions (20 ml) were collected from mature boars, and after filtration the sperm were held at 20°C for 16 hours. Thereafter, 1 ml of semen was washed three times by centrifugation (500 g) in PBS supplemented with BSA (3 mg/ml) to remove seminal plasma. After the final wash, the sperm were resuspended at 4 × 108 cells/ml in a prein- cubation medium consisting of TCM199 + 0.25 mM Hepes sodium pyruvate, 3.05 mM D-glucose and antibiotics. The sperm suspension was preincubated for 90 minutes at 38.5 ° C under 5% CO2 in air. After that, aliquots of the preincubated sperm were added to the fertilization medium consisting of TCM199 supplemented with the same additives as the preincubation medium. The final sperm concentration in the medium was 1 ×105 cells/ml.

Radiolabelling and electrophoretic analysis of labelled proteins in eggs and zygotes

Comparisons were made of the pattern of proteins synthesised by control eggs and eggs after activation by sperm penetration or electrostimulation. Fertilised eggs at 8 hours postinsemination were transferred to BOCM-2 medium (Cheng, 1985) and cultured for a further 12 hours before radiolabelling. Groups of 10 to 15 denuded eggs or zygotes were labelled at 38.5 ° C for 3 hours in 50 μl of labelling medium (Moor et al., 1981) containing [35S]methionine (1000 Ci/mmol, Amersham) at a radioactive concentration of 500 μCi/ml. After labelling, the eggs and zygotes were washed in 10 mM Tris-HCl, pH 7.4, and transferred to plastic tubes in a minimal volume of the Tris-buffer (<2 μl), and then lyophilized and frozen at –70 ° C until required for electrophoresis.

Radiolabelled proteins in the eggs and zygotes were analysed on one-dimensional gels as described by Sun and Moor (1991). The scanning of the gels and quantitative analysis of protein bands were carried out by using a Chromoscan 3 densitometer (Joyce-Loebl, England).

Measurement of intracellular calcium

Changes in [Ca2+]i were measured in (1) control eggs, (2) electrically activated eggs and (3) eggs activated by sperm penetration. In all groups, newly matured eggs were incubated at 38.5 ° C with the calcium-sensitive fluorescent dye, fura-2 acetoxymethyl ester (fura-2/AM; 4 μM) for 45 minutes in TCM199 supplemented with 0.25 mM Hepes and 10% FCS. After that, they were washed five times in culture medium and then maintained in culture at 38.5 ° C pending further treatments. For measuring [Ca2]i change after electroactivation, fura-2/AM loading eggs were extensively washed in a pulsing medium before being placed in a modified Krüss stimulation chamber (Krüss, Hamburg, Germany). The modifications involved replacing the supporting glass in the chamber with a thin glass coverslip (22 mm diameter, ARH, England) and introducing a culture well (Sun and Moor, 1989) into the chamber. This modified chamber enabled eggs to be electroacti-vated and their [Ca2+]i changes monitored under the same set of conditions. For the analysis of [Ca2+]i change after sperm penetration, the fura-2/AM prelabelled eggs were briefly exposed to prewarmed acid tyrode solution for zona removal (Nicholson et al., 1975) and then extensively washed before transfer either to normal fertilisation medium or to the fertilisation chamber, which consisted of a thin coverslip (22 mm diameter, ARH) fitted into a stainless steel well that was maintained at 38.5 ° C by a thermostatically controlled heating block. The volume of fertilisation medium was typically 300 μl covered by light liquid paraffin (BDH, UK).

Dynamic video imaging was performed using the Magiscan hardware and TARDIS software supplied by Joyce Loebl (Dukesway, Team Valley, Gateshead, UK) following the procedures as described by Neylon et al. (1990). All the fluorescent measurements were performed on an inverted Nikon Diaphot epifluorescence microscope. Ratios (340 nm/380 nm) of 0.30, 1.82 and 2.25 were estimated to correspond to [Ca2+]i of 0 nM, 1000 nM and 2000 nM respectively.

(1) Electrical field parameters and external Ca2+ control the rate of egg activation induced by electrostimulation

As a prelude to investigations on the intracellular changes induced by electroactivation, optimal electrical field parameters and pulsing media were determined. Successful egg activation was measured by the formation of morphologically normal pronuclei by 15 hours poststimulation; eggs with MII chromosomes at this stage were classified as non- activated.

Fig. 1A shows both the relationship between DC field strength and activation rates and the importance of the presence of extracellular Ca2+ in the pulsing medium on egg activation. It is apparent that for a given pulse duration (60 μseconds) and in the presence of extracellular Ca2+ in the pulsing medium activation rate increases with the elevation of the field strength (in the range of 0.3 kV/cm to 1.0 kV/cm). It is further clear that under our experimental conditions the optimal DC field strength for high activation rates coupled with a low incidence of egg lysis is between 1.0 kV/cm and 1.5 kV/cm. The results in Fig. 1A also show that some eggs exposed to a field strength below 0.5 kV/cm are activated while the others remain at MII, illustrating that individual eggs require different threshold field strength to induce activation. Moreover, the results in Fig. 1A demonstrate that despite adequate electrostimulation, treated eggs will fail to undergo activation in the absence of Ca 2+ in the pulsing medium.

Fig. 1.

(A) Effect of DC field strength and extracellular Ca2+ on egg activation rates. The eggs were stimulated at different DC field strength with two pulses of 60 μseconds duration separated by 0.2 second pulse interval either in the basic pulsing medium consisting of 0.3 M mannitol, 100 μM Ca2+, 100 μM Mg2+ and 0.1% of PVA (●) or in Ca2+-free pulsing medium (◼). Each point represents 20 to 30 eggs. (B) The relationship between DC pulse duration and field strength on the rate of activation. Eggs were stimulated with two pulses of varying pulse duration using weak (◼, 0.7 kV/cm), medium (0, 1.0 kV/cm) and high (●, 2.5 kV/cm) external DC field strength separated by 0.2 second pulse interval in the basic pulsing medium.

Fig. 1.

(A) Effect of DC field strength and extracellular Ca2+ on egg activation rates. The eggs were stimulated at different DC field strength with two pulses of 60 μseconds duration separated by 0.2 second pulse interval either in the basic pulsing medium consisting of 0.3 M mannitol, 100 μM Ca2+, 100 μM Mg2+ and 0.1% of PVA (●) or in Ca2+-free pulsing medium (◼). Each point represents 20 to 30 eggs. (B) The relationship between DC pulse duration and field strength on the rate of activation. Eggs were stimulated with two pulses of varying pulse duration using weak (◼, 0.7 kV/cm), medium (0, 1.0 kV/cm) and high (●, 2.5 kV/cm) external DC field strength separated by 0.2 second pulse interval in the basic pulsing medium.

The effect of pulse duration on egg activation rate is summarized in Fig. 1B. Firstly, these results show that, using a single field strength (e.g 1.0 kV/cm) and a standard pulsing medium, the number of eggs that undergo activation increases with the increase of pulse duration (in the range of 10 μseconds to 80 μseconds). Secondly, the results demonstrate that high rates of activation are the product of an inverse relationship between DC field strength and pulse duration. Therefore, when the field strength is low (0.7 kV/cm), a long pulse duration is required to induce activation, while at a high field strength (2.5 kV/cm) a short pulse duration is sufficient to induce a high rate of activation, indicating that a delicate balance must exist between the pulse duration and the field strength.

In summary, the combined results demonstrate that, under adequate experimental conditions, eggs can be efficiently activated by electrical stimuli and that the rate of activation is primarily determined both by the electrical field parameters and the presence of extracellular Ca2+ in the pulsing medium.

(2) Electrostimulation induces both cortical granule exocytosis and the reprogramming of protein synthesis in a manner analogous to that at fertilization

Cortical granule exocytosis

Cortical granules are membrane-bound secretory granules located immediately beneath the cell membrane in the mammalian eggs. Typically, sperm penetration triggers the fusion of the cortical granule membrane with the resultant release of the granule’s content. An ultrastructural analysis of the efficiency of electrostimulation for the induction of cortical granule exocytosis was undertaken by fixing eggs at various times poststimulation.

Fig. 2 shows the ultrastructural changes that occur in eggs following stimulation using conditions that induce 100% egg activation. Fig. 2A shows the distribution, before stimulation, of numerous cortical granules which appear as membrane-bound vesicles containing electron-dense material located immediately beneath the plasma membrane. A sharp reduction in the number of cortical granules is apparent in groups of comparably treated eggs by 1 minute poststimulation (Fig. 2B) and by 10 minutes poststimulation almost all the cortical granules are absent from just beneath the egg membranes (Fig. 2C). The ultrastructural changes observed after electrostimulation both to the cortical granules and plasma membrane appear similar to those observed in porcine eggs after fertilization (see Cran and Cheng, 1986 for details).

Fig. 2.

Electron micrographs of porcine eggs showing (A) the presence of cortical granules (CG) in control eggs, (B) CG exocytosis at 1 minute postelectrostimulation, and (C) complete release of CGs at 10 minutes poststimulation. The eggs in B and C were stimulated at 1.5 kV/cm, two pulses of 60 μseconds duration in the basic pulsing medium.

Fig. 2.

Electron micrographs of porcine eggs showing (A) the presence of cortical granules (CG) in control eggs, (B) CG exocytosis at 1 minute postelectrostimulation, and (C) complete release of CGs at 10 minutes poststimulation. The eggs in B and C were stimulated at 1.5 kV/cm, two pulses of 60 μseconds duration in the basic pulsing medium.

Reprogramming of protein synthesis in activated eggs

To determine whether electrostimulation induces the same programme of protein reprogramming as that elicited by sperm penetration, comparisons were made between polypeptide profiles from unfertilized eggs, zygotes and parthenogenetically activated eggs. The results in Fig. 3 show that sperm penetration induces changes in the polypeptide profile which differs sharply from those of unfertilized eggs. There is, however, a close similarity confirmed by densitometry, between polypeptide profiles from zygotes and parthenogenetic eggs activated by electrostimulation.

Fig. 3.

Fluorgraph of SDS-PAGE gel showing polypeptides synthesised by porcine MII control eggs (1-2), parthenogenetically activated eggs (3-4) and normally fertilised zygotes (5-6). At 45 hours of maturation, the MII eggs were either treated by electrostimulation or inseminated by sperm, the control eggs were cultured without any further treatment. After stimulation for 15 hours or inseminated for 21 hours, both the eggs and zygotes were labelled with [35S]methionine for 3 hours, and then run on 8-15% SDS gradient gels. The detectable differences among them in polypeptide compositions are indicated by arrows. Each track in these groups represented 5 eggs or zygotes.

Fig. 3.

Fluorgraph of SDS-PAGE gel showing polypeptides synthesised by porcine MII control eggs (1-2), parthenogenetically activated eggs (3-4) and normally fertilised zygotes (5-6). At 45 hours of maturation, the MII eggs were either treated by electrostimulation or inseminated by sperm, the control eggs were cultured without any further treatment. After stimulation for 15 hours or inseminated for 21 hours, both the eggs and zygotes were labelled with [35S]methionine for 3 hours, and then run on 8-15% SDS gradient gels. The detectable differences among them in polypeptide compositions are indicated by arrows. Each track in these groups represented 5 eggs or zygotes.

In summary, the combined results of this series of experiments reveal that adequate electrostimulation causes porcine eggs to undergo both cortical granule exocytosis and protein reprogramming in a manner that mimics that induced by sperm during fertilization.

(3) Egg activation induced by electrostimulation is attributed to a transient increase in [Ca2+]i immediately following stimulation

The purpose of this series of experiments was to determine some of the extracellular factors involved in regulating [Ca2+]i changes and activation of eggs following electrostimulation.

Effect of extracellular Ca2+ on [Ca2+]i levels following stimulation

Analysis of 80 eggs showed that before electrostimulation the basal [Ca2+]i level is low (20-60 nM) and that the sharp rise after stimulation depends both on the presence of extracellular Ca2+ and on a Ca2+ influx. Thus, under the same electrical field conditions the [Ca2+]i rise after stimulation increases in parallel with increases in the concentration of extracellular Ca2+ (Fig. 4A). Secondly, despite adequate electrostimulus, eggs show no elevation of [Ca2+]i in a pulsing medium devoid of Ca2+. Moreover, using standardised electrostimulation procedures all eggs stimulated in pulsing medium containing 100 μM Ca 2+ were activated as judged by pronuclear formation whilst none of those stimulated in the absence of extracellular Ca2+ formed pronuclei. We suggest that the results of these different experiments indicate (i) that the intracellular free Ca2+ rise after stimulation directly triggers the activation programme and (ii) that a Ca2+ influx immediately following stimulation plays an essential role in raising the [Ca2+]i levels.

Fig. 4.

(A) The effect of extracellular Ca2+ concentrations on [Ca2+]i changes in electrosimulated eggs. Following intensive washing in a pulsing medium, Fura-2 labelled eggs were transferred to the stimulation chamber and their basal [Ca2+]i levels were recorded. Thereafter, the eggs were electrically activated and the [Ca2+]i changes were measured. All the eggs in this study were treated under the same electrical field conditions (1.0 kV/cm, two pulses of 60 μseconds duration separated by 0.2 second interval). The pulsing media had common basic components (0.3 M mannitol, 100 μM Mg2+, and 0.1% PVA), but differed in their Ca2+ concentrations. Each bar represents the mean ± s.e.m. of the maximum [Ca2+]i levels detected in three groups of eggs (n=15-20). (B) Effect of DC field strength on [Ca2+]i in porcine eggs. Each group of eggs was stimulated at a defined DC field strength with two pulses of 60 μseconds duration in the basic pulsing medium containing 100 μM Ca2+. Each bar represents the mean ± s.e.m. of the maximum [Ca2+]i levels of the treated eggs (n=12-18). (C) Effect of DC pulse duration on [Ca2+]i changes in eggs following stimulation in a basic pulsing medium containing 100 μM Ca2+. The eggs were stimulated with two pulses of varying pulse duration using a field strength of 1.0 kV/cm. Each bar represents the mean ± s.e.m. of the maximum [Ca2+]i levels in three groups of eggs (n=12-18).

Fig. 4.

(A) The effect of extracellular Ca2+ concentrations on [Ca2+]i changes in electrosimulated eggs. Following intensive washing in a pulsing medium, Fura-2 labelled eggs were transferred to the stimulation chamber and their basal [Ca2+]i levels were recorded. Thereafter, the eggs were electrically activated and the [Ca2+]i changes were measured. All the eggs in this study were treated under the same electrical field conditions (1.0 kV/cm, two pulses of 60 μseconds duration separated by 0.2 second interval). The pulsing media had common basic components (0.3 M mannitol, 100 μM Mg2+, and 0.1% PVA), but differed in their Ca2+ concentrations. Each bar represents the mean ± s.e.m. of the maximum [Ca2+]i levels detected in three groups of eggs (n=15-20). (B) Effect of DC field strength on [Ca2+]i in porcine eggs. Each group of eggs was stimulated at a defined DC field strength with two pulses of 60 μseconds duration in the basic pulsing medium containing 100 μM Ca2+. Each bar represents the mean ± s.e.m. of the maximum [Ca2+]i levels of the treated eggs (n=12-18). (C) Effect of DC pulse duration on [Ca2+]i changes in eggs following stimulation in a basic pulsing medium containing 100 μM Ca2+. The eggs were stimulated with two pulses of varying pulse duration using a field strength of 1.0 kV/cm. Each bar represents the mean ± s.e.m. of the maximum [Ca2+]i levels in three groups of eggs (n=12-18).

Effect of electrical field strength and pulse duration on [Ca2+]i changes in eggs

The results presented in Fig. 4B and 4C extend those presented earlier (Fig. 1) by showing that field strength and pulse duration strongly affect not only activation rate but also [Ca2+]i in porcine eggs. Thus, in a standard pulsing medium, the greater the field strength or the longer the pulse duration the higher the [Ca2+]i in treated eggs. Interestingly, all eggs showed similar [Ca2+]i changes under optimal stimulation conditions (100% activation). By contrast, marked individual variations in [Ca2+]i were observed in eggs subjected to sub-optimal electrostimulation conditions (below 60% activation), thus, it was found that under the same stimulation condition some eggs have shown significant increase in [Ca2+]i while the others showed very little response.

In summary, the results in this part of the investigation illustrate that [Ca2+]i increase following electrostimulation is the direct trigger that initiates the activation programme and that the [Ca2+]i changes are determined by both an influx of Ca2+ and the field parameters.

(4) Electrostimulation is not able to reproduce the pattern of [Ca2+]i changes initiated by sperm penetration

An important physiological characteristic of the way in which sperm activate eggs during hamster fertilisation is the induction of a series of repetitive transient rises in [Ca2+]i that persist for more than an hour after penetration (Miyazaki, 1990). Our present results extend these observations both by studying calcium transients in the eggs of another species (pig) and by comparing the pattern and characteristics of the [Ca2+]i changes in eggs after fertilization and after electroactivation.

To determine the [Ca2+]i changes initiated by sperm penetration, different groups of the fura-2 prelabelled zona-free eggs were examined by dynamic imaging immediately postinsemination. The results show that the basal [Ca2+]i level in these labelled eggs remained very low (at the range of 20 to 75 nM) during the first 4 hours postinsemination. Cytological assessment showed that no sperm had penetrated the eggs during this period of time. This observation is in agreement with the results of Cheng (1985) who reported that sperm require approximately 4 hours of preincubation before becoming capable of fertilising zona-free eggs. Indeed, the imaging studies showed that Ca2+ oscillations were initiated in some of the eggs from 4 hours postinsemination whilst others were delayed for a further 0.5-1 hour before responding. The results in Fig. 5 show that sperm penetration induces a series of Ca2+ spikes in matured pig eggs. The oscillations in these fertilising eggs will last for more than 3 hours. However, individual eggs showed differences in both the spiking frequency and amplitude. Characteristically, there was always a gradual increase in basal [Ca2+]i level before each Ca2+ spike. It appears that the spike was triggered only when the basal [Ca2+]i reached a threshold level. The analysis shows further that the Ca2+ rise reached a peak (579 ± 101 nM) in 9.7 ± 2.5 seconds, remained at this high level for 7.4 ± 4.1 seconds and then declined to the basal level in 35.2 ± 13.9 seconds (n=14 eggs). The spiking interval was 18.5 ± 5.8 minutes (n=14 eggs). Interestingly, it was found that if the basal [Ca2+]i level in an egg was high then the threshold [Ca2+]i required to initiate Ca2+ spiking was also high. Nevertheless, in the same egg during the first 2 hours of the oscillation both the threshold required to induce spiking and the amplitude of the spikes remained consistent. Moreover, our preliminary results show that even in the same eggs (n=8 eggs) the pattern of Ca2+ oscillations in the later stages of fertilisation (approximately 2.5 hours post the initiation of Ca2+ oscillation) differed from that of the early stage (Fig. 6). Miyazaki (1986) has reported that in hamster eggs a Ca2+ wave begins near the site of sperm attachment, and then spreads over the entire eggs within seconds.Our imaging analysis shows that the first Ca2+ rise observed in the fertilizing eggs was also initiated on one side and then spread quickly over the entire eggs.

Fig. 5.

Pattern of Ca2+ transient rises observed in porcine eggs activated by sperm penetration. A and B show two different fertilised eggs which were oscillating at different spiking frequencies and amplitudes.

Fig. 5.

Pattern of Ca2+ transient rises observed in porcine eggs activated by sperm penetration. A and B show two different fertilised eggs which were oscillating at different spiking frequencies and amplitudes.

Fig. 6.

An example of Ca2+ oscillation patterns of the same egg at different times after fertilization (A, within 1.5 hour of sperm penetration; B, 3.0-3.2 hours after sperm penetration).

Fig. 6.

An example of Ca2+ oscillation patterns of the same egg at different times after fertilization (A, within 1.5 hour of sperm penetration; B, 3.0-3.2 hours after sperm penetration).

In contradiction to the spiking pattern observed after sperm penetration, Fig. 7 shows that a single electrical stimulus is only capable of eliciting a single Ca2+ transient in pig eggs. Further analysis revealed that the stimulation caused the eggs to increase their [Ca2+]i levels from the resting level (in the range of 20-50 nM) to a peak of 1.0- 2.0 μM in 10-20 seconds which then declined slowly over several minutes (3-10 minutes); the precise decay time was dependent upon the DC field parameters and the extracellular Ca2+ concentration in the pulsing medium. Further Ca2+ transients do not occur in electrically activated eggs unless additional electrical pulses are administered. More over, the results showed that the [Ca2+]i peaks induced by the same stimulation declined following repeated treatments, although the pattern of the Ca2+ rise remained similar. Imaging further revealed that the Ca2+ transient induced by electrostimulation travels in the form of a wave (Fig. 8) which is always initiated in the region adjacent to the positive electrode and then speads over the entire eggs at a velocity of 5-10 μm/second. However, if more than two pulses are applied during a stimulation, a second Ca2+ wave is then also initiated adjacent to the negative electrode. The two opposing waves travel towards each other until they meet.

Fig. 7.

A typical example of the pattern of Ca2+ transient increase in porcine eggs induced by a single electrostimulation (A) and by multiple ones (B). The eggs were stimulated at 1.0 kV/cm with two pulses of 60 μseconds duration in the basic pulsing medium. The stimulations were given at the times indicated by the arrows. Detection of the [Ca2+]i changes following the stimulation(s) was also performed in the same pulsing medium at room temperature.

Fig. 7.

A typical example of the pattern of Ca2+ transient increase in porcine eggs induced by a single electrostimulation (A) and by multiple ones (B). The eggs were stimulated at 1.0 kV/cm with two pulses of 60 μseconds duration in the basic pulsing medium. The stimulations were given at the times indicated by the arrows. Detection of the [Ca2+]i changes following the stimulation(s) was also performed in the same pulsing medium at room temperature.

Fig. 8.

Propagation of a Ca2+ transient in a porcine egg following electrostimulation. The egg was stimulated by a single pulse of 60 μseconds duration at the field strength of 1.0 kV/cm in the basic pulsing medium. The stimulation was given at the second frame. The frame interval is 1.8 seconds.

Fig. 8.

Propagation of a Ca2+ transient in a porcine egg following electrostimulation. The egg was stimulated by a single pulse of 60 μseconds duration at the field strength of 1.0 kV/cm in the basic pulsing medium. The stimulation was given at the second frame. The frame interval is 1.8 seconds.

In summary, the results of these experiments demonstrate that the pattern and characteristics of [Ca2+]i transients induced by electrostimulation differs sharply from those observed in fertilising eggs. It is, therefore, concluded that the pattern of [Ca2+]i changes initiated by sperm penetration can not be reproduced by electrostimulation.

The results of our experiments show that pig eggs activated using optimal electrostimulation protocols not only complete meiosis but also undergo cortical granule exocytosis and protein reprogramming in a manner analogous to that observed after fertilization. Dynamic video imaging using Fura-2 as a Ca2+ probe has extended our results by showing that electrical stimulation activates porcine eggs by inducing a transient increase in intracellular free Ca2+. These findings are principally in agreement with those observed both in sea urchin eggs stimulated by electrical shock (Baker et al., 1980; Rossignol et al., 1983) and in mouse eggs activated by specific protein kinase C stimulators (Colonna et al., 1989). Moreover, our results show that the amplitude of the [Ca2+]i increases following electrostimulation is determined by both the electrical field parameters and the concentration of extracellular Ca2+ in the pulsing media. In the absence of extracellular Ca2+, despite adequate stimulation, no transient increase in [Ca2+]i has been observed in these eggs and none have activated. Our data suggest that an influx of Ca2+ across the egg’s plasma membrane following electrostimulation is the direct trigger that initiates both the changes in [Ca2+]i and the activation process of porcine eggs.

This investigation also shows a striking difference in intracellular Ca2+ characteristics between eggs activated by electrostimulation and those activated by sperm penetration. Our results demonstrate clearly that a single electrical stimulus can only induce a single transient [Ca2+]i increase which shows a quick increase phase and a very slow recovery phase lasting for minutes; further transient increases are not observed unless additional stimuli are applied. More over, when multiple stimulations are given each resultant Ca2+ transient is similar to that which occurs during the first stimulation. By contrast during fertilization, the sperm activate the eggs by inducing multiple Ca2+ spikes of short duration and with a fairly regular frequency. The entire Ca2+ oscillatory activity induced by fertilization can persist for at least 3 hours following sperm-egg fusion. These results therefore suggest that, although repeated electrical stimulations can induce multiple Ca2+ transients, the pattern of Ca2+ oscillations differ markedly from that induced by sperm at fertilization. We believe that the absence of Ca2+ oscillations after electroactivation is a reflection of an entirely different form of intracellular signalling from that observed at fertilization. At present, there are two fundamentally different ideas on how sperm activate eggs by generating transient Ca2+ rises. One hypothesis is that sperm act by binding to external receptors that are linked to the egg’s GTP-binding proteins, these in turn activate a phospholipase C that leads both to the generation of Ins(1,4,5)P3, and the release of Ca2+ from intracellular stores (Jaffe et al., 1988; Whitaker, 1989). The alternative hypothesis proposes that the sperm trigger Ca2+ release by first fusing with the eggs and then delivering a soluble factor into the egg cytoplasm (Dale, 1988; Swann, 1990). Despite disagreements about the proposed models, it is generally accepted that Ins(1,4,5)P3 plays a key role in regulating oscillatory activity in living cells. As to electrostimulation, it has been demonstrated in human red blood cells that intensive electrical pulses can cause transient volcanoshaped membrane openings which may represent the membrane pathways for entry of molecules and ions into elec-tropermeabilized cells (Chang and Reese, 1990). It appears probable but not yet proved that electrostimulation may induce similar temporary pore formation in the egg membrane. Thus, an influx of extracellular Ca2+ via ion diffusion through the pores occurs; this in turn causes the increase in [Ca2+]i level and may also induce a consequent single release of Ca2+ from intracellular stores. Successive electrical pulses each induce temporary pore formation and a resultant single Ca2+ transient rise. The Ca2+ waves observed in the electrically activated eggs may represent the pattern of Ca2+ diffusion in the eggs following stimulation; the velocities of the waves may reflect the speed of Ca2+ diffusion in the egg cytoplasm. However, it is still an open question as to whether, accompanying the Ca2+ influx, Ca2+ release from intracellular stores can also be induced in those electrically activated eggs. Neverthless, it is clear from our results that in the absence of extracellular Ca2+ the eggs show no detectable [Ca2+]i increase even after intensive stimulation, indicating that under those conditions Ca2+ release from intracellular stores was not induced.

A comparison of our dynamic imaging with that of Miyazaki (1990) shows a number of interesting similarities and differences in the way in which the Ca 2+ signal is propagated after sperm penetration as compared with electrostimulation. In fertilised hamster eggs the Ca2+ wave is initiated at the site of sperm-egg fusion and then spreads across the entire egg within seconds (Miyazaki et al., 1986). Our results show that after a single electrical stimulus a Ca2+ wave is always initiated at a point adjacent to the positive electrode (Fig. 8); the Ca2+ wave spreads across the egg but more slowly than that induced by sperm fusion. The administration of multiple pulses during a stimulation can also initiate a second Ca2+ wave with an initiation site adjacent to the negative electrode, the two waves propagated from both directions and then converged. It appears that the pattern of the Ca2+ wave observed in the porcine eggs activated by multiple electrical pulses is very similar to that reported in the sea urchin eggs (Kinosita et al., 1991).

A number of other interesting cell biological observations have been made during the course of these experiments. (i) An inverse relationship has been established between field strength and pulse duration in determining the rate of egg activation. (ii) Marked heterogeneity in the responsiveness of eggs to electrical stimuli has been recorded; under the same electric field conditions some eggs will activate while others will not. (iii) The [Ca2+]i level increases as the field strength or pulse duration is increased. A ready explanation for these observations can be provided by linking the need for a threshold [Ca2+]i level to initiate egg activation with the variation in membrane pores induced by the electrical pulses. It appears that with a given Ca2+ concentration in a pulsing medium, the overall [Ca2+]i increase following stimulation is primarily determined by the extent of membrane disruption and that activation will occur in only those eggs whose [Ca2+]i exceeds a trigger threshold. Individual egg responsiveness could result from differences in either pore formation or intracellular Ca2+ sensitivity.

The role in development of the sequence of Ca2+ oscillations that occur in eggs after fertilization is still to be determined. However, our present findings show that a single Ca2+ transient is sufficient to induce both protein reprogramming and cortical granule exocytosis in a manner that appears to mimic precisely that induced by sperm at fertilization. It is therefore unlikely that multiple Ca2+ transients play a critically important role in these two biological processes. In the rabbit, Ozil (1990) has examined the influence of the level of electrostimulation on embryo preimplantation development by using two treatments of 22 pulses with two different total pulse durations. His experiments demonstrated for the first time that some developmental events like compaction and blastocyst formation can be determined by the process of egg activation by the Ca2+ stimuli two or four days earlier. In the present investigation, we have shown the [Ca2+]i increase following electrostimulation is controlled by both the electrical field parameters and the extracellular Ca2+ concentrations, which clearly support Ozil’s suggestion that altering the pulse duration may in turn influence [Ca2+]i level following stimulation. Future studies will determine whether the level of [Ca2+]i induced in porcine eggs following electrostimulation will influence cell cycle and early embryo developmental events.

The inability of electroactivation to initiate Ca2+ oscillation in the eggs following electrostimulation, together with the results of Ozil (1990), has raised our interest in the relationship between the developmental capacity of nucleartransplanted eggs and the lack of appropriate calcium oscillations in these eggs following electrofusion. It seems that if calcium oscillations in the eggs are indeed essential for normal preimplantation development, the lack of this important biological process in the artificially activated eggs may well explain why the development of the nucleartransplanted eggs obtained by electrofusion is generally very poor; it also means that the current electrofusion procedures used for nuclear transfer in the mammalian species are inadequate and should be supplemented with other approaches in order to mimic those normally initiated by sperm during fertilization. Work in this direction is now under way in our laboratory. Moreover, it is also evident from the present investigation that the Ca2+ transient rise resulting from electrostimulation is the direct trigger of inducing egg activation. Therefore, despite adequate elec-trostimulation, the treated eggs will not be activated in a pulsing medium devoid of Ca2+. By applying this observation to nuclear transplantation studies, we have found that fusion between karyoplasts and MII cytoplasts can be efficiently achieved without inducing egg activation (Sun, unpublished data). This enables us, therefore, to develop a modified nuclear transplantation procedure in which donor nuclei can be conditioned for various periods of time, before initiation of egg activation by a second set of electrical stimuli in a pulsing medium containing Ca2+. We postulate that such a modified procedure, by conditioning the donor nuclei in the MII cytoplasm for adequate periods of time, may enhance complete nuclear reprogramming and therefore improve the developmental capacity of the nuclear-transplanted eggs.

F.Z.S. and R.M.M. wish to thank Drs Robin Irvine and Michael Berridge for the discussions on the results of our electrostimulation studies, Mr Leandro Christman for help with the pig in vitro fertilization experiments and Linda Notton for typing the manuscript. This work was supported by the AFRC Pig Science Programme.

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