Role of glucose in mouse preimplantation embryo development

Mouse preimplantation embryos consume pyruvate preferentially during the early developmental stages, before glucose becomes the predominant energy substrate in the blastocyst. To investigate the importance of the switch to glucose utilization at the later developmental stages, mouse embryos from F1 hybrid mice (CBA/Ca × C57BL/6) were cultured from the one-and two-cell stages (22 and 46 h post hCG, respectively) for 5 days in a modified medium, M16, containing 0.33 mM pyruvate and 5 or 23 mM D+L-lactate, in the presence and absence of 1 mM glucose (M16+G and M16-G, respectively). Nutrient uptakes were also determined over this time. Some embryos cultured in M16-G were transferred to M16+G at 94 or 118 h post hCG. Embryos cultured from the two-cell stage in M16+G exhibited the characteristic fall in pyruvate consumption between the morula and the blastocyst stage; those cultured from the two-cell stage in M16-G compensated for the lack of glucose by consuming increasing amounts of pyruvate, from 2.78 pmol/embryo/h at 58 h post hCG to 5.21 pmol/embryo/h at 154 h post hCG. However, the percentage of embryos developing to the blastocyst stage, the hatching rate, and blastocyst cell numbers (50.6 ± 2.5 [28] vs. 105 ± 3.8 [37]) were all lower in this group. When exposed to glucose at 94 or 118 h post hCG, embryos cultured from the two-cell stage in M16-G readily consumed glucose in preference to pyruvate, although the characteristic fall in pyruvate consumption was not observed. One-cell embryos cultured continuously in M16-G were only able to develop to the morula stage, after which time they degenerated. In these embryos pyruvate was readily consumed between 22 and 94 h post hCG, before falling from 2.77 pmol/embryo/h at 83 h post hCG to 0.045 pmol/embryo/h at 130 h post hCG. Transfer of these embryos to M16+G at 94 and 118 h post hCG did not support development to the hatching blastocyst stage. The results show that mouse preimplantation embryos from F1 hybrid mice (CBA/Ca × C57BL/6) need only be exposed to glucose for less than 24 h between 22 and 94 h post hCG in order to develop from the morula to the blastocyst stage in vitro. However, the exposure time needs to be increased to between 24 and 72 h in order that blastocyst cell numbers reach control levels. The importance of glucose before the morula stage may relate to the need to synthesize glycogen for later use. If the obligatory requirement for glucose is fulfilled, embryos are able to utilize pyruvate in the absence of glucose at the later stages of development. These results show that the mouse preimplantation embryo can, to some extent, adapt metabolically to changes in its external environment. © 1995 Wiley-Liss, Inc.     

Metabolite availability as a window to view the early embryo microenvironment in vivo

A preimplantation embryo exists independent of blood supply, and relies on energy sources from its in vivo environment (e.g., oviduct and uterine fluid) to sustain its development. The embryos can survive in this aqueous environment because it contains amino acids, proteins, lactate, pyruvate, oxygen, glucose, antioxidants, ions, growth factors, hormones, and phospholipids—albeit the concentration of each component varies by species, stage of the estrous cycle, and anatomical location. The dynamic nature of this environment sustains early development from the one‐cell zygote to blastocyst, and is reciprocally influenced by the embryo at each embryonic stage. Focusing on embryo metabolism allowed us to identify how the local environment was deliberately selected to meet the dynamic needs of the preimplantation embryo, and helped reveal approaches to improve the in vitro culture of human embryos for improved implantation rates and pregnancy outcome.     

Oxygen consumption and energy metabolism of the early mouse embryo

Oxygen consumption of preimplantation and early postimplantation mouse embryos has been measured using a novel noninvasive ultramicrofluorescence technique, based on an oil-soluble, nontoxic quaternary benzoid compound pyrene, whose fluorescence is quenched in the presence of oxygen. Pyruvate and glucose consumption, lactate production, and glycogen formation from glucose were also measured. Preimplantation mouse embryos of the strain CBA/Ca × C57BL/6 were cultured in groups of 10–30 in 2 μl of modified M2 medium containing 1 mmol l⁻¹ glucose, 0 mmol l⁻¹ lactate, and 0.33 mmol l⁻¹ pyruvate, for between 4–6 hr. Day 6.5 and 7.5 embryos were cultured singly in 40 μl M2 medium for between 2–3 hr. Oxygen consumption was detected at all stages of development, including, for the first time, in the early postimplantation embryo. Consumption remained relatively constant from zygote to morula stages before increasing in the blastocyst and day 6.5–7.5 stages. When expressed as QO₂ (μl/mg dry weight/hr), oxygen consumption was relatively constant from the one-cell to morula stages before increasing sharply at the blastocyst stage and declining to preblastocyst levels on days 6.5 and 7.5. Pyruvate was consumed during preimplantation stages, with glucose uptake undetectable until the blastocyst stage. Glucose was the main substrate consumed by the 6.5 and 7.5 day embryo. The proportions of glucose accounted for by lactate appearance were 81%, 86%, and 119% at blastocyst, day 6.5, and day 7.5 stages, respectively. The equivalent figures for glucose incorporated into glycogen were 10.36%, 0.21%, and 0.19%, respectively. The data are consistent with a switch from a metabolism dependent on aerobic respiration during early preimplantation stages to one dependent on both oxidative phosphorylation and aerobic glycolysis at the blastocyst stage, a pattern which is maintained on days 6.5 and 7.5. Our technique for measuring oxygen consumption may have diagnostic potential for selecting viable embryos for transfer following assisted conception techniques in man and domestic animals. © 1996 Wiley-Liss, Inc.     

Oxidative phosphorylation-dependent and -independent oxygen consumption by individual preimplantation mouse embryos

The self-referencing electrode technique was employed to noninvasively measure gradients of dissolved oxygen in the medium immediately surrounding developing mouse embryos and, thereby, characterized changes in oxygen consumption and utilization during development. A gradient of depleted oxygen surrounded each embryo and could be detected >50 microm from the embryo. Blastocysts depleted the surrounding medium of 0.6+/-0.1 microM of oxygen, whereas early cleavage stage embryos depleted the medium of only 0.3+/-0.1 microM of oxygen, suggesting a twofold increase in oxygen consumption at the blastocyst stage. Mitochondrial oxidative phosphorylation (OXPHOS) accounted for 60-70% of the oxygen consumed by blastocysts, while it accounted for only 30% of the total oxygen consumed by cleavage-stage embryos. The amount of oxygen consumed by non-OXPHOS mechanisms remained relatively constant throughout preimplantation development. By contrast, the amount of oxygen consumed by OXPHOS in blastocysts is greater than that consumed by OXPHOS in cleavage-stage embryos. The amount of oxygen consumed by one-cell embryos was modulated by the absence of pyruvate from the culture medium. Treatment of one-cell embryos and blastocysts with diamide, an agent known to induce cell death in embryos, resulted in a decline in oxygen consumption, such that the medium surrounding dying embryos was not as depleted of oxygen as that surrounding untreated control embryos. Together these results validate the self-referencing electrode technique for analyzing oxygen consumption and utilization by preimplantation embryos and demonstrate that changes in oxygen consumption accompany important physiological events, such as development, response to medium metabolites, or cell death.     

The development of preimplantation mouse parthenogenones in vitro in absence of glucose: Influence of the maternally inherited components

Mouse preimplantation embryo development is characterized by a switch from a dependence on the tricarboxylic acid cycle pre-compaction to a metabolism based on glycolysis post-compaction. In view of this, the role of glucose in embryo culture medium has come under increased analysis and has lead to improved development of outbred mouse embryos in glucose free medium. Another type of embryo that has proven difficult to culture is the parthenogenetic (PN) mouse embryo. With this in mind we have investigated the effect of glucose deprivation on PN embryo development in vitro. Haploid and diploid PN embryos were grown in medium M16 with or without glucose (M16-G) and development, glycolytic rate, and methionine incorporation rates assessed. Haploid PN and normal embryo development to the blastocyst stage did not differ in either M16 or M16-G. In contrast, although diploid PN embryos formed blastocysts in M16 (28.3%), they had difficulty in undergoing the morula/blastocyst transition in M16-G (7.6%). There was no significant difference in mean cell numbers of haploid PN, diploid PN and normal embryos cultured in M16 and M16-G at the morula and blastocyst stage. Transfer of diploid PN embryos from M16-G to M16 at the four- to eight-cell stage dramatically increased blastocyst development. At the morula stage diploid PN embryos grown in M16-G exhibited a higher glucose metabolism and protein synthesis compared to those grown in M16 and to haploid PN embryos. Difficulties of diploid PN embryos in undergoing the morula/blastocyst transition in absence of glucose infer the existence of a link between the maternally inherited components and the preimplantation embryos dependence on glucose. © 1996 Wiley-Liss, Inc.     

Oxygen regulates amino acid turnover and carbohydrate uptake during the preimplantation period of mouse embryo development

Oxygen is a powerful regulator of preimplantation embryo development, affecting gene expression, the proteome, and energy metabolism. Even a transient exposure to atmospheric oxygen can have a negative impact on embryo development, which is greatest prior to compaction, and subsequent postcompaction culture at low oxygen cannot alleviate this damage. In spite of this evidence, the majority of human in vitro fertilization is still performed at atmospheric oxygen. One of the physiological parameters shown to be affected by the relative oxygen concentration, carbohydrate metabolism, is linked to the ability of the mammalian embryo to develop in culture and remain viable after transfer. The aim of this study was, therefore, to determine the effect of oxygen concentration on the ability of mouse embryos to utilize both amino acids and carbohydrates both before and after compaction. Metabolomic and fluorometric analysis of embryo culture media revealed that when embryos were exposed to atmospheric oxygen during the cleavage stages, they exhibited significantly greater amino acid utilization and pyruvate uptake than when cultured under 5% oxygen. In contrast, postcompaction embryos cultured in atmospheric oxygen showed significantly lower mean amino acid utilization and glucose uptake. These metabolic changes correlated with developmental compromise because embryos grown in atmospheric oxygen at all stages showed significantly lower blastocyst formation and proliferation. These findings confirm the need to consider both embryo development and metabolism in establishing optimal human embryo growth conditions and prognostic markers of viability, and further highlight the impact of oxygen on such vital parameters.     

Glutamine uptake and utilization by preimplantation mouse embryos in CZB medium

At least 71% of CF1 x B6SJLF1/J embryos developed from the 1-cell stage to the blastocyst stage in an optimum glutamine concentration of 1 mM, as long as glucose was present after the first 48 h of culture. Blastocysts raised under these conditions had significantly more cells than did blastocysts raised in CZB medium alone (glutamine present, glucose absent). Embryos raised in vivo accumulated 170-200 fmol glutamine/embryo/h at the unfertilized egg and 1-cell stages with a decline to 145 fmol/embryo/h at the 2-cell stage, followed by sharp increases to 400 and 850 fmol/embryo/h at the 8-cell and blastocyst stages. The presence or absence of glucose in the labelling medium had no effect on glutamine uptake by these embryos. Embryos raised in vitro accumulated 2-3 times more glutamine at stages comparable to those of embryos raised in vivo. In all cases in which 1-cell to blastocyst development in vitro was successful, glucose was present in the culture medium and the incremental uptake of glutamine between the 8-cell stage and the blastocyst stage was approximately 2-fold. This was also the increment for in-vivo raised embryos. When glucose was not present after the first 48 h, the 8-cell to blastocyst glutamine increment was not significant, and development into blastocysts was reduced. The results also show that glutamine can be used as an energy source for the generation of CO2 through the TCA cycle by all stages of preimplantation mouse development, whether raised in vivo or in vitro from the 1-cell stage. Two-cell embryos raised in vivo converted as much as 70% of the glutamine uptake into CO2, consistent with an important role for glutamine in the very earliest stages of preimplantation development. Cultured blastocysts appeared to convert less glutamine and the presence of glucose in the culture medium seemed to inhibit this conversion.     

Analysis of HLA-G in maternal plasma, follicular fluid, and preimplantation embryos reveal an asymmetric pattern of expression

Soluble HLA-G (sHLA-G) secretion by human preimplantation embryos in culture has been associated with successful embryo development, and therefore has potential to serve as a noninvasive marker of embryo viability. We have examined the spatial and temporal expression of HLA-G in embryos of varying developmental competence and the role of maternal factors in human embryonic HLA-G expression. Embryos that reached blastocyst stage on day 5 showed a higher frequency of sHLA-G secretion than those at morula or arrested stages (p < 0.05). There was no significant difference in sHLA-G secretion between normal embryos and those diagnosed as chromosomally abnormal by preimplantation genetic diagnosis. HLA-G detected in maternal plasma and follicular fluid did not appear to correlate with HLA-G expressed in the embryo or embryo supernatants. Confocal microscopy analysis indicated that HLA-G protein expression in embryos was not homogeneous; mostly, it was confined to blastocysts localized on trophectoderm and trophectoderm projections. Single-particle fluorescent imaging analysis of HLA-G on the cell surface of JEG-3 cells showed that HLA-G particles were mostly monomeric, but dimeric and higher order oligomers were also observed. These results suggest that HLA-G play an important role in preimplantation embryo development. However, the observed expression of HLA-G in arrested and chromosomally abnormal embryos indicates that HLA-G testing should be used with caution and in conjunction with conventional methods of embryo screening and selection.     

Glucose and pyruvate metabolism of preimplantation goat blastocysts following in vitro fertilization and parthenogenetic activation

The energy metabolism of preimplantation embryos can be used to predict viability and postimplantation development. Although preimplantation development and mean blastocyst cell numbers of goat in vitro-fertilized (IVF) embryos and chemically activated parthenogenotes are comparable, mammalian parthenogenotes are not viable, with most dying shortly after implantation. The objective of this study was to compare glucose and pyruvate metabolism of IVF goat blastocysts with that of parthenogenetic blastocysts developing from chemically activated oocytes. Embryos derived from IVF and parthenogenotes produced by exposing oocytes to either ionomycin or ethanol followed by 6-dimethylaminopurine (6-DMAP) were cultured in G1.2/G2.2 sequential culture media. Metabolism was determined for individual blastocysts using [5-3H]glucose and [2-14C]pyruvate to determine glycolytic and Kreb's cycle activity, respectively. Data were analyzed by ANOVA. A significantly higher percentage of activated oocytes underwent cleavage and developed to the blastocyst stage compared to IVF oocytes (p < 0.05). There was no significant difference in glucose or pyruvate metabolism between IVF and parthenogenetically activated blastocysts. Mean glucose metabolism through glycolysis was 154.9 +/- 29.1, 130.3 +/- 17.1, and 129 +/- 16.5 pmol/embryo/3 h for IVF, ethanol-activated, and ionomycin-activated blastocysts, respectively. Mean pyruvate metabolism through the Kreb's cycle was 28.1 +/- 8.0, 15.8 +/- 4.2, and 24.4 +/- 4.4 in pmol/embryo/3 h for IVF, ethanol-activated, and ionomycin-activated blastocysts, respectively. Our results suggest that known differences in postimplantation development observed in IVF versus parthenogenetic embryos cannot be attributed to differences in pyruvate or glucose metabolism in the preimplantation blastocysts. Thus, these activation protocols result in embryos capable of appropriate regulation of key metabolic enzymes.     

The influence of insulin on the in vitro development of mouse and bovine embryos

To further investigate the role of insulin during preimplantation embryo development, we compared the effects of insulin on the development of mouse and bovine preimplantation embryos and on cell proliferation during culture in vitro in simplex media. The influence of insulin on the development of mouse zygotes was determined during cultivation in mSOF medium, alone or supplemented with glucose. Similarly, the effects of insulin on the bovine preimplantation embryo development were studied in mSOF medium. The addition of insulin into mSOF medium enhanced significantly the number of cells per mouse blastocyst. Moreover, when mSOF medium was supplemented with insulin and 0.2 mmol x l(-1) glucose, the percentage of hatched blastocysts and the mean cell number of mouse blastocysts were significantly higher. Insulin had no significant effect on the development of bovine embryos, produced by in vitro fertilization of in vitro matured oocytes. Neither the rates of developing embryos nor the mean number of cells in blastocysts were different in comparison with control embryos. Our results suggest that the in vitro development of mouse embryos could be enhanced by the addition of insulin to the culture medium and is further improved by the addition of glucose. In contrast to this our results indicate that insulin has no detectable beneficial effect on the preimplantation development of bovine embryos in mSOF medium.     

Expression and regulation of genes associated with cell death during murine preimplantation embryo development

The newly fertilized preimplantation embryo depends entirely on maternal mRNAs and proteins deposited and stored in the oocyte prior to its ovulation. If the oocyte is not sufficiently equipped with maternally stored products, or if zygotic gene expression does not commence at the correct time, the embryo will die. One of the major abnormalities observed during early development is cellular fragmentation. We showed previously that cellular fragmentation in human embryos can be attributed to programmed cell death (PCD). Here, we demonstrate that the PCD that occurs during the 1-cell stage of mouse embryogenesis is likely to be regulated by many cell death genes either maternally inherited or transcribed from the embryonic genome. We have demonstrated for the first time the temporal expression patterns of nine cell death regulatory genes, and our preliminary experiments show that the expression of these genes is altered in embryos undergoing fragmentation. The expression of genes involved in cell death (MA-3, p53, Bad, and Bcl-xS) seems to be elevated, whereas the expression of genes involved in cell survival (Bcl-2) is reduced. We propose that PCD may occur by default in embryos that fail to execute essential developmental events during the first cell cycle. Mol. Reprod. Dev. 51:243–253, 1998. © 1998 Wiley-Liss, Inc.     

Somatic cell-like features of cloned mouse embryos prepared with cultured myoblast nuclei

Cloning by somatic cell nuclear transfer requires silencing of the donor cell gene expression program and the initiation of the embryonic gene expression program (nuclear reprogramming). Failure to silence the donor cell program could lead to altered embryonic phenotypes. Cloned mouse embryos produced using myoblast nuclei fail to thrive in standard embryo culture media but flourish in somatic cell culture media favored by the donor myoblasts themselves, forming blastocysts at a significant rate, with robust morphologies, high total cell number, and a normal allocation of cells to the inner cell mass in most embryos. Myoblast cloned embryos continue expressing the GLUT4 glucose transporter, which is typically expressed in muscle but not in preimplantation stage embryos. Myoblast clones also exhibit precocious enrichment of GLUT1 at the cell surface. Both myoblast and cumulus cell cloned embryos exhibit enhanced rates of glucose uptake. These observations indicate that silencing of the donor cell genome during cloning either is incomplete or occurs progressively over the course of preimplantation development. As a result, cloned embryos initially exhibit many somatic cell-like characteristics. Tetraploid constructs, which possess a transplanted somatic cell genome plus the oocyte-derived chromosomes, exhibit a more embryonic-like pattern of gene expression and culture preference. We conclude that preimplantation stage cloned embryos have profoundly altered characteristics that are donor cell type specific and that exposure of cloned embryos to standard embryo culture conditions may lead to disruptions in basic homeostasis and inhibition of a range of essential processes including further nuclear reprogramming, contributing to cloned embryo demise.     

Expression of beta-galactosidase in preimplantation ovine and porcine embryos

Knowledge regarding the timing of embryonic expression of the mammalian genome is of relevance for the development of preimplantation diagnostic methods for human genetic diseases. For development of preimplantation diagnosis of lysosomal storage diseases, it will be necessary to know at which embryonic stage the genes for lysosomal enzymes are expressed. In previous studies by other investigators, it has been shown that lysosomal alpha- and beta-galactosidase and beta-glucuronidase in murine embryos increase 50- to 100-fold in activity between the two-cell and late blastocyst stage. We describe here expression of lysosomal beta-galactosidase in preimplantation ovine (two-cell through midblastocyst) and porcine (two-cell through late blastocyst) embryos. Expression of beta-galactosidase in ovine and porcine preimplantation embryos followed a similar rate of increase as that described for murine embryos. Activity of beta-galactosidase increased over 10-fold between the two- to four-cell and midblastocyst stages in ovine embryos, and 300-fold between the two- to four-cell and late blastocyst stages in porcine embryos. Activity expressed on a per cell basis was relatively constant in ovine embryos, as has been described in murine embryos, and increased approximately 5-fold on a per cell basis in porcine embryos. Activity of beta-galactosidase in ovine and porcine embryos initially was greater than 12-fold on a per cell or per embryo basis than in murine embryos evaluated. The knowledge of beta-galactosidase embryonic expression may provide the basis for preimplantation diagnosis of genetic beta-galactosidase deficiency in these species.     

Sex-related growth rate differences in mouse preimplantation embryos in vivo and in vitro

Sex-related growth rate differences in preimplantation mouse embryos were investigated. In experiment I, Day 3 embryos were recovered from reproductive tracts, classified according to developmental stage, and cultured for 24 hr in CZB medium containing glucose. Each embryo was then reclassified and stained for measurement of number of nuclei and finally sexed using the polymerase chain reaction. In experiment II, Day 4 embryos were recovered, classified, stained, and sexed as in experiment I immediately after recovery. Morphologically, there were no differences between the sexes in either of the experiments on Day 4. However, based on number of nuclei, the data showed that in vitro conditions support the development of male embryos to the blastocyst stage compared to female embryos. Furthermore, growth rate differences were observed in vivo on Day 3, as females compacted earlier than males. These results suggest that the increased cell proliferation in cultured male embryos is an artifact caused by the in vitro environment. The variation may be due to sex differences in embryonal energy metabolism during the preimplantation stage. The growth difference implies different in vitro requirements of male and female embryos. © 1995 Wiley-Liss, Inc.     

Effects of oocyte quality, oxygen tension, embryo density, cumulus cells and energy substrates on cleavage and morula/blastocyst formation of bovine embryos

Various factors, such as quality of the oocyte, oxygen tension, embryo density, and kind of energy substrate during in vitro production of embryos may affect the rate of preimplantation embryo development. In the present study we used 12553 bovine oocytes aspirated from slaughterhouse ovaries to evaluate various culture conditions that would increase in vitro production of advanced stages of preimplantation embryos. The morphological quality of the oocyte based on the compactness and number of layers of cumulus cells had significant positive effects on the rates of in vitro maturation, fertilization and development to the morula and blastocyst stages. None of the corona-enclosed or nude oocytes progressed beyond the 8- to 16-cell stage. The level of oxygen (5 or 20%) did not affect the proportion of one-cell embryos undergoing cleavage or progressing to morula and blastocyst stages. The rate of development of one-cell embryos originating from inferior quality oocytes was significantly improved when cultured in groups of 40 instead of 20 embryos per 0.5 mL medium. In the presence of cumulus cells, glucose had beneficial effects on in vitro maturation and subsequent development of IVM-IVF zygotes. The presence of serum improved the rate of in vitro development of one-cell embryos. Minimum Essential Medium supplemented with energy substrates according to the findings of metabolic studies was less effective in supporting in vitro maturation and subsequent development than TCM-199. In conclusion, morphological grading of immature oocytes is an appropriate selection criterion for their developmental ability. Embryo yields from low quality oocytes can be increased by culturing them in large groups. Serum is not essential for in vitro generation of embryos but its addition improves rates of success.     

Amino acid transport regulation and early embryo development

Amino acids are essential components of media utilized to culture fertilized human eggs to the blastocyst stage in vitro. Use of such media has led to a significant increase in the proportion of embryos that implant upon transfer to the uterus and to a decrease in the number that need to be transferred to achieve pregnancy. Little is known about the mechanisms by which amino acids foster development of healthy human blastocysts. Indications are, however, that many of these mechanisms are the same in human and mouse embryos. Both essential and nonessential amino acid transport benefit preimplantation mouse embryo development, albeit at different stages. Nonessential amino acid transport improves development primarily during cleavage, whereas essential amino acid transport supports development of more viable embryos, especially subsequent to the eight-cell stage. This review discusses likely mechanisms for these beneficial effects.     

Glucose and phosphate inhibit respiration and oxidative metabolism in cultured hamster eight-cell embryos: evidence for the "crabtree effect"

The development of hamster eight-cell embryos is inhibited by glucose in culture medium containing inorganic phosphate (Pi). This is hypothetically attributed to the "Crabtree effect," in which enhanced glycolysis inhibits respiratory activity and oxidative metabolism. To examine this hypothesis, oxygen consumption of hamster eight-cell embryos was measured using a microelectrode. A two- to three-fold decrease in oxygen consumption was observed in embryos cultured with glucose and Pi. Oxidizable substrates and intermediates of the Krebs cycle supported embryo development only in the absence of glucose and Pi; Krebs cycle inhibitors (fluoroacetate and arsenite) arrested embryo development. Under anaerobic conditions, pyruvate and lactate did not support embryo development. Inhibition of both respiration and oxidative activity in cultured hamster embryos by glucose and Pi is consistent with the existence of a Crabtree effect and indicates that the metabolic properties of preimplantation embryonic cells differ markedly from those of most somatic cells and resemble some cancer cells.