Regulation of mouse preimplantation development: differential effects of CZB medium and Whitten's medium on rates and patterns of protein synthesis in 2-cell embryos

CRISPR-based Genome Editing of a Diurnal Rodent, Nile Grass Rat ( Arvicanthis niloticus)

Diurnal and nocturnal mammals have evolved distinct pathways to optimize survival for their chronotype-specific lifestyles. Conventional rodent models, being nocturnal, may not sufficiently recapitulate the biology of diurnal humans in health and disease. Although diurnal rodents are potentially advantageous for translational research, until recently, they have not been genetically tractable. Here, we address this major limitation by demonstrating the first successful CRISPR genome editing of the Nile grass rat ( Arvicanthis niloticus ), a valuable diurnal rodent. We establish methods for superovulation; embryo development, manipulation, and culture; and pregnancy maintenance to guide future genome editing of this and other diurnal rodent species.

Genotypic divergence in mouse oocyte transcriptomes: possible pathways to hybrid vigor impacting fertility and embryogenesis

What causes hybrid vigor phenotypes in mammalian oocytes and preimplantation embryos? Answering this question should provide new insight into determinants of oocyte and embryo quality and infertility. Hybrid vigor could arise through a variety of mechanisms, many of which must operate through posttranscriptional mechanisms affecting oocyte mRNA accumulation, stability, translation, and degradation. The differential regulation of such mRNAs may impact essential pathways and functions within the oocyte. We conducted in-depth transcriptome comparisons of immature and mature oocytes of C57BL/6J and DBA/2J inbred strains and C57BL/6J × DBA/2J F1 (BDF1) hybrid oocytes with RNA sequencing, combined with novel computational methods of analysis. We observed extensive differences in mRNA expression and regulation between parental inbred strains and between inbred and hybrid genotypes, including mRNAs encoding proposed markers of oocyte quality. Unique BDF1 oocyte characteristics arise through a combination of additive dominance and incomplete dominance features in the transcriptome, with a lesser degree of transgressive mRNA expression. Special features of the BDF1 transcriptome most prominently relate to histone expression, mitochondrial function, and oxidative phosphorylation. The study reveals the major underlying mechanisms that contribute to superior properties of hybrid oocytes in a mouse model.

The role of animal studies in supporting human assisted reproductive technology

Although average success rates of human IVF have increased progressively during the past two decades, the efficiency of this technique, based on each embryo produced or transferred, is still low. High success rates are usually achieved by transferring several embryos to the patient, which is often associated with multiple pregnancies. The quality of in vitro produced embryos is a major area that needs attention. Because there is no in vivo database for human embryos, the properties of normal embryos are not known, and so it is difficult to know how to improve quality and viability. In addition, selection of the most viable embryos for transfer is a rather subjective process. The origins of human assisted reproductive technology (ART) are based on animal ART; however, the two areas of research (animal and human ART) appear to have become disconnected. Re-examination of progress in animal ART could help improve human embryo quality and thereby assist efforts to sustain high pregnancy rates with only one or two embryos transferred. Some key areas in which animal ART can help guide progress in human ART are discussed.

Delay of ZGA initiation occurred in 2-cell blocked mouse embryos

One-cell mouse embryos from KM strain and B6C3F1 strain were cultured in M16 medium, in which 2-cell block generally occurs. Embryos of KM strain exhibited 2-cell block, whereas B6C3F1 embryos, which are regarded as a nonblocking strain, proceeded to the 4-cell stage in our culture condition. It is often assumed that the block of early development is due to the failure of zygotic gene activation (ZGA) in cultured embryos. In this study we examined protein synthesis patterns by two-dimensional gel electrophoresis of [35S] methionine radiolabeled 2-cell embryos. Embryos from the blocking strain and the nonblocking strain were compared in their development both in vitro and in vivo. The detection of TRC expression, a marker of ZGA, at 42 h post hCG in KM embryos developed in vitro suggested that ZGA was also initiated even in the 2-cell arrested embryos. Nevertheless, a significant delay of ZGA was observed in KM strain as compared with normally developed B6C3F1 embryos. At the very beginning of major ZGA as early as 36 h post hCG, TRC has already been expressed in B6C3F1 embryos developed in vitro and KM embryos developed in vivo. But for 2-cell blocked KM embryos, TRC was still not detectable even at 38 h post hCG. These evidences suggest that 2-cell-blocked embryos do initiate ZGA, and that 2-cell block phenomenon is due not to the disability in initiating ZGA, but to a delay of ZGA.

The essential role of RI alpha in the maintenance of regulated PKA activity

Cloning of the individual regulatory (R) and catalytic (C) subunits of the cAMP-dependent protein kinase (PKA) and expression of these subunits in cell culture have provided mechanistic answers about the rules for PKA holoenzyme assembly. One of the central findings of these studies is the essential role of the RI alpha regulatory subunit in maintaining the catalytic subunit under cAMP control. The role of RI alpha as the key compensatory regulatory subunit in this enzyme family was confirmed by gene knockouts of the three other regulatory subunits in mice. In each case, RI alpha has demonstrated the capacity for significant compensatory regulation of PKA activity in tissues where the other regulatory subunits are expressed, including brain, brown and white adipose tissue, skeletal muscle, and sperm. The essential requirement of the RI alpha regulatory subunit in maintaining cAMP control of PKA activity was further corroborated by the knockout of RI alpha in mice, which results in early embryonic lethality due to failed cardiac morphogenesis. Closer examination of RI alpha knockout embryos at even earlier stages of development revealed profound deficits in the morphogenesis of the mesodermal embryonic germ layer, which gives rise to essential structures including the embryonic heart tube. Failure of the mesodermal germ layer in RI alpha knockout embryos can be rescued by crossing RI alpha knockout mice to C alpha knockout mice, supporting the conclusion that inappropriately regulated PKA catalytic subunit activity is responsible for the phenotype. Isolation of primary embryonic fibroblasts from RI alpha knockout embryos reveals profound alterations in the actin-based cytoskeleton, which may account for the failure in mesoderm morphogenesis at gastrulation.

Nuclear-cytoplasmic "tug of war" during cloning: effects of somatic cell nuclei on culture medium preferences of preimplantation cloned mouse embryos

Cloning by somatic cell nuclear transfer is critically dependent upon early events that occur immediately after nuclear transfer, and possibly additional events that occur in the cleaving embryo. Embryo culture conditions have not been optimized for cloned embryos, and the effects of culture conditions on these early events and the successful initiation of clonal development have not been examined. To evaluate the possible effect of culture conditions on early cloned embryo development, we have compared a number of different culture media, either singly or in sequential combinations, for their ability to support preimplantation development of clones produced using cumulus cell nuclei. We find that glucose is beneficial during the 1-cell stage when CZB medium is employed. We also find that potassium simplex optimized medium (KSOM), which is optimized to support efficient early cleavage divisions in mouse embryos, does not support development during the 1-cell or 2-cell stages in the cloned embryos as well as other media. Glucose-supplemented CZB medium (CZB-G) supports initial development to the 2-cell stage very well, but does not support later cleavage stages as well as Whittten medium or KSOM. Culturing cloned embryos either entirely in Whitten medium or initially in Whittens medium and then changing to KSOM at the late 4-cell/early 8-cell stage produces consistent production of blastocysts at a greater frequency than using CZB-G medium alone. The combination of Whitten medium followed by KSOM resulted in an increased number of cells per blastocyst. Because normal embryos do not require glucose during the early cleavage stages and develop efficiently in all of the media employed, these results reveal unusual culture medium requirements that are indicative of altered physiology and metabolism in the cloned embryos. The relevance of this to understanding the kinetics and mechanisms of nuclear reprogramming and to the eventual improvement of the overall success in cloning is discussed.

The mechanism of action of coculture on embryo development in the mouse model: direct embryo-to-cell contact and the removal of deleterious components

OBJECTIVE

To elucidate the mechanism for the mode of action of coculture by the use of a coculture system for mouse one-cell embryos with human oviductal epithelial cells.

DESIGN

Prospective, controlled in vitro experimental study.

SETTING

Academic research laboratory.

ANIMAL(S)

Female ICR strain mice aged between 6 and 8 weeks.

INTERVENTION(S)

Flushed one-cell embryos were cultured in human tubal fluid medium alone (control), in coculture system with human oviductal cells, in five kinds of conditioned media, and in a contactless coculture system using a cell-culture insert.

MAIN OUTCOME MEASURE(S)

The percentage of the embryos developed to hatching blastocyst stage and the level of superoxide anion in the supernatant from each culture condition.

RESULT(S)

The rates of embryo development to the hatching blastocyst stage were significantly higher in the coculture group (43%) than in the control group (none) (P <.05). The embryo development rate in the control group was similar to that of the embryos in the five kinds of conditioned media. The effects of coculture on embryo development disappeared in the contactless coculture group. The level of superoxide anion was significantly reduced in the coculture group compared to the control group.

CONCLUSION(S)

The present coculture system overcomes the two-cell block in vitro and improves the embryo development. The beneficial effect may be a result of direct cell-to-cell contact between the embryo and helper cells and the removal of deleterious components from medium, rather than a result of embryotrophic factors.

Analysis of glutaminase activity and RNA expression in preimplantation mouse embryos

Glutamine is utilized as an energy substrate in preimplantation mouse embryos. Glutaminase is the enzyme responsible for the conversion of glutamine to glutamic acid, which then enters the trichloro acetic acid (TCA) cycle as alpha-ketoglutarate. Glutaminase enzyme activity was assessed in preimplantation embryos that developed in vivo, and glutaminase RNA expression was examined in embryos that developed in vivo or were cultured in CZB medium to various preimplantation stages between 1-cell and blastocyst. Glutaminase activity in 1-8-cell-stage mouse embryos that developed in vivo ranged from 0.009-0.01 U/mg protein (2.39-2.95 x 10(-7) U per embryo) and increased 3-4 fold to 0.034 U/mg protein (8.13 x 10(-7) U per embryo) at the blastocyst stage. Relative stage-specific expression of glutaminase RNA was assessed by reverse transcription polymerase chain reaction (RT-PCR) in embryos that developed both in vivo and in CZB culture. In vivo, glutaminase RNA was expressed at the 1-cell stage, declined to 23% of 1-cell levels at the early 2-cell stage, and reaccumulated from late 2-cell through blastocyst stage, where it reached a high of 204% of 1-cell levels. CZB-cultured embryos exhibited a similar pattern of developmental RNA expression, declining to 30% of 1-cell levels at the early 2-cell stage, and increasing RNA expression at the blastocyst stage to 191% of the 1-cell level.

Glutamine as a regulator of DNA and protein biosynthesis in human solid tumor cell lines

OBJECTIVE

The transport of glutamine by six different human solid tumor-derived cell lines (e.g., breast, colon, liver) was characterized and the impact of glutamine deprivation on rates of tumor cell proliferation and DNA and protein synthesis was assayed.

SUMMARY BACKGROUND DATA

Glutamine is added routinely to cell culture media and its importance for cellular growth has been established. However, carrier-mediated glutamine transport by solid tumors has not been studied extensively, and the mechanisms by which glutamine contributes to cell growth regulation require further investigation.

METHODS

In a panel of different human solid tumor-derived cells, sodium-dependent glutamine transport was characterized in vitro and rates of cell proliferation, protein and DNA synthesis, as well as thymidine transport, were correlated with glutamine concentrations in the culture media.

RESULTS

In all cells, regardless of tissue origin, sodium-dependent glutamine transport was mediated almost exclusively by a single carrier. There was a range of Michaelis constants (Km) and maximal transport velocities (Vmax) for the glutamine transporter in each cell type, but the amino acid inhibition profiles were nearly identical, consistent with uptake by the System ASC family of transporters. Rates of cell growth, DNA and protein synthesis, and thymidine transport correlated with the glutamine concentration in the culture media, indicating the central role of this amino acid in regulating cellular proliferation.

CONCLUSIONS

These data indicate that glutamine transport by all solid tumors is mediated by the System ASC family of transporters. The variation in Km values suggests that some cancers may be better suited to survive in a low glutamine environment than others. The mechanism by which glutamine supports cell proliferation and regulates cell cycle kinetics involves its modulation of DNA and protein biosynthetic rates.

The microenvironment created by non-blocking embryos in aggregates may rescue blocking embryos via cell-embryo adherent contacts

Under our culture conditions, mouse embryos from the BALB/c inbred mouse strain develop successfully in culture only from the late 2-cell stage onwards (so-called 2-cell block), whether or not EDTA is added to the culture medium. (CBA x C57BL) F2 embryos do not exhibit a 2-cell block. Medium conditioned by culture of non-blocking embryos from the 2-cell to the 8-cell stage did not improve the development of blocking embryos, nor did co-culture of blocking and non-blocking embryos, with or without conditioned medium. On the other hand phytohaemagglutinin (PHA)-assisted aggregation of an early 2-cell BALB/c embryo with five surrounding non-blocking F2 embryos (2-cell or 8-cell) or five BALB/c 8-cell embryos allowed the early 2-cell BALB/c embryos to develop into blastocysts within 72 h. Aggregation of blocking BALB/c 2-cell embryos with each other had no 'rescue' effect. When blocking and non-blocking 2-cell embryos were aggregated together, an integrated blastocyst was formed; but when the early 2-cell BALB/c embryos were aggregated with non-blocking 8-cell embryos, the blocking embryos formed a separate small blastocyst, which nonetheless retained adherent contact with the non-blocking embryos throughout the culture period. Ultrastructural analysis showed that 2-cell embryos aggregated with the aid of PHA form close adherent cell contacts up to several micrometres in length.

Co-culture of the early human embryo: factors affecting human blastocyst formation in vitro

Co-culture systems have been designed to overcome the embryonic developmental arrest observed in vitro in conventional culture media. Oviduct and uterine epithelial cells can sustain embryonic development, as can trophoblastic tissue and transport epithelia of non-genital origin. Its benefits involve neither hormone dependency nor histo-specificity. Fibroblasts do not overcome the developmental arrest in most mammalian species, but whether they do in humans is still unsure. In all systems, the quality of the feeder cells and the co-culture medium are very important. Using the Vero cell line, 60% of human IVF embryos reach the blastocyst stage. The quality of the sperm seems to affect results. We have observed: For 10% of the patients with unexplained fertility, blastocyst stage is not attained; this probably involves a maternal (ovarian) problem. When at least one blastocyst is transferred, the pregnancy rate per transfer is 31%. The implantation rate in pregnant women is higher than after transfer at day 2. After repeated failures of transfer at early stages (2-6 cells), transfer at the blastocyst stage gives high pregnancy rates (40%). This indicates an in vitro selection. There is a strong paternal effect on blastocyst formation: poor quality sperm give lower rates of blastocyst. Co-culture helps to understand treatment failures related to male factors. Around 60% of the patients having spare embryos have had blastocysts frozen. Transfers of frozen-thawed blastocysts give a 20% pregnancy rate and an implantation rate per embryo of 11%. Co-culture is a new tool which has to be carefully evaluated in human IVF programs. It does not impair "a minima" embryo viability and it allows in vitro selection.(ABSTRACT TRUNCATED AT 250 WORDS)

Radical solutions and cultural problems: could free oxygen radicals be responsible for the impaired development of preimplantation mammalian embryos in vitro?

A major obstacle to the study of mammalian development, and to the practical application of knowledge gained from it in the clinic during therapeutic in vitro fertilisation and embryo transfer (IVF-ET), is the propensity of embryos to become retarded or arrested during their culture in vitro. The precise developmental cell cycle in which embryos arrest or delay is characteristic for the species and coincides with the earliest period of embryonic gene expression. Much evidence reviewed here implicates free oxygen radicals (FORs) in the process of arrest. Thus, studies on the development of mouse preimplantation embryos in vitro have shown that (i) FORs are elevated in vitro, but not in vivo, at the time at which embryos become arrested or delayed, (ii) systems for removing reactive oxygen species to limit the formation of hydroxy radicals are present, although they have not yet been assessed quantitatively and may differ qualitatively from those in adult cells, (iii) metabolic and possibly genetic adaptations to oxidative damage are evident, (iv) published procedures for overcoming in vitro arrest are explicable in terms of FOR-mediated damage or responses and (v) the arrest or delay of most embryos in vitro can be reduced or prevented experimentally by addition of metal chelators to limit hydroxy radical formation and lipid hydroperoxidation.

Regulation of zygotic gene activation in the mouse

Zygotic gene activation (ZGA) is the critical event that governs the transition from maternal to embryonic control of development. In the mouse, ZGA occurs during the 2-cell stage and appears to be regulated by the time following fertilization, i.e. a zygotic clock, rather than by progression through the first cell cycle. The onset of ZGA must depend on maternally inherited proteins, and post-translational modification of these maternally derived proteins is likely to play a role in ZGA. Consistent with this prediction is that protein phosphorylation catalyzed by the cAMP-dependent protein kinase is involved in ZGA and that protein synthesis is not required for ZGA. Recent results suggest that ZGA may occur earlier than previously thought, i.e. not during the 2-cell stage, but rather in G2 of the 1-cell embryo. Thus ZGA may comprise a period of minor gene activation in the 1-cell embryo that is followed by a period of major gene activation in the 2-cell embryo. Following ZGA, the expression of constitutively activated genes may require an enhancer.

Effects of media NaCl concentration and osmolarity on the culture of early-stage porcine embryos and the viability of embryos cultured in a selected superior medium

One- and 2-cell porcine embryos were obtained from oviductal flushes and cultured for 96 hours in media with varied osmolarity that resulted from alterations in NaCl and sorbitol content. The viability of experimental embryos cultured to advanced stages was determined by comparison with that of the controls, noncultured embryos transferred to recipient gilts. The data suggest that variation in embryonic development in the experimental media is related to the NaCl concentration rather than to osmolarity. Increased NaCl concentration impairs development of the embryos to the advanced morula/blastocyst stages (P<0.001). There was no difference in the pregnancy rate between the recipients of cultured (45%) and noncultured (57%) embryos on Day 25. There was, however, a higher embryonic survival rate (P<0.05) within the control gilts.

In vitro development of ovine embryos in CZB medium

One- to four-cell embryos were collected from multiparous crossbred ewes and were cultured in vitro for 120 hours in CZB medium. A 2x2 factorial treatment arrangement was used to examine the effects of glucose and ethylenediaminetetraacetic acid (EDTA) on in vitro embryo development. The embryos were examined every 12 hours, and all of the embryos were stained with a DNA-specific fluorochrome after the 120-hour evaluation to enable the counting of cell nuclei. Embryo development was analyzed for cleavage beyond 16 cells as well as for cleavage to at least the compact morula stage based upon both the 120-hour morphological evaluation and nuclear counts. Forty-eight percent of the embryos passed through the in vitro developmental block (i.e., cleaved beyond 16 cells), and 26% developed to 30 or more cells. Neither EDTA nor glucose affected in vitro embryo development based on the nuclear counts.

An improved medium for long-term culture of human embryos overcomes the in vitro developmental block and increases blastocyst formation

OBJECTIVE

To determine if a culture medium, designated CZB after the authors who first described it, which is supplemented with 0.11 mM ethylenediaminetetraacetic acid, 1.0 mM glutamine, 31.30 mM lactate, and 0.27 mM pyruvate and is lacking glucose for the initial stages of culture that overcomes the in vitro two-cell block of mouse embryos, can improve the rate of blastocyst formation of human embryos in long-term cultures and increase the pregnancy rate (PR) when used in a clinical in vitro fertilization (IVF) program.

DESIGN

The study is in two parts. Initially, excess oocytes from IVF and gamete intrafallopian transfer patients were fertilized in vitro and then placed in long-term culture of either CZB plus 10% heat-inactivated human serum (32 zygotes) or Earle's balanced salt solution (EBSS) supplemented with 0.45 mM pyruvate plus 10% human serum (28 zygotes) to determine if CZB medium could enhance in vitro development and increase blastocyst formation when compared with EBSS. Subsequently, CZB or EBSS medium was used for short-term cultures of embryos in a clinical IVF program to determine if the use of CZB could increase the clinical IVF PR.

SETTING

Private practice of one author (M.D.).

PATIENTS

The excess oocytes were donated by couples not wishing to have cryopreservation. In the clinical trial, 49 couples presenting with tubal or male factor infertility and who had three or more fertilized zygotes were randomly assigned to one of the culture media being used.

RESULTS

In long-term cultures, embryos were observed at 42, 66, 90, 114, and 138 hours after fertilization and scored for blastomere number, degree of fragmentation, and developmental arrest. When CZB- and EBSS-cultured embryos were evaluated over 138 hours, there was a significant increase in the number of CZB-cultured embryos reaching the blastocyst stage (56% versus 20%; P less than 0.009) and less fragmentation of CZB-cultured embryos (18.8% versus 50%; P less than 0.01). In short-term cultures, pronuclear stage embryos from patients undergoing IVF were cultured in CZB or EBSS for 24 hours, graded, and then used in embryo transfers. Of the 28 patients assigned to EBSS, 6 became pregnant (21.4%), and of the 21 assigned to CZB, 5 attained pregnancy (23.8%). These results were not significantly different.

CONCLUSIONS

The use of CZB for the long-term culture of human embryos is highly beneficial and increases the rate of blastocyst formation, but its use in an IVF program does not increase the clinical PR.

Stimulatory effect of okadaic acid, an inhibitor of protein phosphatases, on nuclear envelope breakdown and protein phosphorylation in mouse oocytes and one-cell embryos

Treatment of one-cell mouse embryos with okadaic acid (OA), which is an inhibitor of protein phosphatases 1 and 2A, induces a concentration-dependent precocious nuclear envelope breakdown (NEBD) of the pronuclei; at 10 microM okadaic acid, NEBD starts to occur after 1 hr and the embryos become committed to NEBD after about 45 min. Correlated with NEBD is the conversion of a protein of Mr 32,000 (p32) to more highly phosphorylated forms. One-cell embryos cultured continuously in OA-containing medium do not cleave, whereas one-cell embryos incubated for 15-60 min prior to transfer to OA-free medium reveal a time-dependent inhibition in their ability to cleave. OA treatment of oocytes that are arrested from resuming spontaneous maturation by either a phosphodiesterase inhibitor or biologically active phorbol diester results in germinal vesicle breakdown and the maturation-associated changes in the pattern of protein phosphorylation, which include the apparent phosphorylation of p32. Results of these experiments implicate protein phosphatases in the G2 to M transition of the cell cycle in both meiotic and mitotic cells.

Stage-specific expression of a family of proteins that are major products of zygotic gene activation in the mouse embryo

Transcriptional activation of the embryonic genome occurs during the two-cell stage in the mouse embryo and is marked by the synthesis of a set of alpha-amanitin-sensitive proteins of Mr 73,000, 70,000, and 68,000. We have characterized these three proteins by two-dimensional gel electrophoresis of [35S]methionine radiolabeled two-cell embryos. Their isoelectric points range from 6.2 to 6.8 and their synthesis, which can constitute 5-10% of total protein synthesis, is restricted to the two-cell stage. These proteins are not heat shock proteins that have previously been reported as major products of transcriptional activation. Peptide mapping by limited proteolysis indicates that these three proteins are highly related to one another and the results of pulse-chase experiments indicate that they are likely to be degraded by the eight-cell stage. These proteins are nuclear-associated and insoluble in 2% Triton X-100/0.3 M KCl. Although these proteins share some features with somatic lamins--they exhibit solubility properties similar to somatic lamins--they do not cross-react with polyclonal antibodies to either lamins A/C or B, nor do they comigrate with somatic lamins on two-dimensional gels. Additional evidence that these proteins are not lamins is that although treatment of two-cell embryos with okadaic acid, which is an inhibitor of protein phosphatases 1 and 2A, results in precocious nuclear envelope breakdown, the proteins remain insoluble in 2% Triton X-100/0.3 M KCl.

A consideration of the factors which influence and control the viability and developmental potential of the preimplantation embryo

The advent of the new reproductive technologies (including in vitro fertilization) has led to a revolution in the treatment of infertility. It has not yet led to a marked improvement in our understanding of the control of development and of viability of the early embryo. It is the poor viability of embryos, with consequent implantation failure, which is the major limiting factor to successful outcomes. While much of the research in this area has concentrated on strategies of ovulation induction, experimental models have shown that the major cause of reduced embryo viability is due to the actual process of fertilization in vitro and subsequent culture of the preembryo in synthetic culture medium. It is likely that this is due to the absence of critical nutrients or trophic factors of maternal origin and work with co-culture of embryos with somatic cells suggests improved viability can be achieved. Such co-culture is not an option for routine clinical use, however. It is essential therefore to understand by detailed study of the physiology of embryonic development their requirements for optimal development. The empirical approach of comparing different formulations of culture media is unlikely to be successful because of the vast range of parameters to be tested and the large number of pregnancies required to demonstrate a significant improvement in outcome. The strategy that is most likely to be successful in the future, therefore, is the use of appropriate experimental models, such as the developing rodent embryo, to understand the essential physiological changes in the embryo during its development, the control processes in place, and the effect of the embryo's environment on the processes. This will allow the rational design of culture media which can then be rigorously tested for improved outcome. An example of successful application of this approach is the discovery of embryo-derived platelet activating factor (PAF). The production of embryo-derived PAF was first described and validated in the rodent. In the same species it was shown to have an essential role in pregnancy and to act as an autocrine mediator of embryo viability. This fundamental observation in rodents was then confirmed in humans, and recent work has shown that supplementation of culture human embryo media with PAF results in a dramatic increase in their developmental and pregnancy potential. This example should be the first of many such improvements based on a more fundamental understanding of the embryo's developmental requirements.