Cumulus oophorus as a sperm sequestering device, in vivo

High-Fat Diet and Female Fertility across Lifespan: A Comparative Lesson from Mammal Models

Female reproduction focuses mainly on achieving fully grown follicles and competent oocytes to be successfully fertilized, as well as on nourishing the developing offspring once pregnancy occurs. Current evidence demonstrates that obesity and/or high-fat diet regimes can perturbate these processes, leading to female infertility and transgenerational disorders. Since the mechanisms and reproductive processes involved are not yet fully clarified, the present review is designed as a systematic and comparative survey of the available literature. The available data demonstrate the adverse influences of obesity on diverse reproductive processes, such as folliculogenesis, oogenesis, and embryo development/implant. The negative reproductive impact may be attributed to a direct action on reproductive somatic and germinal compartments and/or to an indirect influence mediated by the endocrine, metabolic, and immune axis control systems. Overall, the present review highlights the fragmentation of the current information limiting the comprehension of the reproductive impact of a high-fat diet. Based on the incidence and prevalence of obesity in the Western countries, this topic becomes a research challenge to increase self-awareness of dietary reproductive risk to propose solid and rigorous preventive dietary regimes, as well as to develop targeted pharmacological interventions.

Mysteries and unsolved problems of mammalian fertilization and related topics

Mammalian fertilization is a fascinating process that leads to the formation of a new individual. Eggs and sperm are complex cells that must meet at the appropriate time and position within the female reproductive tract for successful fertilization. I have been studying various aspects of mammalian fertilization over 60 years. In this review, I discuss many different aspects of mammalian fertilization, some of my laboratory's contribution to the field, and discuss enigmas and mysteries that remain to be solved.

Active peristaltic movements and fluid production of the mouse oviduct: their roles in fluid and sperm transport and fertilization†

To study how the oviduct behaves in relation to fluid secretion and sperm transport, ovary–oviduct–uterus complexes of the mouse were installed in a fluid-circulating chamber without disturbing the blood circulation or parasympathetic innervation. Injection of a bolus of Indian ink into the lower isthmus revealed very active adovarian peristalsis of the isthmus, which was most prominent during the periovulatory period. Oviduct fluid, secreted by the entire length of the isthmus, was rapidly transported to the ampulla and ovarian bursa before draining into the peritoneal cavity. The upper isthmus, in particular the isthmic–ampullary junction, was responsible for this adovarian fluid flow. Peristalsis of the oviduct, undisturbed flow of oviduct fluid from the isthmus to the peritoneal cavity, and the spermatozoon's own motility all contribute to efficient sperm ascent and to fertilization within the oviduct. Therefore, chemotaxis, rheotaxis, and thermotaxis of spermatozoa toward oocyte–cumulus complexes in the ampulla are all unlikely mechanisms for explaining sperm–oocyte contact and successful fertilization, given the rapid adovarian flow of oviduct fluid in this species.

The effect of elevated non-esterified fatty acid concentrations on bovine spermatozoa and on oocyte in vitro fertilisation

Elevated non-esterified fatty acid (NEFA) concentrations, present in follicular and oviductal fluid, have been postulated as a causative link between metabolic disorders and subfertility. High NEFA conditions can directly disrupt oocyte maturation and developmental capacity after fertilisation. However, their influence on sperm function and the fertilisation process is not known. This study investigated the fertilisation process under high NEFA conditions. To differentiate between effects on both spermatozoa and oocytes or on spermatozoa only, different experiments were conducted. In the first experiment both gametes were simultaneously incubated during IVF under different conditions: (1) NEFA-free, solvent-free control conditions, (2) solvent control, (3) physiological concentrations of oleic (OA), palmitic (PA) and stearic (SA) acids or (4) pathophysiological concentrations of OA, PA and SA. In the second experiment spermatozoa were incubated (4 h) under the same treatment conditions prior to routine IVF. Gamete co-incubation resulted in reduced fertilisation and cleavage rates and increased prevalence of polyspermy. In the second experiment embryo developmental capacity and quality were not affected, although sperm motility and plasma membrane integrity were decreased. In conclusion, lipolytic conditions affected the fertilisation process mainly through an effect on the oocyte. Spermatozoa were still able to fertilise even though these conditions reduced sperm function.

The effect of cumulus cells on domestic cat (Felis catus) oocytes during in vitro maturation and fertilization

The aim of this study was to evaluate the effect of co-culture of denuded oocytes with cumulus cells (CC) or cumulus-oocyte complexes (COCs) on in vitro maturation (IVM) and in vitro fertilization (IVF). Immature oocytes were collected from ovaries of domestic cats following a routine ovariectomy. Oocytes were matured in vitro for 24 hr within four groups: (i) denuded oocytes (DO), (ii) DO co-cultured with CC, (iii) DO co-cultured with COC and (iv) COC as a control group. In further experiments, COCs were matured in vitro for 24 hr, and then, oocytes were randomly divided into four groups as previously described and fertilized in vitro. Embryos were cultured for up to 7 days. At the end of each experiment, oocytes/embryos were stained with Hoechst 33342 solution and observed under an inverted fluorescence microscope. The results of oocyte maturation showed that their meiotic competence decreased significantly in all experimental groups, compared to the control group. The maturation rates were approximately 45%, 24%, 43% and 76% in experiment 1, and 21%, 14%, 33% and 50% in experiment 2 in groups (i), (ii), (iii) and (iv), respectively. Examination of in vitro fertilization revealed that embryos developed up to the morula stage in all experimental groups. DO and oocytes cultured with COC during fertilization showed a lower cleavage rate-36% and 25% as opposed to those co-cultured with loose CC and the control group-43% and 42%, respectively. Results of this study indicate that cumulus cells connected with an oocyte into a cumulus-oocyte complex are irreplaceable for the maturation of domestic cat oocyte, but that the addition of loose CC may be beneficial for IVF.

The epididymis re-visited: a personal view

The sperm maturation and storage functions of the epididymis are important determinants of ejaculate quality, and perhaps provide an avenue to male contraception. In the last 50 years, the creation of epididymal fertility profiles in laboratory animals was followed by recognition of new sperm maturation-related parameters (organization of the acrosome, of the sperm plasmalemma, and –S–S– -based structural change) which made it possible to confirm that a similar pattern of sperm maturation obtains in man. The novel sperm storage function of the cauda epididymidis in therian mammals is regulated by androgen, usually in conjunction with the low temperature of the scrotum. The temperature-dependence of the scrotal cauda is reflected in the secretory and ion transport functions of the epithelium, in its duct dimensions and so in sperm storage capacity. Moreover, a variety of indirect evidence suggests that an elevated temperature of the cauda created by clothing may be compromising its function in man. The pattern of change in the sperm plasmalemma involving sterols, and also glycosylphosphatidylinositol-linked macromolecules as spermatozoa enter the cauda region, may underlie the need for their capacitation subsequently in the female tract. Further, in a variety of taxa the anatomy of the scrotum, together with the U-shaped configuration of the epididymis/vas deferens, suggests that the cauda's storage function may also underlie the evolution of the scrotum. Finally, despite the still relative paucity of comparative evidence, we can consider now why the epididymis has come to be organized as it is.

The beneficial effects of cumulus cells and oocyte-cumulus cell gap junctions depends on oocyte maturation and fertilization methods in mice

Cumulus cells are a group of closely associated granulosa cells that surround and nourish oocytes. Previous studies have shown that cumulus cells contribute to oocyte maturation and fertilization through gap junction communication. However, it is not known how this gap junction signaling affects in vivo versus in vitro maturation of oocytes, and their subsequent fertilization and embryonic development following insemination. Therefore, in our study, we performed mouse oocyte maturation and insemination using in vivo- or in vitro-matured oocyte-cumulus complexes (OCCs, which retain gap junctions between the cumulus cells and the oocytes), in vitro-matured, denuded oocytes co-cultured with cumulus cells (DCs, which lack gap junctions between the cumulus cells and the oocytes), and in vitro-matured, denuded oocytes without cumulus cells (DOs). Using these models, we were able to analyze the effects of gap junction signaling on oocyte maturation, fertilization, and early embryo development. We found that gap junctions were necessary for both in vivo and in vitro oocyte maturation. In addition, for oocytes matured in vivo, the presence of cumulus cells during insemination improved fertilization and blastocyst formation, and this improvement was strengthened by gap junctions. Moreover, for oocytes matured in vitro, the presence of cumulus cells during insemination improved fertilization, but not blastocyst formation, and this improvement was independent of gap junctions. Our results demonstrate, for the first time, that the beneficial effect of gap junction signaling from cumulus cells depends on oocyte maturation and fertilization methods.

Metabolites involved in cellular communication among human cumulus-oocyte-complex and sperm during in vitro fertilization

BACKGROUND

Fertilization is a key physiological process for the preservation of the species. Consequently, different mechanisms affecting the sperm and the oocyte have been developed to ensure a successful fertilization. Thus, sperm acrosome reaction is necessary for the egg coat penetration and sperm-oolema fusion. Several molecules are able to induce the sperm acrosome reaction; however, this process should be produced coordinately in time and in the space to allow the success of fertilization between gametes. The goal of this study was to analyze the metabolites secreted by cumulus-oocyte-complex (COC) to find out new components that could contribute to the induction of the human sperm acrosome reaction and other physiological processes at the time of gamete interaction and fertilization.

METHODS

For the metabolomic analysis, eighteen aliquots of medium were used in each group, containing: a) only COC before insemination and after 3 h of incubation; b) COC and capacitated spermatozoa after insemination and incubated for 16-20 hours; c) only capacitated sperm after 16-20 h in culture and d) only fertilization medium as control. Six patients undergoing assisted reproduction whose male partners provided normozoospermic samples were included in the study. Seventy-two COC were inseminated.

RESULTS

The metabolites identified were monoacylglycerol (MAG), lysophosphatidylcholine (LPC) and phytosphingosine (PHS). Analysis by PCR and in silico of the gene expression strongly suggests that the cumulus cells contribute to the formation of the PHS and LPC.

CONCLUSIONS

LPC and PHS are secreted by cumulus cells during in vitro fertilization and they could be involved in the induction of human acrosome reaction (AR). The identification of new molecules with a paracrine effect on oocytes, cumulus cells and spermatozoa will provide a better understanding of gamete interaction.

Ejaculated mouse sperm enter cumulus-oocyte complexes more efficiently in vitro than epididymal sperm

The mouse is an established and popular animal model for studying reproductive biology. Epididymal mouse sperm, which lack exposure to secretions of male accessory glands and do not precisely represent ejaculated sperm for the study of sperm functions, have been almost exclusively used in studies. We compared ejaculated and epididymal sperm in an in vitro fertilization setting to examine whether ejaculated sperm enter cumulus-oocyte complexes more efficiently. In order to prepare sperm for fertilization, they were incubated under capacitating conditions. At the outset of incubation, ejaculated sperm stuck to the glass surfaces of slides and the incidences of sticking decreased with time; whereas, very few epididymal sperm stuck to glass at any time point, indicating differences in surface charge. At the end of the capacitating incubation, when sperm were added to cumulus-oocyte complexes, the form of flagellar movement differed dramatically; specifically, ejaculated sperm predominantly exhibited increased bending on one side of the flagellum (a process termed pro-hook hyperactivation), while epididymal sperm equally exhibited increased bending on one or the other side of the flagellum (pro-hook or anti-hook hyperactivation). This indicates that accessory sex gland secretions might have modified Ca²⁺ signaling activities in sperm, because the two forms of hyperactivation are reported to be triggered by different Ca²⁺ signaling patterns. Lastly, over time, more ejaculated than epididymal sperm entered the cumulus oocyte complexes. We concluded that modification of sperm by male accessory gland secretions affects the behavior of ejaculated sperm, possibly providing them with an advantage over epididymal sperm for reaching the eggs in vivo.

The importance of having zinc during in vitro maturation of cattle cumulus-oocyte complex: role of cumulus cells

The aim of this study was to investigate the influence of zinc (Zn) on the health of cumulus-oocyte complex (COC) during in vitro maturation (IVM). Experiments were designed to evaluate the effect of Zn added to IVM medium on: DNA integrity, apoptosis, cumulus expansion and superoxide dismutase (SOD) activity of cumulus cells (CC). Also, role of CC on Zn transport during IVM was evaluated on oocyte developmental capacity. DNA damage and early apoptosis were higher in CC matured with 0 μg/ml Zn compared with 0.7, 1.1 and 1.5 μg/ml Zn (p < 0.05). Cumulus expansion did not show differences in COC matured with or without Zn supplementation (p > 0.05). Superoxide dismutase activity was higher in COC matured with 1.5 μg/ml Zn than with 0 μg/ml Zn (p < 0.05). Cleavage and blastocyst rates were recorded after IVM in three maturation systems: intact COCs, denuded oocytes with cumulus cells monolayer (DO + CC) and denuded oocytes (DO). Cleavage rates were similar when COC, DO + CC or DO were matured with 1.5 μg/ml Zn compared with control group (p > 0.05). Blastocyst rates were significantly higher in COC than in DO + CC and DO with the addition of 1.5 μg/ml Zn during IVM (p < 0.01). Blastocyst quality was enhanced in COC and DO + CC compared with DO when Zn was added to IVM medium (p < 0.001). The results of this study indicate that Zn supplementation to IVM medium (i) decreased DNA damage and apoptosis in CC; (ii) increased SOD activity in CC; (iii) did not modify cumulus expansion and cleavage rates after in vitro fertilization; (iv) improved subsequent embryo development up to blastocyst stage; and (v) enhanced blastocyst quality when CC were present either in intact COC or in coculture during IVM.

Sperm competition risk generates phenotypic plasticity in ovum fertilizability

Theory predicts that sperm competition will generate sexual conflict that favours increased ovum defences against polyspermy. A recent study on house mice has shown that ovum resistance to fertilization coevolves in response to increased sperm fertilizing capacity. However, the capacity for the female gamete to adjust its fertilizability as a strategic response to sperm competition risk has never, to our knowledge, been studied. We sourced house mice (Mus domesticus) from natural populations that differ in the level of sperm competition and sperm fertilizing capacity, and manipulated the social experience of females during their sexual development to simulate conditions of either a future 'risk' or 'no risk' of sperm competition. Consistent with coevolutionary predictions, we found lower fertilization rates in ova produced by females from a high sperm competition population compared with ova from a low sperm competition population, indicating that these populations are divergent in the fertilizability of their ova. More importantly, females exposed to a 'risk' of sperm competition produced ova that had greater resistance to fertilization than ova produced by females reared in an environment with 'no risk'. Consequently, we show that variation in sperm competition risk during development generates phenotypic plasticity in ova fertilizability, which allows females to prepare for prevailing conditions during their reproductive life.

Chemosensory Ca2+ dynamics correlate with diverse behavioral phenotypes in human sperm

In the female reproductive tract, mammalian sperm undergo a regulated sequence of prefusion changes that "prime" sperm for fertilization. Among the least understood of these complex processes are the molecular mechanisms that underlie sperm guidance by environmental chemical cues. A "hard-wired" Ca(2+) signaling strategy that orchestrates specific motility patterns according to given functional requirements is an emerging concept for regulation of sperm swimming behavior. The molecular players involved, the spatiotemporal characteristics of such motility-associated Ca(2+) dynamics, and the relation between a distinct Ca(2+) signaling pattern and a behavioral sperm phenotype, however, remain largely unclear. Here, we report the functional characterization of two human sperm chemoreceptors. Using complementary molecular, physiological, and behavioral approaches, we comparatively describe sperm Ca(2+) responses to specific agonists of these novel receptors and bourgeonal, a known sperm chemoattractant. We further show that individual receptor activation induces specific Ca(2+) signaling patterns with unique spatiotemporal dynamics. These distinct Ca(2+) dynamics are correlated to a set of stimulus-specific stereotyped behavioral responses that could play vital roles during various stages of prefusion sperm-egg chemical communication.

Male infertility, female fertility and extrapair copulations

Females that are socially bonded to a single male, either in a social monogamy or in a social polygyny, are often sexually polyandrous. Extrapair copulations (EPC) have often been suggested or rejected, on both empirical and theoretical grounds, as an important mechanism that enables females to avoid fertility risks in case their socially bonded male is infertile. Here, we explore this possibility in two steps. First, we present a mathematical model that assumes that females have no precopulatory information about male fertility, and shows that a female EPC strategy increases female reproductive success only if certain specific conditions are upheld in the nature of male infertility. In particular, these conditions require both (i) that fertile sperm precedence (FSP) is absent or incomplete within ejaculates of the same male (i.e. that an infertile male is, at least partly, truly infertile), and (ii) the existence of FSP among ejaculates of different males (such that infertile spermatozoa of the infertile male are at a disadvantage when competing against spermatozoa of a fertile male). Second, to evaluate their potential role in the evolution of female EPC, we review the abundance and FSP patterns of the different male infertility types. The conclusion is drawn that some common infertility types, such as poor sperm count or motility, contribute to the evolution of female EPC, whereas other common infertility types, such as sperm depletion or allocation in a social monogamy (but not in a social polygyny), and in particular male driven polyspermy, do not. Also, a deeper look at the arms race between sperm fertilization efficiency and female barriers to sperm may answer the non-trivial question: "why are some types of infertility so common?"

Fertilization and early embryonic development in the porcine fallopian tube

Fertilization and early embryo development relies on a complex interplay between the Fallopian tube and the gametes before and after fertilization. Thereby the oviduct, as a dynamic reproductive organ, enables reception, transport and maturation of male and female gametes, their fusion, and supports early embryo development. This paper reviews current knowledge regarding physiological processes behind the transport of boar spermatozoa, their storage in and release from the functional sperm reservoir (SR), and of the interactions that newly ovulated oocytes play within the tube during their transport to the site of fertilization. Experimental evidence of an ovarian control on sperm release from the SR is highlighted. Furthermore, the impact of oviductal secretion on sperm capacitation, oocyte maturation, fertilization and early embryo development is stressed.

The sperm chemoattractant secreted from human cumulus cells is progesterone

BACKGROUND

Human spermatozoa appear to be guided by chemotaxis to the oocyte in the female genital tract. While one of the sources of sperm chemoattractants is the cumulus cells that surround the oocyte, the identity of the chemoattractant secreted from them is unknown. Progesterone, recognized to be secreted from cumulus cells, was demonstrated, at the pM concentration range, to be a chemoattractant for human spermatozoa. Here, we examined whether this steroid is the cumulus-originated chemoattractant for human spermatozoa.

METHODS

Human cumulus cells were cultured, and the cultured medium was demonstrated to be chemotactically active. Progesterone was then eliminated from the medium by a specific anti-progesterone antibody, and the residual chemotactic activity was assessed.

RESULTS

The rate of progesterone secretion from the cells decreased with time. Removal of progesterone from the cumulus-cultured medium resulted in total loss of the chemotactic activity of the medium. Furthermore, the cumulus-cultured medium could substitute for progesterone in stimulating changes in the intracellular Ca(2+) concentration in the spermatozoa, and the changes were very similar to those caused by measured progesterone concentrations in the medium.

CONCLUSIONS

Taken together, the results suggest that progesterone is the main, if not the sole, chemoattractant secreted by human cumulus cells.

Use of the capillary–cumulus oophorus model for evaluating the selection of spermatozoa

OBJECTIVE

To establish the action of the cumulus mass on the function of human spermatozoa.

DESIGN

A prospective, controlled clinical study.

SETTING

Academic infertility clinic.

PATIENT(S)

Semen donors and IVF patients.

INTERVENTION(S)

Donor human cumulus oophorus complex was aspirated into a glass Pasteur pipette. The semen of selected IVF patients was then loaded into the pipette, and the spermatozoa were allowed to transverse the cumulus matrix. The penetrated spermatozoa were then collected and underwent sperm functional tests, namely chlortetracycline, fluorescein isothiocyanate-labeled Pisum sativum, and chromomycin A(3). These were then compared with the fertilization rates of the IVF patients used in the study.

MAIN OUTCOME MEASURE(S)

The cumulus oophorus' action on capacitational processes of human spermatozoa.

RESULT(S)

The penetrated spermatozoa showed a higher tendency to be capacitated and acrosome-reacted and to have superior chromosomal packaging quality, compared with the initial semen sample.

CONCLUSION(S)

Cumulus oophorus cells play a vital role in the fertilization process, especially during the capacitational stages. This method could physiologically select spermatozoa to be used in assisted reproductive techniques, especially intracytoplasmic sperm injection, in which sperm usually are immobile (because of low motility or because spermatozoa were testicularly retrieved).

Enigmas of mammalian gamete form and function

The gametes of man and some other Eutheria have been manipulated successfully for practical reasons, but many gaps remain in our basic understanding of the way that they function. This situation stems not least from a failure to recognize the extent to which eutherian spermatozoa and eggs, and elements related to their operation, have come to differ from those of other groups. Novel features in the male that reflect this include a radical design of the sperm head with the acrosome seeming to function primarily in egg-coat binding rather than its lysis, a multifaceted post-testicular sperm maturation and an androgen/low-temperature-regulated system of sperm storage--both tied to the epididymis, a variable male accessory sex gland complex, and descent of the testis and epididymis to a scrotum. In the female, such novelties are represented in a need for sperm capacitation, in an unusual regulation of sperm transport within the oviduct, in the cumulus oophorus and character of the zona pellucida around the small egg, and in a unique configuration of gamete fusion. The collective evidence now suggests that many of these features reflect a new fertilisation strategy or its consequences, with most being causally linked. One initial 'domino' in this regard appears to be the small yolkless state of the egg and its intolerance for polyspermy, as determinants of the unusual mode of oviductal sperm transport and possibly the existence and form of the cumulus oophorus. However, a particularly influential first 'domino' appears to be the physical character of the eutherian zona pellucida. This differs from the egg coats of other animal groups by virtue of a resilient elasticity and thickness. These qualities allow this primary and often only coat to stretch and so persist during later expansion of the blastocyst, usually until close to implantation. At the same time, the dimensions, physical character, and particularly the relative protease-insensitivity of the zona appear to have had profound effects on sperm form and function and, more indirectly, on sperm-related events in the male and the female tract. Marsupials display some similarities and also some strikingly different features, against which the enigmas of the eutherian situation can be evaluated.

Sperm guidance in mammals - an unpaved road to the egg

Contrary to the prevalent view, there seems to be no competition in the mammalian female genital tract among large numbers of sperm cells that are racing towards the egg. Instead, small numbers of the ejaculated sperm cells enter the Fallopian tube, and these few must be guided to make the remaining long, obstructed way to the egg. Here, we review the mechanisms by which mammalian sperm cells are guided to the egg.

Relationship between urinary estrogen levels before conception and sex ratio at birth in a primate, the gray mouse lemur

BACKGROUND

In primates, including humans, bias of sex ratio at birth has been reported to depend on maternal condition at conception. In a Malagasy primate, the gray mouse lemur, male-biased sex ratio at birth occurred in captive parous females. The male bias was previously demonstrated to be pre-conceptual and independent of the female's nutritional state. It was hypothesized to be related to changes in hormonal state at ovulation.

METHODS AND RESULTS

The relationship between urinary estradiol (E2) levels during the follicular phase until estrus and litter production (number and sex of newborns) was assessed in 91 females mated under controlled conditions. Changes in E2 levels prior to ovulation followed the typical primate pattern characterized by a gradual rise during the 10 days preceding the sharp increase at estrus. A clear decline in E2 levels occurred with ageing. Direction of the sex ratio bias was unrelated to E2 levels at ovulation time but was significantly dependent on E2 levels during the follicular phase. Reduced E2 levels prior to estrus led to male-biased litters.

CONCLUSION

This study suggests that hormonal stimulation during the follicular phase plays a role in shifting sex ratio at conception through changes in the local environment of the ova. This hypothesis deserves testing by assessing estrogen levels throughout the follicular phase in other primate species including humans.

Human sperm chemotaxis: both the oocyte and its surrounding cumulus cells secrete sperm chemoattractants

BACKGROUND

Human sperm chemotaxis to pre-ovulatory follicular fluid is well established in vitro. However, it is not known whether the female's oocyte-cumulus complex secretes sperm chemoattractants subsequent to ovulation (for enabling sperm chemotaxis within the Fallopian tube) and, if so, which of these cell types--the oocyte or the cumulus oophorus--is the physiological origin of the secreted chemoattractant.

METHODS

By employing a directionality-based chemotaxis assay, we examined whether media conditioned with either individual, mature (metaphase II) human oocytes or the surrounding cumulus cells attract human sperm by chemotaxis.

RESULTS

We observed sperm chemotaxis to each of these media, suggesting that both the oocyte and the cumulus cells secrete sperm chemoattractants.

CONCLUSIONS

These observations suggest that sperm chemoattractants are secreted not only prior to ovulation within the follicle, as earlier studies have demonstrated, but also after oocyte maturation outside the follicle, and that there are two chemoattractant origins: the mature oocyte and the surrounding cumulus cells.

Effect of homologous follicular fluid from medium-sized and large follicles on in vitro maturation of equine cumulus - oocyte complexes

The in vitro maturation (IVM) of equine oocytes is typically performed using various synthetic media; however, an optimal IVM system for equine oocytes has not been developed. The aim of the present study was to evaluate the effects of two types of follicular fluid (FF) obtained from cyclic mares and two incubation intervals for the IVM of equine cumulus–oocyte complexes (COCs). Follicular fluid was collected from medium-sized (20–29 mm diameter) and large (≥30 mm; post-human chorionic gonadotrophin administration) follicles using transvaginal ultrasound-guided follicle aspiration. Compact (n = 232) and non-compact (n = 183) COCs obtained from a slaughterhouse were incubated separately in the following groups: (1) FF from medium follicles for 24 h; (2) FF from large follicles for 24 h; (3) control (synthetic) medium for 24 h; (4) FF from medium follicles for 24 h then FF from large follicles for an additional 24 h; (5) FF from large follicles for 48 h; and (6) control medium for 48 h. For compact COCs, there was a tendency (P = 0.06) for more COCs incubated in FF from large follicles for 24 h to reach metaphase II compared with those incubated in control medium for 24 h (58% v. 35%, respectively). More (P < 0.05) compact COCs had degenerated after incubation in control medium for 48 h compared with all other groups (51% v. 14–24%, respectively). For non-compact COCs, incubation in FF from medium follicles for 24 h resulted in more (P = 0.05) COCs at metaphase II compared with control medium for 48 h (58% v. 29%, respectively). These results indicate that homologous FF from cyclic mares is a suitable alternative for the IVM of equine COCs and that it may be superior to conventional media for longer (i.e. >24 h) incubation intervals.

Specificity of the tumor necrosis factor-induced protein 6-mediated heavy chain transfer from inter-alpha-trypsin inhibitor to hyaluronan: implications for the assembly of the cumulus extracellular matrix

The formation of the hyaluronan-rich cumulus extracellular matrix is crucial for female fertility and accompanied by a transesterification reaction in which the heavy chains (HCs) of inter-alpha-trypsin inhibitor (IalphaI)-related proteins are covalently transferred to hyaluronan. Tumor necrosis factor-induced protein-6 (TNFIP6) is essential for this transfer reaction. Female mice deficient in TNFIP6 are infertile due to the lack of a correctly formed cumulus matrix. In this report, we characterize the specificity of TNFIP6-mediated HC transfer from IalphaI to hyaluronan. Hyaluronan oligosaccharides with eight or more monosaccharide units are potent acceptors in the HC transfer, with longer oligosaccharides being somewhat more efficient. Epimerization of the N-acetyl-glucosamine residues to N-acetyl-galactosamines (i.e. in chondroitin) still allows the HC transfer although at a significantly lower efficiency. Sulfation of the N-acetyl-galactosamines in dermatan-4-sulfate or chondroitin-6-sulfate prevents the HC transfer. Hyaluronan oligosaccharides disperse cumulus cells from expanding cumulus cell-oocyte complexes with the same size specificity as their HC acceptor specificity. This process is accompanied by the loss of hyaluronan-linked HCs from the cumulus matrix and the appearance of oligosaccharide-linked HCs in the culture medium. Chondroitin interferes with the expansion of cumulus cell-oocyte complexes only when added with exogenous TNFIP6 before endogenous hyaluronan synthesis starts, supporting that chondroitin is a weaker HC acceptor than hyaluronan. Our data indicate that TNFIP6-mediated HC transfer to hyaluronan is a prerequisite for the correct cumulus matrix assembly and hyaluronan oligosaccharides and chondroitin interfere with this assembly by capturing the HCs of the IalphaI-related proteins.

The extracellular matrix of porcine mature oocytes: origin, composition and presumptive roles

The extracellular matrix (ECM) of porcine mature oocytes was revealed by transmission electron microscopy (TEM) after treatment with tannic acid and ruthenium red. Present in the perivitelline space (PVS) and on the surface of the zona pellucida (ZP), it appeared to be composed of thin filaments and granules at the interconnections of the filaments, which were interpreted respectively as hyaluronic acid chains and bound proteoglycans. In order to determine whether this material is produced by the corona cells (the same ECM was found also on the surface of the zona pellucida and between cumulus cells) or by the oocyte itself, the synthesis of glycoproteins and glycosaminoglycans was checked by autoradiography on semi-thin and thin sections observed by light and electron microscopy. Immature oocytes within or without cumulus cells, were incubated with L [3H-] fucose or L [3H-] glucosamine--precursors respectively of glycoproteins and hyaluronic acid or hyaluronan (HA) bound to proteoglycans--for various times (with or without chase) and at different stages during in vitro maturation. In the first case, incorporation was found in both cumulus cells and ooplasm (notably in the Golgi area for 3H-fucose) and labeled material accumulated in the ECM of the PVS and of the ZP surface. Labeling in the PVS with both precursors was maximum between metaphase I (MI) and metaphase II (MII) and was partially extracted by hyaluronidase but not by neuraminidase. Tunicamycin, an inhibitor of glycoprotein synthesis, significantly decreased the amount of 3H-fucose labeled molecules in the PVS and increased the incidence of polyspermic penetration during subsequent in vivo fertilization. Since cumulus-free oocytes also secreted 3H-glucosamine containing compounds, both oocyte and cumulus cells probably contribute to the production of the ECM found in the PVS of mature oocytes. ECM and particularly its HA moiety present on both sides of the ZP may constitute a favourable factor for sperm penetration.

Reflections upon sperm-endosalpingeal and sperm-zona pellucida interactions in vivo and in vitro

In terms of experimental studies of the process of fertilization in mammals, this essay attempts to give a balanced assessment of current interpretations and to raise questions of direct relevance to two fields of research. First, the nature of pre-ovulatory sperm-endosalpingeal binding is set in a physiological perspective, and examination of this dynamic process in vitro is shown to suffer from serious shortcomings. In particular, sperm-epithelial binding reactions demonstrated in cultures of endosalpingeal cells may represent only a portion of the binding reaction in vivo or, as revealed by sperm binding to the tracheal epithelium, could be largely non-specific. Second, concerning the subsequent phase of binding of a fertilizing spermatozoon to the zona pellucida, the nature of the initial contact arrest of the sperm head is evaluated, especially in the light of the highly uneven or 'pitted' surface of this outer coating of the oocyte. Bearing in mind the active phase of oviduct macromolecular secretion that continues after ovulation, the question is posed as to how functional sperm binding sites are preserved on the surface of the zona pellucida rather than masked by epithelial and cumulus cell secretions. Finally, it is recalled that initial sperm : egg ratios at the site of fertilization in the ampullary-isthmic region of the oviduct are close to unity in diverse species of mammal, such fertilizing spermatozoa being released from prominent reserves in the caudal isthmus shortly before the time of ovulation. On strictly quantitative grounds, therefore, the extent to which biochemical or molecular studies portray changes in the vanguard of fertilizing spermatozoa needs to be questioned. Such studies are more likely to be descriptive of overall changes in very large numbers of sperm cells at diverse stages of maturation.

Function of the cumulus oophorus before and during mammalian fertilization

CONTENTS

Fertilization encompasses a series of different steps which have to be performed in a well-orchestrated way to create a new individual. They include sperm capacitation, sperm binding and penetration of the zona pellucida, traversing the perivitelline space, binding and fusion with the oolemma, activation of the oocyte and decondensation of the sperm head to form the male pronucleus. In most mammalian species, cumulus cells surround the oocyte at the time of fertilization. Removal of the cumulus oophorus at this point of time often leads to a drop in fertilization rates. It is not yet known how cumulus cells interact with the oocyte or with spermatozoa to promote fertilization. There are different possibilities: 1 cumulus cells cause mechanical entrapment of spermatozoa and guide hyperactivated spermatozoa towards the oocyte, while preventing abnormal spermatozoa to enter the cumulus matrix; 2 cumulus cells create a micro-environment for the spermatozoa which favours their capacitation and penetration into the oocyte; 3 cumulus cells prevent changes in the oocyte which are unfavourable for normal fertilization; these changes can be located in the zona pellucida or in the cytoplasm. In this review, studies in several species are listed to prove the importance of these three cumulus cell functions and the current lines of research are highlighted. Moreover, different ways to improve in vitro fertilization of bovine cumulus-denuded oocytes are discussed.

The effects of oocyte storage and cumulus cell presence on canine zona penetration by domestic dog spermatozoa

Zona penetration assays (ZPAs) have been developed in numerous species to evaluate sperm fertilizing potential. Preservation methods to stockpile oocytes would be beneficial because of the difficulty in obtaining sufficient numbers of fresh oocytes. Using a canine ZPA, the objectives of this study were to evaluate: (1) two methods of storing canine oocytes (salt storage and intrafollicular cooling) and (2) the effects of cumulus cells on oocyte penetration. In experiment 1. oocytes from fresh ovaries were assigned at random to 3 categories: fresh control (FRE), salt storage in solution 1 (1.5 M MgCl2.6H2O; SS1) and salt storage in solution 2 (0.5 M (NH4)2SO4, 0.75 M MgCl2.6H2O, 0.2 mM ZnCl2; SS2). Each category was subdivided into two treatments: cumulus cells intact (intact) and cumulus cells removed (denuded), resulting in a total of six treatments with n > 15 oocytes per treatment for each ejaculate. Fresh (FRE) intact oocytes demonstrated greater sperm-oocyte interaction than other treatments, including FRE denuded oocytes (11.7 +/- 0.6 versus <4.1 +/- 0.5 sperm-oocyte and 94.9 versus <55.6% penetration; P < 0.01). Poor sperm-oocyte interaction was demonstrated with all salt-stored oocytes (< or = 1.6 +/- 0.2 sperm-oocyte and < 51% penetration), but was further attenuated in the absence of cumulus cells. In experiment 2, oocytes obtained from fresh (FRESH) or cooled (24 h COOL, 48 h COOL) ovaries were used with cumulus cells intact for a total of three treatments with n > 15 oocytes per treatment for each ejaculate. No significant difference was observed in sperm interaction between oocytes from fresh, 24 and 48 h COOL ovaries ( 12.3 +/- 0.5 to 13.1 +/- 0.4 sperm-oocyte and 92.2-97.7% penetration; P > 0.01). These results indicate that salt storage may cause damage to canine oocytes, subsequently impairing sperm penetration, whereas short-term intrafollicular cooling does not affect the oocyte's penetrability. Furthermore, greater sperm interaction in oocytes with an intact cumulus suggests a possible role for cumulus cells in canine gamete interaction.

Spermatozoa of the shrew, Suncus murinus, undergo the acrosome reaction and then selectively kill cells in penetrating the cumulus oophorus

In the musk shrew, Suncus murinus (and other shrews), the cumulus oophorus is ovulated as a discrete, compact, matrix-free ball of cells linked by specialized junctions. In examining how they penetrate the cumulus, Suncus spermatozoa were observed to first bind consistently by the ventral face over the acrosomal region to the exposed smooth surface of a peripheral cumulus cell. This was apparently followed by point fusions between the plasma and outer acrosomal membranes. Thereafter, spermatozoa without acrosomes were observed within cumulus cells that displayed signs of necrosis, as did some radially neighboring cumulus cells linked by zona adherens and gap junctions. Eventually, penetration of spermatozoa as far as the perizonal space around the zona pellucida left linear tracks of locally necrotic cells flanked by normal cumulus cells. Based on these and previous observations, we conclude that the acrosome reaction in Suncus is always induced by cumulus cells, and that reacted spermatozoa penetrate the cumulus by selective invasion and killing of cumulus cells along a linear track. Loss of the acrosome also exposes an apical body/perforatorium that is covered with barbs that appear to assist reacted fertilizing spermatozoa in binding to the zona pellucida. Because fertilized eggs displayed no other spermatozoa within or bound to the zona, an efficient block to polyspermy must prevent such binding of additional spermatozoa.

Effects of maturational stage, cumulus cells and coincubation of mature and immature cumulus-oocyte complexes on in vitro penetrability of porcine oocytes

The in vitro penetrability of porcine oocytes is conditioned by several factors, some of which remain unclear. Knowledge of the different effects of the cellular components involved in penetrability would no doubt serve to simplify laboratory IVF methods. This study was designed to evaluate the effects of the following factors on penetrability: oocyte maturational stage, the presence of isolated or oocyte-attached cumulus cells, and coincubation of in vitro-matured and immature oocytes. Immature oocytes and oocytes matured in Waymouth medium were obtained from non atretic follicles and fertilized in TCM 199 medium. Sperm-rich fractions were collected by the gloved hand method and semen was used for IVF at a final concentration of 1 x 10(6) cells/mL in all experiments. Under the same conditions of IVF, the penetrability of the immature cumulus-oocyte complexes (COCs) was significantly lower than that of mature COCs, in terms of penetration rate and mean number of sperm per penetrated oocyte. This difference was abolished when the oocytes were denuded, leading to similar penetration rates. Coincubation of mature and immature COCs reduced the penetrability of immature COCs compared with that observed when these were incubated in isolation. However, neither the addition of isolated cumulus cells from decumulated mature oocytes nor the addition of denuded mature oocytes to immature COCs modified the penetration rate. These findings suggest that the presence of surrounding cumulus cells is mainly responsible for the differences observed in penetrability, regardless of the maturational stage of the oocyte. Moreover, when mature and immature COCs are coincubated, penetrability of immature COCs is diminished by the effects of the mature COC and not by the independent actions of the cellular components.

Assay and importance of adhesive interaction between hamster (Mesocricetus auratus) oocyte-cumulus complexes and the oviductal epithelium

Adhesion between the oocyte-cumulus complex and infundibulum plays an important, but poorly understood, role in oocyte pick-up. The purposes of this study were to determine which components of the oocyte-cumulus complex and oviductal epithelium function in adhesion, to measure adhesion under physiological conditions, and to examine the effect of modulation of adhesion on oocyte-cumulus complex pick-up rate. Oocyte-cumulus complexes containing an expanded matrix were readily transported into the oviduct, while unexpanded complexes lacking an extracellular matrix were not picked up, indicating that the matrix is necessary for pick-up. Transmission electron microscopy revealed that during pick-up, adhesion occurred specifically between the ciliary crowns of the oviduct and the granules and filaments of the cumulus matrix. An assay was developed using vacuum from a low-flow peristaltic precision pump, modified for bi-directional flow, to measure the strength of adhesion between the oocyte-cumulus complex and the oviductal epithelium, and adhesion was measured during physiological conditions. The lectin wheat germ agglutinin and the polycation poly-L-lysine were then used to modulate adhesion, and the effects of increasing or decreasing adhesion on oocyte pick-up rate and ciliary beat frequency were examined. The data show that 1) the matrix of the oocyte-cumulus complex and the ciliary crowns of the oviduct function in adhesion during pick-up and that adhesion is necessary for pick-up, 2) adhesion can be assayed quantitatively and is very uniform among control infundibula, and 3) decreasing or increasing adhesion decreases oocyte pick-up rate and in some cases prevents pick-up without affecting ciliary beat frequency.

Mammalian sperm chemotaxis and its association with capacitation

Much progress has been made in recent years in establishing mammalian sperm chemotaxis and understanding sperm capacitation. Thus far, chemotaxis to follicular fluid has been established by a variety of means in human and mouse spermatozoa. It was found that only a small fraction of a given sperm population (averaging around 10%) is chemotactically responsive and that this fraction constitutes capacitated (ripe) spermatozoa. Both the chemotactic responsiveness and the capacitated state are transient (with a lifetime of 50 min to 4 h) and they occur only once in the sperm's lifetime. It has been proposed that the role of sperm chemotaxis in mammals (at least in humans) is selective recruitment of capacitated spermatozoa for fertilizing the egg, and that the role of the continuous replacement of chemotactic/capacitated spermatozoa is to prolong the time during which capacitated spermatozoa are available in the female reproductive tract. The sperm chemoattractants have not been identified, but they appear to be heat-stable peptides. Although the molecular mechanism and the in vivo location of sperm chemotaxis are not known, a number of possible mechanisms and locations are discussed.

Inter-alpha-inhibitor binding to hyaluronan in the cumulus extracellular matrix is required for optimal ovulation and development of mouse oocytes

This report characterizes the effects of excess hyaluronan (HA) upon the expansion of the cumulus oocyte complex (COC) within intact follicles and upon ovulation and oocyte viability in mice. Covalent linkage between heavy chains of the inter-alpha-inhibitor (IalphaI) family of serum glycoproteins and HA is necessary for optimal cumulus extracellular matrix (cECM) stabilization and cumulus expansion. Intravenous administration of HA oligosaccharides inhibited the binding of IalphaI to endogenous HA, disrupting the process of expansion and resulting in a reduction in the size of the cumulus mass. Western blot and immunocytochemical analyses of COCs from HA-treated animals demonstrated a reduction of IalphaI heavy chains within the cECM. Additionally, HA-treated immature animals ovulated 56.3% fewer COCs compared to control animals. The developmental potential of COCs in HA-treated animals was also tested. Extended periods of oviductal storage of COCs ovulated by HA-injected adult mice resulted in a reduction of normal embryos and a significant increase in the proportion of fragmented oocytes/embryos. These observations support the view that covalent binding of IalphaI heavy chains to HA is required for optimal cumulus expansion, extrusion of the COCs from the follicle at ovulation, and maintenance of oocyte viability within the oviduct.

Do human eggs attract spermatozoa?

A key process in human fertilization is bringing the two gametes together, so that the complex molecular events involved in sperm and egg interaction can begin. Does nature allow fertilization to occur only as a consequence of a chance collision, or is there a precontact sperm-egg communication? This review summarizes the bioassays used in testing human spermatozoa for chemotaxis, emphasizing the necessity to distinguish between chemotaxis and other accumulation-causing processes, and the results obtained. It demonstrates that human sperm chemotaxis to a follicular factor(s) does occur, at least in vitro, and that only capacitated spermatozoa are chemotactically responsive. Substances that have been proposed as attractants for human spermatozoa are reassessed. The potential role of sperm chemotaxis in vivo is discussed. Faulty precontact sperm-egg communication may be one of the causes of male infertility, female infertility, or both. On the other hand, interfering with human sperm chemotaxis may represent an exciting new approach to contraception.

Effects of reducing insemination time in human in vitro fertilization and embryo development by using sibling oocytes

PURPOSE

Recent studies showed a beneficial effect of reducing the time of sperm-oocyte interaction on fertilization, division, and implantation rates of the oocytes obtained from randomized patients. In the present study, the effects of reduced insemination time on fertilization and embryo development were evaluated by using sibling oocytes from the same patient.

METHODS

A total of 464 oocytes from 36 patients was randomly allocated to be inseminated for either 1 hr (reduced) or 18 hr (regular).

RESULTS

Fertilization rates were not significantly different between reduced (135/229; 59%) and regular (150/235; 64%) groups. Cleavage rates and embryo quality were similar in both groups. A total of 135 embryos (73 from the reduced and 62 from the regular group) was transferred to 36 patients. Thirty-four embryos implanted in 18 patients (25.2% implantation and 50.0% pregnancy rates).

CONCLUSIONS

Fertilization, cleavage, and embryo development from 1-hr insemination is comparable, not superior, to those from an 18-hr insemination time, which is commonly used in in vitro fertilization programs. These data suggest that reduced insemination time can be used during in vitro fertilization to avoid unnecessarily longer exposure to spermatozoa.

Growth factors and components for extracellular proteolysis are differentially expressed during in vitro maturation of bovine cumulus-oocyte complexes

In the present study, the time-dependent collagenolytic potential and mRNA transcription of extracellular matrix (ECM)-degrading components, transforming growth factor beta1 (TGFbeta1), and both basic fibroblast growth factor (bFGF) and FGF receptors (FGFR) in bovine cumulus-oocyte complexes (COCs) were investigated during 24 h of in vitro maturation (IVM). COCs were collected from 2- to 6-mm follicles, cultured in maturation medium, and sequentially removed at 3-h intervals for analysis. From 285 oocytes matured under these conditions, 114 (40.0%) developed to blastocysts on Day 9 after fertilization. Gelatin zymograms revealed protease activity at about 55 kDa for COCs matured for at least 3 h and two additional proteolytic zones at about 70 kDa after at least 9 h of IVM. The mRNAs of ECM-degrading components urokinase-type plasminogen activator (uPA), plasminogen activator inhibitor 1 (PAI1), matrix metalloproteinase 1 (MMP1), and tissue inhibitor of metalloproteinases 1 (TIMP1), as well as TGFbeta1, bFGF, and FGFR, were detected during IVM in a factor-specific pattern: all transcript levels found at COC 0 generally increased after 3 h of maturation and either remained high or decreased thereafter. On the basis of the chosen reverse transcription-polymerase chain reaction technique, one could suggest relative higher mRNA concentrations for TIMP1, PAI1, and both growth factors compared to uPA, MMP1, and FGFR. Our results suggest a finely tuned extracellular proteolysis during IVM of bovine COCs, for which the concerted action of modulating growth factors like bFGF and TGFbeta1 may be essential.

Egg-cumulus-oviduct interactions and fertilization

In this communication we approach the events leading to fertilization in mammals by examining the triangle of egg, sperm and oviductal cell taking account of the local physiology and focussing on auto/paracrine interactions. The expression of growth factors and extra-cellular matrix (ECM)-components in bovine ovarian granulosa- and theca-cells, the oocyte-cumulus complex (OOC) and oviductal epithelium, as well as some of the corresponding secreted proteins can be detected through the estrous cycle. Components of the insulin-like (IGF), fibroblast (FGF) and transforming (TGF) growth factor systems, and also metalloproteinase 1 (MMP1) and urokinase (uPA) are found to be modulated in these cells prior to fertilization. Different expression levels between the cell types are found, each representative of a specific reaction window within that particular stage of the cycle. Our findings support the concept that most of the observed tissue in the reproductive tract is dependent upon on the effects of gonadotropins or steroids, but that the fine-regulation is conveyed by, for example, growth factors and ECM-components. We suggest a sophisticated, auto/paracrine and species-specific crosstalk of growth factors and ECM components between the different cell types involved, enabling fertilization and development of the embryo at the right time and in the right location.

Potential mechanisms for sex ratio adjustment in mammals and birds

Sex ratio skews in relation to a variety of environmental or parental conditions have frequently been reported among mammals and, though less commonly, among birds. However, the adaptive significance of such sex ratio variation remains unclear. This has, in part, been attributed to the absence of a low-cost physiological mechanism for sex ratio manipulation by the parent. It is shown here that several recent findings in reproductive biology are suggestive of many potential pathways by which gonadotropins and steroid hormones could interfere with the sex ratio at birth. And these hormone levels are well-known to be influenced by many parameters which have been invoked in correlating with offspring sex ratios. Hence, it is argued that the significant, but inconsistent sex ratio biases reported in mammalian and avian populations are coherent with current knowledge on reproductive physiology in those species. However, whether such variations can be viewed at as a consequence of physiological constraint or as adaptive sex ratio adjustment, has still to be determined.

Effects of cumulus cells on sperm penetration of bovine oocytes in protein-free medium

The present study was conducted to examine the effects of cumulus cells on sperm capacitation, acrosome reaction and penetration of bovine oocytes in vitro in a protein-free medium. In vitro matured oocyte-cumulus complexes (OCCs) and denuded oocytes were co-incubated with spermatozoa in the medium with or without bovine serum albumin (BSA). Higher fertilization rates were obtained in the OCCs (92 and 89%, respectively) than denuded oocytes (57 and 6%, respectively) in the medium with or without BSA (P<0.01). Higher proportion of the denuded oocytes were fertilized in the medium with BSA (57%) than without BSA (6%; P<0.01). These results suggest that the cumulus cells are more effective for increasing fertilization rate than BSA (P<0.05). Both the percentages of capacitated and acrosome-reacted spermatozoa incubated for 4 h with isolated cumulus cells were not significantly different in the medium without cumulus cells in the presence or absence of BSA. The denuded oocytes were inseminated with isolated cumulus cells taken from OCCs matured with or without hormones, follicle stimulating hormone (FSH) and estradiol-17beta (E(2)), and from immature OCCs in a protein-free medium. Presence of the cumulus cells matured with hormones enhanced sperm penetration of denuded oocytes more effectively (81%) than either of the cells matured without hormones (41%) or the immature cells (26%; P<0.01). The conditioned medium of cumulus cells matured with hormones was not effective for sperm penetration of denuded oocytes (2%), while a high proportion (82%) of the oocytes were fertilized when they were inseminated with isolated cumulus cells (P<0.01). In conclusion, the presence of cumulus cells matured with FSH and E(2) was effective for sperm penetration but not for sperm capacitation or acrosome reaction.

Sperm:egg ratios and putative molecular signals to modulate gamete interactions in polytocous mammals

Reasoning from the premises that 1) the sperm:egg ratio at the time of activation of the secondary oocyte in mammals is close to unity under conditions of spontaneous mating, 2) a majority of eggs within the cumulus oophorus of a polytocous species is fertilized in a reasonably short interval of time, and 3) spermatozoa would find it difficult to reverse their approach to the zona pellucida, it is proposed that molecular gradients exist to divert spermatozoa penetrating the cumulus mass away from eggs already activated and towards eggs as yet unfertilized. Possible sources of such molecular cues are considered, as is the event that triggers their release.