Introduction
It is now well established that the risk of developing cardiovascular and metabolic disease in adulthood may stem not only from lifestyle factors and genetic predisposition, but can also be attributed to the nature of development in utero. Retrospective epidemiological observations made by Barker and colleagues in the late 1980s identified a causal association between sub-optimal foetal growth and an increased propensity for cardiovascular disease in later life.Reference Barker and Osmond1, Reference Barker, Osmond, Golding, Kuh and Wadsworth2 Since then, similar observations have been reported from several large independent human cohort studies from around the world,Reference de Rooij, Painter and Roseboom3–Reference Ravelli, van der Meulen and Michels6 identifying a link between sub-optimal foetal growth and increased risk of developing type 2 diabetes, hypertension, elevated cholesterol and cardiovascular disease in adulthood. Interestingly, studies have revealed both short and long term effects on development, dependent on the timing and nature of the altered intrauterine environments. Analysis of women who became pregnant during the Dutch Hunger Winter, a period of 5 months famine in the winter of 1944–1945 in Amsterdam, revealed that maternal undernutrition during embryonic or early foetal development induced an enhanced risk of coronary heart disease, increased body mass index, and glucose intolerance in their offspring.Reference de Rooij, Painter and Roseboom3, Reference Painter, de Rooij and Bossuyt7, Reference Ravelli, van der Meulen, Osmond, Barker and Bleker8 However, mothers who became pregnant prior to the famine and experienced malnutrition during the later stages of pregnancy gave birth to children of reduced birth weights but who then became obese in adulthood.Reference Ravelli, van der Meulen, Osmond, Barker and Bleker8, Reference McMillen and Robinson9 Similarly, protein-derived energy levels during early pregnancy have been positively associated with placental and birth weights,Reference Moore and Davies10 whilst low maternal dietary protein intake during late pregnancy results in babies that are thinner at birth.Reference Godfrey, Barker, Robinson and Osmond11
The use of animal models provides an invaluable tool with which it is possible to precisely control and target maternal nutrition to specific gestational periods, given the obvious barriers to such studies in humans. Numerous animal studies have demonstrated that the earliest stages of development display particular sensitivity to the effects of altered maternal nutrition, response to which may influence the pattern of future growth and physiology via several developmental pathways. Early gestational nutrient restriction in sheep alters cardiovascular function and circulating cortisol levels in the foetus,Reference Gardner, Pearce and Dandrea12, Reference Gardner, Van Bon and Dandrea13 and results in the hypertrophy of the cardiac muscle in adult offspring.Reference McMillen and Robinson9, Reference Cleal, Poore and Boullin14, Reference Gluckman, Hanson and Beedle15 Our own studies have revealed that whilst maternal low protein diet (LPD) given throughout gestation in the mouse induces hypertension within the adult offspring, targeting the LPD exposure exclusively to the preimplantation period exacerbates the effect, inducing a spectrum of alterations in postnatal cardiovascular and behavioural phenotype.Reference Watkins, Ursell and Panton16 Similarly, in the rat model, maternal protein restriction exclusive to the preimplantation period is sufficient to cause adult hypertension.Reference Kwong, Wild, Roberts, Willis and Fleming17 In response to studies like these, attention is now beginning to focus on understanding the molecular mechanisms by which environmental conditions can affect long-term development, such as epigenetic change within the genome of the developing offspring.
Epigenetics and developmental programming
Epigenetic modifications result in specific packaging of chromatin to generate either a transcriptionally permissive or repressed state at a given locus, which can vary in a temporal or tissue-specific manner. As such, these modifications can influence the gene expression status of specific regions of DNA, without altering the underlying nucleotide sequence, thus providing a causal mechanism linking a single genotype with multiple phenotypic outcomes.Reference Waddington18 In mammals, these modifications include the methylation of DNA at cytosine residues in CpG dinucleotides, addition of methyl, phospho or acetyl groups to specific residues within histone tails and the recruitment of multiprotein chromatin remodelling complexes.Reference Kim, Samaranayake and Pradhan19 During both gametogenesis and preimplantation development the mammalian genome undergoes an extensive reorganisation of epigenetic marks, in order to establish the appropriate patterns for differentiation and tissue-specific gene expression.Reference Santos and Dean20 Investigations into environmentally induced epigenetic changes during development frequently focus on imprinted genes, wherein parent-specific expression is achieved by epigenetic silencing of one allele and monoallelic expression subsequently occurs from the active allele. However, genes expressed in a tissue-specific manner and regions of non-coding repetitive DNA, both of which are frequently epigenetically modified, are just as likely to be influenced by environmental factors during development.Reference Thurston, Lucas, Allegrucci, Steele and Young21In vitro or in vivo environmental conditions may feasibly impact on the epigenetic remodelling which takes place during germline and early embryonic development, thus altering the methylation status of gene promoters and potentially resulting in downstream alterations in their expression and implications for offspring long-term health.Reference Kwong, Miller and Ursell22, Reference Young, Fernandes and McEvoy23
Gametogenesis
Mammalian gametogenesis is a highly orchestrated series of events, resulting in the differentiation and production of highly specialized, developmentally competent haploid gametes. However, despite the critical function of gametogenesis in reproduction and development, the mechanisms regulating this process are at present poorly defined. Human and animal gametes are routinely subjected to non-physiological conditions both in vitro and in vivo, despite our limited knowledge about their sensitivity to such manipulations and the potential long-term impacts on the resultant offspring.
Mammalian oogenesis has been shown to display sensitivity to its immediate environment, such that gene expression patterns within the ovary,Reference Martin, Pearson and Brenneman24 oocyteReference Watson, De and Caveney25 and resultant blastocystsReference Russell, Baqir, Bordignon and Betts26 can be altered. Both in vitro culture and suboptimal maternal diet during oocyte development and maturation have been shown to impact on embryo developmentReference Wakefield, Lane and Schulz27 and the future health of offspring in several species.Reference Howie, Sloboda, Kamal and Vickers28–Reference Watkins, Wilkins and Cunningham30 In domestic animals, follicular growth and oocyte quality have been shown to be altered by a range of dietary manipulations.Reference Armstrong, McEvoy and Baxter31 Follicle morphology, number and the composition of follicular fluid in ewes and heifers are all affected by maternal dietary energy levels.Reference Boland, Lonergan and O’Callaghan32, Reference O’Callaghan, Yaakub, Hyttel, Spicer and Boland33 Increased intake of dietary protein and urea elevate ammonia levels in follicular fluid and lead to reduced blastocyst development.Reference McEvoy, Robinson, Aitken, Findlay and Robertson34, Reference Sinclair, Kuran, Gebbie, Webb and McEvoy35 In mice, significantly decreased cell numbers were observed in the blastocyst inner cell mass (ICM) after feeding the females with diets of high or low in protein for several weeks prior to fertilization.Reference Mitchell, Schulz, Armstrong and Lane36 These morphological changes occurred in unison with elevated markers of metabolic stress in high protein diet embryos, whilst an apparent reduction of metabolic activity was observed in LPD embryos.Reference Mitchell, Schulz, Armstrong and Lane36 Such changes may have far-reaching consequences for the developing offspring. Maternal periconceptional undernutrition in the sheep increases foetal blood pressure, alters growth dynamics of the fetus and placenta and increases the activation of the foetal hypothalamic-pituitary-adrenal axis.Reference Edwards and McMillen37–Reference McMillen, MacLaughlin, Muhlhausler, Gentili, Duffield and Morrison40 Feeding sheep diets deficient in B vitamins and methionine, which are essential factors for the methyl cycle, during the periconceptional period resulted in offspring who became heavier and fatter than controls, displayed altered immune responses, became insulin-resistant and had elevated blood pressure.Reference Sinclair, Allegrucci and Singh29 These authors also reported wide spread changes in methylation status of the foetal liver genome, demonstrating the interaction between maternal environment and epigenetic modification.Reference Sinclair, Allegrucci and Singh29 While the findings of such studies have resulted from dietary intervention for several weeks preceding conception, the window of oocyte sensitivity has been refined even further to encompass just the final stages of maturation with the demonstration that maternal LPD provided for just 3.5 days prior to fertilisation resulted in offspring displaying elevated blood pressure, impaired vascular responsiveness, altered anxiety-related behaviour and changes in kidney glomerular numbers.Reference Watkins, Wilkins and Cunningham30
One manipulation routinely used in human assisted reproduction and in domestic animal production and conservation, as well as in experimental settings, is the process of superovulation. Large doses of hormones are used to induce the maturation and ovulation of multiple oocytes at the same time. However, this practice has been reported to affect the developmental competence of the embryo in mice,Reference Shi and Haaf41 as well as leading to the appearance of abnormal DNA methylation patterns within embryonic and foetal DNA,Reference Fortier, Lopes, Darricarrere, Martel and Trasler42 and altered DNA methylation patterns in the sperm of male offspring, with subsequent transgenerational inheritance.Reference Stouder, Deutsch and Paoloni-Giacobino43 The establishment of appropriate methylation patterns at imprinted gene loci takes place during meiosis I in the growing oocyte,Reference Allegrucci, Thurston, Lucas and Young44 thus the premature recruitment of oocytes by superovulation may result in incomplete establishment of genomic imprints and subsequent alterations in allelic expression of these genes. Temporal differences have been reported in the timing of DNA methylation establishment both at maternally imprinted genes during oocyte maturation,Reference Song, Min, Pan, Shi and Shen45 as well as at paternally imprinted genes during different stages of spermatogenesis,Reference Boyano, Andollo, Zalduendo and Arechaga46 thus the specific timing of adverse environmental stimuli may affect different genes and result in varied phenotypic outcomes. In vitro maturation of mouse, sheep and human oocytes has also been shown to reduce viability and developmental competence,Reference Barnes, Kausche, Tiglias, Wood, Wilton and Trounson47–Reference Mikkelsen, Smith and Lindenberg50 and has been shown to alter foetal growth,Reference Bertolini, Mason and Beam48 again likely due to altering the temporal dynamics of the maturation process and the exposure to non-physiological culture conditions.
Less attention has been focused on the environmental sensitivity of spermatogenesis, however, several studies have shown adverse effects of in vitro and in vivo environments on sperm motility,Reference Polyzos, Schmid, Pina-Guzman, Quintanilla-Vega and Marchetti51, Reference Si, Men, Benson and Critser52 DNA breakageReference Polyzos, Schmid, Pina-Guzman, Quintanilla-Vega and Marchetti53 and apoptosis.Reference Anway, Rekow and Skinner54 These changes have been further associated with alterations in DNA methyltransferase expression and transgenerational inheritance of altered phenotype.Reference Anway, Rekow and Skinner54, Reference Lucas and Fleming55 The impact of maternal gestational nutritional status has also been highlighted as being fundamental for successful spermatogenesis. In rats, maternal dietary vitamin B12 deficiency results in defects of the testis development and spermatogenesis in male offspring.Reference Watanabe, Ebara and Kimura56 The subsequent feeding of a diet sufficient in B12 to male offspring, following exposure to a maternal vitamin B12 deficient diet, did lead to some recovery in seminiferous tubule development and spermatogenesis, although still not comparable to control animals.Reference Watanabe, Ebara and Kimura56In vitro organ culture experiments have also demonstrated specific windows of oestrogen-sensitivity in rat testicular development,Reference Delbes, Duquenne, Szenker, Taccoen, Habert and Levacher57 wherein cell proliferation is reduced specifically in testicular samples taken during foetal but not neonatal development, whilst inhibition of testosterone production is suppressed continually by oestrogen exposure.Reference Delbes, Duquenne, Szenker, Taccoen, Habert and Levacher57
The apparent sensitivity of the maturing gametes to environmental factors clearly has implications for the success of embryonic and foetal development, as well as for the health of the resulting adult. However, this sensitivity is certainly not limited to the gametes and post-fertilisation environment may be equally as important to the future phenotype and disease propensity.
Preimplantation embryo development
The relatively short developmental time from fertilisation to implantation encompasses several critical events. These include the first cleavage division, the timing of which can be an indicator of developmental competence;Reference Lonergan, Khatir, Piumi, Rieger, Humblot and Boland58 a switch in the regulation of development from maternally stored transcripts and proteins to the activation of the embryonic genome; and embryo compaction at the morula stage which permits the formation of the blastocyst. At the blastocyst stage, the embryo is composed of two distinct cell populations; the outer polarised epithelial trophectoderm (TE) cells and the inner pluripotent non-polar ICM cells. From these two lineages the chorioallantoic placenta and foetal tissues are derived respectively. The establishment of differential DNA methylation and histone tail modifications of the ICM and TE lineages marks a major differentiation landmark in the developing embryo, and the allocation of cells to these lineages can be predicted by the asymmetry of histone modifications within the chromatin of blastomeres during the cleavage stages.Reference Thurston, Lucas, Allegrucci, Steele and Young59, Reference Torres-Padilla60 Manipulation of the extent of DNA methylation in mouse embryonic stem (ES) cells, which are derived from the ICM, has been shown to be capable of influencing cell fate decisions. ES cells deficient in the maintenance DNA methyltransferase, Dnmt1, and thus hypomethylated, have been reported to differentiate readily into trophoblast derivatives in association with a loss of methylation at the Elf5 promoter region.Reference Ng, Dean and Dawson61 Since Dnmt1 null embryos fail to develop past E8.5, in association with a failure to differentiate during gastrulation,Reference Li, Bestor and Jaenisch62 the importance of establishing appropriate methylation patterns during early embryonic development cannot be underestimated.
During preimplantation development, changing metabolic requirements and sensitivity to the stimulatory effects of essential and non essential amino acids, as well as the expression of different growth factors and their receptors, occur in a spatial and temporal fashion.Reference Gardner, Lane, Stevens and Schoolcraft63–Reference Martin and Leese65 The maternal reproductive tract responds to these changing requirements by providing oviductal and uterine environments that match precisely the needs of the developing embryo,Reference Casslen66, Reference Gardner, Lane, Calderon and Leeton67 with oviductal culture resulting in a embryonic gene expression profile almost identical to that of in vivo derived blastocystsReference Corcoran, Fair and Park68 and culture in oviductal fluid improving cleavage and blastocysts rates.Reference Lloyd, Romar and Matas69 It is therefore imperative that a better understanding of this intimate maternal-embryo interaction and the implications of manipulating are gained in order to minimise the potential for negative consequences during pregnancy and post-gestational life.
Environmental manipulation in vitro and in vivo
In vitro culture and embryo manipulation are widely used for treatment of human infertility, derivation of ES cell lines and the preservation of endangered or important domestic and wild animal species. However, despite advances in these techniques and the development of enhanced culture media, in vitro culture conditions still remain relatively sub-optimal in relation to the in vivo environment when examining blastocyst developmental rates, ICM and TE cell numbers, gene expression and post-transfer viability.Reference Fernandez-Gonzalez, Moreira and Bilbao70–Reference Lonergan, Rizos and Kanka73
Variations in culture media components have been shown to impact on the developmental potential of mouse embryos in vitro in several studies in a strain specific manner.Reference Dandekar and Glass74 The addition of insulin, insulin-like growth factor-1 and albumin to the media stimulates mouse embryo metabolism, increases the number of cells within the ICM and stimulates foetal growth post transfer.Reference Harvey and Kaye75–Reference Mihalik, Rehak and Koppel77 Embryo culture in the presence of granulocyte-macrophage colony-stimulating factor (GM-CSF), when in the absence of albumin, can enhance blastocyst cell numbers and ICM development.Reference Karagenc, Lane and Gardner78 The addition of GM-CSF to the culture medium has also been shown to prevent the negative impacts on adverse foetal, placental and postnatal development associated with embryo culture and transfer and to increase the surface area of placental trophoblast available for nutrient exchange.Reference Sjoblom, Roberts, Wikland and Robertson79 Changes in metabolic and signalling activity in response to in vivo or in vitro environments are more than likely to be accompanied by changes in gene expression patterns. Comparison of bovine blastocysts derived from in vitro or in vivo environments have revealed that culture induces significant changes in transcript levels of genes involved in a wide array of key developmental pathways such as transcription and translation, cellular metabolism,Reference Lloyd, Romar and Matas69 nutrient transport, apoptosis and gap junction formation.Reference Rizos, Gutierrez-Adan, Perez-Garnelo, de la, Boland and Lonergan80 Interestingly, these changes occur in a temporal fashion,Reference Wrenzycki, Herrmann, Carnwath and Niemann81, Reference Lonergan, Rizos and Gutierrez-Adan82 indicating that specific pathways may vary in the timing of their sensitivity to the environment. Survival rates following embryo transfer have also been shown to be compromised following the culture or manipulation of bovine embryos.Reference Dandekar and Glass74, Reference Mansouri-Attia, Sandra and Aubert83 The bovine endometrium has been shown to display a plasticity in its gene expression patterns in response to embryos derived from somatic-cell-nuclear-transfer (SCNT) or in vitro fertilisation (IVF),Reference Mansouri-Attia, Sandra and Aubert83 suggesting that the influence of the in vitro culture environment may persist even after the culture period. Furthermore, the inclusion of non-physiological levels of some nutrients, such as methyl-folate cycle cofactors, may affect the establishment and maintenance of epigenetic modifications such as DNA methylation during embryo cultureReference Steele, Allegrucci and Singh84 with consequences likely to extend into foetal and adult life.
There is now increasing evidence that the effects of in vitro culture and embryo manipulation do in fact extend far beyond preimplantation and foetal development, altering perinatal and adult development. In cattle and sheep, the phenomenon of large offspring syndrome has been recorded following a range of embryo environment manipulations, including in vitro culture in the presence of serum, cloning by SCNT, exposure to a maternal uterine environment high in progesterone, and granulosa cell co-culture.Reference Sinclair, McEvoy and Maxfield85–Reference Yang, Smith, Tian, Lewin, Renard and Wakayama87 As a result a range of foetal and postnatal consequences including enhanced foetal growth and weight at birth, alterations in organ sizing and elevated rates of sudden perinatal death have been reported.Reference Sinclair, McEvoy and Maxfield85, Reference Thompson, Gardner, Pugh, McMillan and Tervit88, Reference Wells, Misica and Tervit89 Studies from our own laboratory have demonstrated that both prolonged (two-cell to blastocyst) or relatively short-term (approximately 2 h immediately prior to blastocyst transfer) periods of in vitro culture significantly elevated offspring postnatal systolic blood pressure.Reference Watkins, Platt and Papenbrock72 Significant changes in the activity of key homoeostatic enzymes regulating both cardiovascular (serum angiotensin converting enzyme) and metabolic (hepatic phosphoenolpyruvate carboxykinase; PEPCK) pathways were also observed in female offspring.Reference Watkins, Platt and Papenbrock72 Significant changes in postnatal memory and growth profiles have also been demonstrated following the in vitro culture and transfer of mouse embryos.Reference Fernandez-Gonzalez, Moreira and Bilbao70, Reference Ecker, Stein and Xu90 Recently, it has been demonstrated that the effects of embryo culture can even perpetuate into a second generation. Significant changes in postnatal growth and organ sizes following mouse embryo culture and transfer were transmitted to the F2 generation, despite no further manipulations being performed.Reference Mahsoudi, Li and O’Neill91 This suggests that a ‘memory’ of the environmental change experienced by the F0 dam is retained in the genome of the offspring, likely by an epigenetic change within the germline.
With approximately 1%–3% of births in developed countries involving some form of assisted reproduction technology (ART),Reference Manipalviratn, DeCherney and Segars92 an increasing number of long-term follow-up studies into the health and development of ART children are being conducted. Increased incidences of prematurity and low birth weight have been widely associated with ART, but often have been attributed to a higher frequency of multiple births following the transfer of several embryos.Reference Fauser, Devroey and Macklon93, Reference Steel and Sutcliffe94 However, there is evidence to suggest that even singleton ART children are at risk from preterm birth and low birth weight.Reference Helmerhorst, Perquin, Donker and Keirse95 Some studies have reported slight but non-significant specific neuro-developmental conditions in ART childrenReference Belva, Henriet, Liebaers, Van, Celestin-Westreich and Bonduelle96–Reference Ponjaert-Kristoffersen, Bonduelle and Barnes98 and several studies have shown an association between IVF and an increased incidence of cerebral palsy.Reference Hvidtjorn, Schieve and Schendel99, Reference Stromberg, Dahlquist, Ericson, Finnstrom, Koster and Stjernqvist100 Additionally, elevated systolic and diastolic blood pressure, and elevated fasting glucose levels have been reported in pre-pubertal IVF children, independent of early life factors or parental characteristics.Reference Ceelen, van Weissenbruch, Vermeiden, van Leeuwen and Delemarre-van de Waal101
In humans, the use of ART has also been associated with increased incidence of several imprinting disorders, including Beckwith-Wiedemann syndrome, Angelman syndrome and Silver-Russel syndrome.Reference Helmerhorst, Perquin, Donker and Keirse95 Abnormal DNA methylation profiles at a cluster of imprinted genes on human chromosome 11p15.5, in association with loss of imprinted mono-allelic expression, have been reported to associate with the incidence of these syndromes.Reference Helmerhorst, Perquin, Donker and Keirse95 However, a recent study has shown altered methylation status of the KvDMR imprinting control region (ICR) at this locus even in clinically normal children after assisted conception.Reference Gomes, Huber, Ferriani, maral Neto and Ramos102 Investigation into the impact of ICR hypomethylation on gene expression in these children would be needed to identify a functional outcome of the altered methylation status, and to tease out the possible role of specific in vitro manipulations. Importantly, imprinted genes are not the only genomic loci that demonstrate epigenetic sensitivity during early development.Reference Thurston, Lucas, Allegrucci, Steele and Young103 The Agouti viable yellow (A vy) allele in mice is one example whereby a repetitive element situated within a gene (in this case controlling coat colour) can be regulated by DNA methylation. Environmental conditions, such as diet or embryo culture, have been used to experimentally alter the methylation status of this locus,Reference Morgan, Jin, Li, Whitelaw and O’Neill104, Reference Waterland and Jirtle105 providing a visual read-out of methylation change via the coat colour of the mouse. In vitro mouse embryo manipulations have been shown to reduce methylation at the A vy locus, shifting coat colours from pseudo-agouti (brown) to mottled or yellow coats, with a significant increase in proportion of animals with yellow or mottled coats observed with increasing duration/extent of manipulation.Reference Morgan, Jin, Li, Whitelaw and O’Neill104 Maternal dietary manipulation can also affect the methylation status at the A vy locus. Supplementation of maternal diet with methyl cycle donors and cofactors, including choline, folic acid and vitamin B12, increases methylation levels resulting in a coat colour shift in offspring towards the pseudo-agouti brown associated with CpG methylation, and thus silencing, of the IAP repetitive element within the A vy gene.Reference Waterland and Jirtle105
The long-term impact of maternal nutrition during preimplantation development on the programming of offspring development has been extensively covered in recent studies.Reference Lucas and Fleming55, Reference Watkins, Papenbrock and Fleming106, Reference Watkins and Fleming107 The large majority of studies examining the role of maternal diet and its effects on subsequent foetal and postnatal health, have centred on the use of diets altered for protein content, and have typically been coupled with the use of rodent models. In studies from our laboratory, maternal LPD diet targeted exclusively to the rodent preimplantation period results in a range of embryonic, foetal and postnatal consequences, including altered blastocyst cell numbers, altered foetal liver gene expression of 11β-hydroxysteroid dehydrogenase type 1 (Hsd11b1) and phosphoenolpyruvate carboxykinase (Pepck, Pck1), postnatal hypertension increased patterns of anxiety-related behaviour in an open field test, and alter the relative sizes of organs.Reference Watkins, Ursell and Panton16, Reference Kwong, Wild, Roberts, Willis and Fleming108, Reference Kwong, Miller and Wilkins109 Interestingly, we further identified a compensatory response mediated by the visceral yolk sac in response to maternal gestational LPD. We observed that endocytosis of maternal proteins mainly via the multi-ligand megalin low-density lipoprotein receptor-related protein 2 receptor was increased in response to LPD.Reference Watkins, Ursell and Panton16 This elevation of yolk sac histiotrophic function appeared subject to developmental plasticity, providing a mechanism by which embryos could respond to maternal undernutrition in order to protect foetal growth and postnatal competive fitness. In the sheep, maternal undernutrition from days 0–30/31 of pregnancy increases circulating plasma cortisol levels in female lambs and altered cardiovascular function of lambs at 1 year of age.Reference Gardner, Pearce and Dandrea12, Reference Gardner, Van Bon and Dandrea13
Conclusions
There is now a growing body of evidence that factors such as maternal nutrition, hormonal imbalance, in vitro culture conditions and other environmental factors can have a profound effect on gamete quality and subsequent embryo development (Fig. 1). This has clear implications and relevance for the Developmental Origins of Health and Disease hypothesis. It is also becoming increasingly apparent that changes to the normal pattern of early development can result in long-term changes in offspring health and physiology. It is therefore imperative that a better understanding of the underlying mechanisms and signalling pathways involved is established. Whilst the mechanisms and pathways involved in postnatal programming of adult health following adverse preimplantation embryo environments are likely to be complex and involve several interacting pathways, epigenetic changes occurring during germline and preimplantation development may be one critical mechanism through which the early-life environment can affect subsequent disease susceptibility, and even result in the transmission of environmentally induced phenotypic change between generations. However, further studies are still required for a better understanding of how the mammalian embryo responds to its immediate environment and the potential long-term programming effects that can occur. Whilst our understanding of certain physiological systems are well characterised (cardiovascular), areas such as digestive function, the immune system and cognative functions are understood to a lesser extent. It is also essential that further development of existing animals models occur if we are to define the limits of the early mammalian embryo, and design improved conditions for the culture and manipulation of human embryos. As most ART children are still relatively young, a wider analysis of postnatal phenotype focusing on cardiovascular and metabolic profiles, as well as substantiation of those effects already well characterised from animal studies is required in these children.
Fig. 1 (Colour online) A summary diagram detailing the impact of environmental conditions on gametogenesis and development, and the short- and long term consequences which may be observed.
Acknowledgements
This work was supported by the BBSRC (grant number BBF007450), the National Institutes of Health, USA as part of the NICHD National Cooperative Program on Female Health and Egg Quality under cooperative agreement (grant number U01 HD04435) and the Gerald Kerkut Charitable Trust.
