Activity of a sperm-borne oocyte-activating factor in spermatozoa and spermatogenic cells from cynomolgus monkeys and its localization after oocyte activation

Phospholipase Cζ (PLCζ) versus postacrosomal sheath WW domain-binding protein (PAWP): Which molecule will survive as a sperm factor?

During mammalian fertilization, sperm is fused with the oocyte's membrane, triggering the resumption of meiosis from the metaphase II arrest, the extrusion of the second polar body, and the exocytosis of cortical granules; these events are collectively called 'oocyte activation.' In all species studied to date, the transient rise in the cytosolic level of calcium (in particular, the repeated calcium increases called 'calcium oscillations' in mammals) is required for these events. Researchers have focused on identifying the factor(s) that can induce calcium oscillations during fertilization. Sperm‐specific phospholipase C, i.e., PLC zeta (PLCζ), is a strong candidate of the factor(s), and several research groups using different species obtained evidence that PLCζ is a sperm factor that can induce calcium oscillations during fertilization. However, postacrosomal sheath Tryptophan‐Tryptophan (WW)—domain‐binding protein (PAWP) was recently shown to have a pivotal role in inducing calcium oscillations in some species. In this review, we focus on PLCζ and PAWP as sperm factors, and we discuss this controversy: Which of these two molecules survives as a sperm factor?

Application of auxin-inducible degron technology to mouse oocyte activation with PLCζ

In mammals, spermatozoa activate oocytes by triggering a series of intracellular Ca²⁺ oscillations with phospholipase C zeta (PLCζ), a sperm-borne oocyte-activating factor. Because the introduction of PLCζ alone can induce oocyte activation, it might be a promising reagent for assisted reproductive technologies. To test this possibility, we injected human PLCζ (hPLCζ) mRNA into mouse oocytes at different concentrations. We observed the oocyte activation and subsequent embryonic development. Efficient oocyte activation and embryonic development to the blastocyst stage was achieved only with a limited range of mRNA concentrations (0.1 ng/μl). Higher concentrations of mRNA caused developmental arrest of most embryos, suggesting that excessive PLCζ protein might be harmful at this stage. In a second series of experiments, we aimed to regulate the PLCζ protein concentration in oocytes by applying auxin-inducible degron (AID) technology that allows rapid degradation of the target protein tagged with AID induced by auxin. Injection of the hPLCζ protein tagged with AID and enhanced green fluorescent protein (hPLCζ-AID-EGFP) demonstrated that high EGFP expression levels at the late 1-cell stage were efficiently reduced by auxin treatment, suggesting efficient hPLCζ degradation by this system. Furthermore, the defective development observed with higher concentrations of hPLCζ-AID-EGFP mRNA was rescued following auxin treatment. Full-term offspring were obtained by round spermatid injection with optimized hPLCζ-AID activation. Our results indicate that this AID technology can be applied to regulate the protein levels in mouse oocytes and that our optimized PLCζ system could be used for assisted fertilization in mammals.

The establishment of appropriate methods for egg-activation by human PLCZ1 RNA injection into human oocyte

Phospholipase C-zeta (PLCZ1), a strong candidate of egg-activating sperm factor, can induce Ca²⁺ oscillations and cause egg activation. For the application of PLCZ1 to clinical use, we examined the pattern of Ca²⁺ responses and developmental rate by comparing PLCZ1 RNA injection methods with the other current methods, such as cytosolic aspiration, electrical stimulation and ionomycin treatment in human oocytes. We found that the pattern of Ca²⁺ oscillations after PLCZ1 RNA injection exhibited similar characteristics to that after ICSI treatment. We also determined the optimal concentration of human PLCZ1 RNA to activate the human oocytes. Our findings suggest that human PLCZ1 RNA is a better therapeutic agent to rescue human oocytes from failed activation, leading to normal and efficient development.

Plasma membrane and acrosome loss before ICSI is required for sheep embryonic development

PURPOSE

This study aims to determine if the integrity of the sperm plasma membrane and acrosome vesicle could be limiting factors in sheep intracytoplasmic sperm injection (ICSI).

METHODS

Prior to in vitro fertilization (IVF) or ICSI, the oocytes were subjected to in vitro maturation (IVM) for 24 h. First, to evaluate the need of artificial activation for ovine ICSI, 226 oocytes were injected with intact spermatozoa (IS), from which 125 were activated by incubation in ionomycin and 101 were cultured without activation. Next, spermatozoa were mechanically (by piezo-electrical pulses) and/or chemically (by ionomycin/Triton X-100) treated to break membranes and acrosomes and were injected into oocytes, grouped as follows: (i) piezo-pulsed spermatozoa (PPS), (ii) PPS pre-treated with ionomycin (PPS-I), (iii) PPS pre-treated with Triton X-100 (PPS-T), and (iv) intact and untreated spermatozoa as a control (CTR-IS).

RESULTS

No differences were observed in the zygote/cleavage/blastocyst rate between chemically activated and non-activated oocytes (50 vs. 45 %, 11.6 vs. 10.1 %; 1.8 vs. 1.1 %, respectively), after ICSI with CTR-IS. Injection of PPS compared to CTR-IS increased the proportion of zygotes and blastocysts (84.6 vs. 45 %, p < 0.01; 15.5 vs. 1.1 %, p < 0.0001, respectively). Moreover, the percentage of PPS-derived blastocysts was not significantly different from that obtained by conventional IVF (15.5 vs. 20.2 %). The ICSI blastocysts' development was also improved with PPS pre-treated with ionomycin (15.6 %), but was completely impeded with PPS pre-treated with Triton X-100 (0 %).

CONCLUSION

Our findings confirm that ICSI with spermatozoa whose plasma membrane and acrosome have been mechanically damaged substantially improves embryonic development until the blastocyst stage.

Egg Activation at Fertilization by a Soluble Sperm Protein

The most fundamental unresolved issue of fertilization is to define how the sperm activates the egg to begin embryo development. Egg activation at fertilization in all species thus far examined is caused by some form of transient increase in the cytoplasmic free Ca2+ concentration. What has not been clear, however, is precisely how the sperm triggers the large changes in Ca2+ observed within the egg cytoplasm. Here, we review the studies indicating that the fertilizing sperm stimulates a cytosolic Ca2+ increase in the egg specifically by delivering a soluble factor that diffuses into the cytosolic space of the egg upon gamete membrane fusion. Evidence is primarily considered in species of eggs where the sperm has been shown to elicit a cytosolic Ca2+ increase by initiating Ca2+ release from intracellular Ca2+ stores. We suggest that our best understanding of these signaling events is in mammals, where the sperm triggers a prolonged series of intracellular Ca2+ oscillations. The strongest empirical studies to date suggest that mammalian sperm-triggered Ca2+ oscillations are caused by the introduction of a sperm-specific protein, called phospholipase C-zeta (PLCζ) that generates inositol trisphosphate within the egg. We will discuss the role and mechanism of action of PLCζ in detail at a molecular and cellular level. We will also consider some of the evidence that a soluble sperm protein might be involved in egg activation in nonmammalian species.

Effects of activation on functional aster formation, microtubule assembly, and blastocyst development of goat oocytes injected with round spermatids

A systematic study was conducted on round spermatids (ROS) injection (ROSI) using the goat model. After ROSI, the oocytes were treated or not with ionomycin (ROSI+I and ROSI-I, respectively) and compared with intracytoplasmic sperm injection (ICSI). After ROSI-I, most oocytes were arrested with premature chromatin condensation and few oocytes formed pronuclei. In contrast, most oocytes formed pronuclei after ROSI+I. Some ROS were observed to form asters that organized a dense microtubule network after ROSI+I, but after ROSI-I, no ROS asters were observed. Whereas most of the oocytes showed Ca(2+) rises and a significant decline in maturation-promoting factor (MPF) and mitogen-activated protein kinase (MAPK) activities after ROSI+I, no such changes were observed after ROSI-I. Due to the lack of Ca(2+) oscillations after ROSI-I, oocytes were injected with more ROS. Interestingly, different from the results observed in a single ROS injection, injection with four ROS effectively activated oocytes by inducing typical Ca(2+) oscillations. Whereas ROSI+I oocytes and ICSI oocytes both showed extensive microtubule networks, no such a network was observed in parthenogenetic oocytes. Together, the results suggest that goat ROS is not able to trigger intracellular Ca(2+) rises and thus to inhibit MPF and MAPK activities, but artificial activation improved fertilization and development of ROSI goat oocytes. Goat ROS can organize functional microtubular asters in activated oocytes. A ROS-derived factor(s) may be essential for organization of a functional microtubule network to unite pronuclei. Goat centrosome is of paternal origin because both ROS and sperm asters organized an extensive microtubule network after intra-oocyte injection.

Birth of normal mice following round spermatid injection without artificial oocyte activation

For fertilization using round spermatid injection (ROSI) in mice, oocytes need to be artificially preactivated because of the lack of oocyte-activating capacity in round spermatids of this species. However, when round spermatids were frozen-thawed before microinjection, 11-71% of injected oocytes developed into 2-cell embryos without any artificial activation. After being transferred into recipient females, 5-27% of these embryos reached term. At least some of the injected oocytes showed intracellular Ca(2+) oscillations, which normally occur after fertilization by mature spermatozoa. Thus, these round spermatids could transmit a sperm-borne oocyte-activating factor, which might have been released from spermatozoa and elongated spermatids in the same suspension by freezing and thawing. This possibility was further supported by activation of intact oocytes following transplantation of the pronuclei from ROSI-generated embryos. Thus, one-step ROSI can be achieved in mice simply by injecting frozen-thawed round spermatids into intact oocytes. Clearly, there is a need for careful interpretation of microinjection experiments when assessing the oocyte-activating capacity of spermatogenic cells, especially when they are derived from frozen-thawed stocks.

The oocyte activation and Ca2+ oscillation-inducing abilities of mouse and human dead (sonicated) spermatozoa

In ICSI procedures, it is well known that the selection of viable (live) spermatozoa and certain types of immobilization prior to injection is very important for obtaining successful results, but unfortunately there are rare situations when only immotile spermatozoa are available (such as in severe asthenozoospermia or necrozoospermia). In such cases, failure of oocyte activation after ICSI often occurs and may be due to the lack of SOAF (sperm-borne oocyte activating factor) activity. In order to investigate the SOAF activities of dead spermatozoa, mouse and human spermatozoa were immobilized (killed by sonication), maintained in THF medium for varying time intervals (up to 72 h) and then injected into mature unfertilized mouse oocytes. Injected mouse oocytes were examined for their activation, development into blastocysts and Ca2+ responses by imaging and confocal laser scanning microscope. The rates of oocyte activation, blastocyst development and normal patterns of Ca2+ oscillation from the killed-sperm-injected oocytes decreased gradually in accordance with the maintenance interval between sonication and injection. For injection with mouse sonicated spermatozoa, the rate of normal Ca2+ oscillations declined first (after a 3 h maintenance interval) and then blastocyst development was gradually obstructed (after approx. 10 h). The oocyte activation-inducing ability of dead spermatozoa was maintained for a relatively long period, but began to decline after 20 h. The activation rates and Ca2+ response of the oocytes that were injected with human sonicated spermatozoa decreased earlier than those injected with mouse spermatozoa. Although the oocyte activation-inducing ability was maintained for a relatively long time after the death of the spermatozoa, embryo development into blastocysts and the rate of normal Ca2+ oscillations declined after a short maintenance interval between sonication and injection. The Ca2+ response seemed to be the most sensitive indicator for the evaluating the SOAF activity of dead (killed) spermatozoa.

Difference in Ca2+ oscillation-inducing activity and nuclear translocation ability of PLCZ1, an egg-activating sperm factor candidate, between mouse, rat, human, and medaka fish

Mouse phospholipase C, zeta 1 (PLCZ1), a strong candidate of egg-activating sperm factor, induces Ca(2+) oscillations and accumulates into formed pronucleus (PN) when expressed by cRNA injection. These activities were compared among mouse and human PLCZ1, newly cloned rat Plcz1, and medaka fish plcz1. The PLCZ1 proteins of the four species have an approximately homologous sequence of nuclear localization signal. However, the nuclear translocation ability was defective in rat, human, and medaka PLCZ1 expressed in mouse eggs. Rat PLCZ1 could not enter rat PN, whereas mouse PLCZ1 could. Mouse and human PLCZ1 translocated into the nucleus of COS-7 cells transfected with cDNA. There was little medaka PLCZ1 accumulated in the nucleus, and rat PLCZ1 was never located in the nucleus. All PLCZ1 proteins including fish could induce Ca(2+) oscillations in mouse eggs, but the activity was variable in the order of human >> mouse > medaka >> rat, estimated from minimal RNA concentration to induce Ca(2+) spikes. Ca(2+) oscillations by human PLCZ1 continued far beyond the time of PN formation (T(PN)), whereas those by mouse PLCZ1 ceased slightly before T(PN). High-frequency Ca(2+) spikes by overexpressed rat PLCZ1 stopped far before T(PN), possibly by feedback inhibition. Ca(2+) oscillations by fertilization of rat eggs stopped at T(PN), despite defective nuclear translocation of rat PLCZ1. Thus, PLCZ1 sequestration into PN participates in termination of Ca(2+) oscillations at the interphase of mouse embryos but does not always operate in other mammals, notably in rat embryos.

Human round spermatids from azoospermic men exhibit oocyte-activation and Ca2+ oscillation-inducing activities

During mammalian fertilization, intracellular Ca2+ oscillations are important for both oocyte activation and embryonic development. As the ability of round spermatids (ROS) to induce Ca2+ oscillations and oocyte activation is different between species, we examined Ca2+ oscillation- and oocyte activation-inducing abilities of human ROS originating from patients with non-obstructive azoospermia. Human ROS from 11 non-obstructive azoospermic patients were collected during their TESE-ICSI cycles. Following injection into mature unfertilized mouse oocytes, we examined the oocyte-activating and Ca2+ oscillation-inducing activities of ROS by using Ca2+ imaging and confocal laser scanning microscopy (mouse test). In these 11 cases, clinical TESE-ICSI using mature testicular spermatozoa was successful, with the exception of one case in which only one sperm-injected oocyte was not fertilized. The mean fertilization rate was 70.1% (40-100%); the mean cleavage rate was 97.9% (46/47). Two pregnancies were established from 10 transfer cycles (PR; 20%). When the ROS from these patients were injected into mouse oocytes, the ROS from all patients induced at least some intracellular Ca2+ oscillations (25-100%). In all patients, 40 out of 82 oocytes injected with ROS exhibited normal oscillation patterns of [Ca2+]i. Human spermatogenetic cells acquired oocyte-activating and Ca2+ oscillation-inducing abilities at the round spermatid stage, an earlier stage than found for rodent cells. These data indicate that human ROS might be useful for clinical treatments of non-obstructive azoospermic patients exhibiting mature spermatozoa in biopsied specimens.

Release of phospholipase C ζand [Ca2+]i oscillation-inducing activity during mammalian fertilization

During fertilization of mammalian eggs a factor from the sperm, the sperm factor (SF), is released into the ooplasm and induces persistent [Ca2+]ioscillations that are required for egg activation and embryo development. A sperm-specific phospholipase C (PLC), PLCz, is thought to be the SF. Here, we investigated whether the SF activity and PLCζare simultaneously and completely released into the ooplasm soon after sperm entry. To accomplish this, we enucleated sperm heads within 90 min of intracytoplasmic sperm injection (ICSI) and monitored the persistence of the [Ca2+]ioscillations in eggs in which the sperm had been withdrawn. We also stained the enucleatedsperm heads to ascertain the presence/absence of PLCζ. Our results show that by 90 min all the SF activity had been released from the sperm, as fertilized enucleated eggs oscillated as fertilized controls, even in cases in which oscillations were prolonged by arresting eggs at metaphase. In addition, we found that the released SF activity became associated with the pronucleus (PN), as induction of PN envelope breakdown evoked comparable [Ca2+]iresponses in enucleated and non-manipulated zygotes. Lastly, we found that PLCzlocalized to the equatorial area of bull sperm and to the post-acrosomal region of mouse sperm and that by 90 min after ICSI all the sperm’s PLCζimmunoreactivity was lost in both species. Altogether, our findings show that during fertilization the SF activity and PLCζimmunoreactivity are simultaneously released from the sperm, suggesting that PLCζmay be the only [Ca2+]ioscillation-inducing factor of mammalian sperm.

The ability of whale haploid spermatogenic cells to induce calcium oscillations and its relevance to oocyte activation

Interspecies microinsemination assay was applied to examine the ability of minke whale haploid spermatogenic cells to induce Ca2+oscillations and oocyte activation. Populations of round spermatids (RS), early-stage elongating spermatids (e-ES), late-stage elongating spermatids (1-ES) and testicular spermatozoa (TS) were cryopreserved in the presence of 7.5% glycerol on board ship in the Antarctic Ocean. Repetitive increases of intracellular Ca2+concentration occurred in 0, 65, 81 and 96% of BDF1 mouse oocytes injected with the postthaw RS, e-ES, 1-ES and TS, respectively. A normal pattern of the Ca2+oscillations was observed in 26–47% of the responding oocytes. Most oocytes that exhibited Ca2+oscillations, regardless of the oscillation pattern, resumed meiosis (83–94%). These results indicate that whale spermatogenic cells acquire SOAF activity, which is closely related to their Ca2+oscillation-inducing ability at the relatively early stage of spermiogenesis.

In vitro development of bovine oocytes reconstructed with round spermatids

The timing between round spermatid(s) (RS) injection and oocyte activation are critical for spermatid remodeling and embryo development in intracytoplasmic injection of round spermatid (ROSI) procedure. The objective of the present study was to develop an appropriate oocyte activation method for producing developmentally competent bovine embryos reconstructed with RS. Embryos reconstructed by ROSI were compared with three activation treatments for the rates of pronuclear formation, development and ploidy. RS were isolated from bull testes by Percoll density gradients. Matured oocytes were divided into three activation groups. In Group 1, oocytes were activated with ionomycin (5 microM, 5 min) before ROSI. In Group 2, oocytes were activated with ionomycin after ROSI. In Group 3, oocytes were activated twice with ionomycin before and after ROSI. All the eggs were then incubated in cycloheximide (CHX, 10 microg/mL) for 5 h and cultured in CR1aa medium for up to 8 days. Three methods of oocyte activation were also compared for the activation and development of parthenotes. Activation rates among the groups were 70-79% and did not differ. Cleavage rates in parthenotes were significantly (P < 0.05) higher in Group 3 than in Groups 1 and 2, but blastocyst rates did not differ among the groups. In ROSI embryos, the rates of cleavage and development into blastocysts were significantly (P < 0.05) greater in Group 3 (82.3% and 13.1%) than in Groups 1 and 2 (53.7, 5.8% and 64.2, 1.7%, respectively). Ploidy analysis by examining the metaphase spreads of ROSI blastocysts displayed greater numbers of diploid chromosomal complements. These results suggest that intracytoplasmic RS injection combined with repeated ionomycin activation followed by CHX treatment is more efficient for producing developmentally competent embryos.

Differential development of rabbit embryos following microinsemination with sperm and spermatids

Microinsemination is the technique of delivering male germ cells directly into oocytes. The efficiency of fertilization after microinsemination and subsequent embryo development may vary with the animal species and male germ cells used. The present study was undertaken to observe the in vitro and in vivo developmental ability of rabbit embryos following microinsemination with male germ cells at different stages. First, we assessed their oocyte-activating capacity by injecting them into mouse and rabbit oocytes. The majority of mouse oocytes were activated irrespective of the type of rabbit male germ cell injected (61-77%), whereas rabbit oocytes were activated differently according to the type of male germ cells (89%, 75%, and 29% were activated by spermatozoa, elongated spermatids, and round spermatids, respectively; P < 0.05). After 120 hr in culture, 66%, 45%, and 13%, respectively, of these activated rabbit oocytes (pronuclear eggs) developed into blastocysts (P < 0.05). Additional electric pulse stimulation of round spermatid-injected oocytes increased the blastocyst rate to 43%. After 24 hr in culture, some four to eight cell embryos were transferred into the oviducts of pseudopregnant females. Normal pups were born from spermatozoa and elongated spermatids, but not from round spermatids. Karyotypic analysis at the morula/blastocyst stage revealed that the majority of round spermatid-derived embryos had abnormal ploidy (8 out of 12 embryos). Our study indicates that rabbit male germ cells acquire the ability to activate oocytes and to support subsequent embryo development as they undergo spermiogenesis. As these differential developmental patterns are similar to those reported for humans in vitro and in vivo, rabbits may provide an alternative small animal model for studying the biological nature and molecular basis of human microinsemination techniques, especially those using immature male germ cells.

Intracytoplasmic injection of spermatozoa and spermatogenic cells: its biology and applications in humans and animals

Intracytoplasmic sperm injection (ICSI) has become the method of choice to overcome male infertility when all other forms of assisted fertilization have failed. Animals in which ICSI has produced normal offspring include many species. Success rate with normal spermatozoa is well above 50% in the mouse but ICSI success rates in other animals have been low, ranging from 0.3 to 16.5%. Mouse ICSI revealed that spermatozoa that cannot participate in normal fertilization can produce normal offspring by ICSI, provided their nuclei are genomically intact. Human ICSI using infertile spermatozoa has been highly successful perhaps because of the intrinsic instability of human sperm plasma membrane. The health of children born after ICSI and other assisted fertilization techniques is of major concern. Careful analyses suggest that higher incidences of congenital malformations and/or low birth weights after assisted fertilization are largely attributable to parental genetic background and increased incidence of multiple births, rather than to the techniques of assisted fertilization. Since the physiological and nutritional environments of developing embryos may cause persisting alteration in DNA methylation, extreme caution must be exercised in handling gametes and embryos in vitro. In the mouse, round spermatid injection (ROSI) has been routinely successful but its use in humans is controversial. Whether human ROSI and assisted fertilization involving younger spermatogenic cells are medically safe must be the subject of further investigations.

Microinsemination and Nuclear Transfer Using Male Germ Cells

Microinsemination has been widely used in basic reproductive research and in human-assisted reproductive technology for treating infertility. Historically, microinsemination in mammals started with research on the golden hamster; since then, it has provided invaluable information on the mechanisms of mammalian fertilization. Thanks to advances in animal genetic engineering and germ-cell technologies, microinsemination techniques are now used extensively to identify the biological significance of genes of interest or to confirm the genetic normality of gametes produced by experimental manipulations in vitro. Fortunately, in mice, high rates of embryo development to offspring can be obtained so long as postmeiotic spermatogenic cells are used as male gametes-that is, round spermatids, elongated spermatids, and spermatozoa. For some other mammalian species, using immature spermatogenic cells significantly decreases the efficiency of microinsemination. Physically unstable chromatin and low oocyte-activating capacity are the major causes of fertilization failure. The youngest male germ cells, including primordial germ cells and gonocytes, can be used in the construction of diploid embryos by nuclear-transfer cloning. The cloned embryos obtained in this way provide invaluable information on the erasure and reestablishment of genomic imprinting in germ cells.

Fertilization and preimplantation development of mouse oocytes after prolonged incubation with caffeine

Background and Aims:  Previous studies have shown that caffeine might cause artificial dephosphorylation at threonine-14 and tyrosine-15 of the p34cdc2 catalytic subunit of maturation-promoting factor (MPF), elevate MPF activity in metaphase II oocytes cultured for a prolonged period, and that caffeine decreases fragmentation in oocytes cultured for up to 96 h. Methods:  Studies were carried out on: (i) the effect of caffeine on the morphological status of oocytes cultured for 96 h; (ii) the parthenogenetic activation and the fertilization of oocytes incubated in a medium that contained caffeine, and (iii) the fertilization and preimplantation development ability of zona-intact and zona-free oocytes by in vitro fertilization (IVF) and by intracytoplasmic sperm injection. Results:  In parthenogenetic activation, the incidence of diploid parthenotes in 24-h caffeine-treated oocytes was significantly higher than 24-h non-treated oocytes. For fertilizability of these oocytes, a significant increase in the fertilization rate resulted from IVF after 12-h caffeine incubation. Although no fertilized eggs were observed after intracytoplasmic sperm injection in 24-h non-treated oocytes, fertilized eggs were observed in caffeine-treated oocytes. MPF activation occurs in relation to nuclear/spindle position, and mitotic spindles and actin filaments determine the site of cleavage during cytokinesis. Spindle disruption does not cause cytofragmentation, but does induce cell cycle arrest. Conclusion:  Based on our results, although caffeine might increase MPF activity, prolonged time in any incubation causes some disruption of cytoskeletal filaments, which might be responsible for the poor development of caffeine-treated oocytes. (Reprod Med Biol 2004; 3: 245-251).

Ca2+ oscillation-inducing phospholipase C zeta expressed in mouse eggs is accumulated to the pronucleus during egg activation

Sperm-specific phospholipase C zeta (PLC zeta) is known to induce intracellular Ca(2+) oscillations and egg activation when expressed in mouse eggs by injection of RNA encoding PLC zeta. We investigated the expression level and spatial distribution of PLC zeta in the egg in real time and in relation to the initiation and termination of Ca(2+) oscillations by monitoring fluorescence of a yellow fluorescent protein 'Venus' fused with PLC zeta. Ca(2+) oscillations similar to those at fertilization were induced at 40-50 min after RNA injection, when expressed PLC zeta reached 10-40 x 10(-15) g in the egg. PLC zeta-Venus increased up to 3 h and attained a steady level at 4-5 h. Interestingly, PLC zeta-Venus is accumulated to the pronucleus (PN) formed at 5-6 h and continuously increased there. Ca(2+) oscillations stopped in most eggs before initiation of the accumulation. A variant of PLC zeta that lacks three EF hand domains was much less effective in induction of Ca(2+) oscillations and little accumulated in the pronucleus, indicating a critical role of those domains. The ability of the accumulation to the pronucleus qualifies PLC zeta for a strong candidate of the Ca(2+) oscillation-inducing sperm factor, which is introduced into the ooplasm upon sperm-egg fusion and concentrated to the pronucleus after inducing egg activation.

Activation of mouse oocytes following intracytoplasmic injection of chicken sperm extract

The cytosolic factor from the sperm of a wide variety of species has been reported to induce [Ca2+]i increase and/or activation in oocytes. Although many species have been studied, there is no data available on the chicken sperm factor responsible for activation of oocytes. This study was aimed to verify the activation of mouse oocyte by intracytoplasmic injection of chicken sperm extract (CSE). Survival rate of oocytes without rupture or lytic degeneration following injection was greater than 80% regardless of the presence or absence of CSE. Among the intact oocytes, activation rates following injection were 27.5% (11 of 40) in the sham operation group. Injection of CSE resulted in significantly higher activation rates in the 1/10 dilution group (57.8%, 26 of 45) compared with the sham operation group (p = 0.0045). Calculated amount of injected sperm extract of 1/10 dilution of CSE was equivalent of 12 chicken sperm. When the 1/10 diluted mouse sperm extract (MSE) was injected, survival rate of injected oocytes and activation rate of the survived oocytes was 82% (41 of 50) and 78% (32 of 41), respectively. Activation rate of MSE-injected oocytes was a little but significantly higher than that of CSE-injected oocytes (p < 0.037). In conclusion, it suggests that CSE can activate the mouse oocyte and that oocyte activating sperm factor(s) may be common between avian and mammal.

Release of the Ca(2+) oscillation-inducing sperm factor during mouse fertilization

A cytosolic sperm protein(s), referred to as the sperm factor (SF), is thought to induce intracellular calcium ([Ca(2+)](i)) oscillations during fertilization in mammalian eggs. These oscillations, which are responsible for inducing complete egg activation, persist for several hours. Nevertheless, whether a protracted release of SF is responsible for the duration of the oscillations is unknown. Using a combination of intracytoplasmic sperm injection (ICSI), in vitro fertilization (IVF), sperm removal, reinjection of the withdrawn sperm, and [Ca(2+)](i) monitoring, we determined that 30 min was necessary for establishing oscillations. Importantly, a significant portion of the Ca(2+) activity became dissociated from the sperm within 15-60 min after entry, and by 120 min post-ICSI or IVF, sperm were unable to induce oscillations. The initiation of oscillations coincided with exposure and solubilization of the perinuclear theca (PT), as evidenced by transmission electron microscopy, although disassembly of the PT was not required for commencement of the [Ca(2+)](i) responses. Remarkably, despite its complete release into the ooplasm, SF associated with nuclear structures at the time of pronuclear formation. Lastly, release of SF was not affected by the cell cycle. We conclude that mouse sperm serves as a carrier for SF, which is rapidly and completely solubilized to establish [Ca(2+)](i) oscillations.

Intracytoplasmic sperm injection: stiletto conception or a stab in the dark

To describe the importance of molecular and cellular analyses in intracytoplasmic sperm injection (ICSI) the authors review the literature on biological challenges in ICSI and associated techniques. Several matters can be proposed in molecular and cellular challenges in ICSI for safety and efficacy: (1) a reliable and convenient animal model for understanding the molecular and cellular basis of human ICSI must be established, and molecular and cellular analysis of the first cell cycle of human fertilization should be better understood; (2) a proper assay for human sperm function that contributes to the indication for ICSI should be developed; and (3) de novo and transmitted genetic security in ICSI should be examined.

Fertilization of oocytes and birth of normal pups following intracytoplasmic injection with spermatids in mastomys (Praomys coucha)

The mastomys is a small laboratory rodent that is native to Africa. Although it has been used for research concerning reproductive biology, in vitro fertilization (IVF) and intracytoplasmic sperm injection are very difficult in mastomys because of technical problems, such as inadequate sperm capacitation and large sperm heads. The present study was undertaken to examine whether mastomys spermatids could be used to fertilize oocytes in vitro using a microinsemination technique, because spermatids are more easily injected than mature spermatozoa into oocytes. Most mastomys oocytes (80%-90%) survived intracytoplasmic injection with either round or elongated spermatids. Round spermatids had little oocyte-activating capacity, similar to those of mice and rats, and exogenous stimuli were needed for normal fertilization. Treatment with an electric pulse in the presence of 50 microM Ca2+ followed by culture in 10 mM SrCl2 led to successful oocyte activation. After injection of round spermatids into preactivated oocytes, 93% of oocytes were normally fertilized (male and female pronuclei formed), and 100% of cultured oocytes developed to the 2-cell stage. However, none reached term after transfer into recipient females. Elongated spermatids, which correspond to steps 9-11 in rats, activated oocytes on injection without additional activation treatment. After embryo transfer, five offspring (6% per transfer) developed to term. These results indicate that microinsemination with spermatids is a feasible alternative in animal species that are refractory to IVF and sperm injection and that using later-stage spermatids may lead to increased production of viable embryos that can develop into normal offspring.