Morphological and Gene Expression Changes in Cattle Embryos from Hatched Blastocyst to Early Gastrulation Stages after Transfer of In Vitro Produced Embryos

Impact of Mycotoxin Metabolites Deepoxy-Deoxynivalenol and Beta-Zearalenol on Bovine Preimplantation Embryo Development in the Presence of Acetonitrile

The quality of animal feed is increasingly affected by weather conditions, high humidity, and damage to grains, which have led to various mycotoxin-producing moulds. The aim of this study was to determine the effects of the combination of deepoxy-deoxynivalenol and beta-zearalenol on the development of preimplantation bovine embryos, the extent to which the presence of both mycotoxin metabolites affects the development of in vitro cultured bovine embryos, or whether the effect of both toxins enhances embryotoxicity. Ovaries were transported from the abattoir to the laboratory and, after maturation and fertilisation, zygotes were placed in an embryo culture medium (IVC) with different mycotoxin metabolite concentrations diluted in acetonitrile. It was found that the blastocyst rate of cleaved embryos was affected by 1 μL acetonitrile in 400 μL medium (0.25%) compared to the group without acetonitrile. For this reason, it was decided to use acetonitrile as a control group, and the desired mycotoxin metabolite concentrations were diluted in the lowest possible amount of acetonitrile (0.5 μL) that could be accurately added to the study groups. There was no statistical difference when the higher mycotoxin metabolite concentrations were added.

Single-cell transcriptional landscapes of bovine peri-implantation development

Supporting healthy pregnancy outcomes requires a comprehensive understanding of the molecular and cellular programs of peri-implantation development, when most pregnancy failure occurs. Here, we present single-cell transcriptomes of bovine peri-implantation embryo development at day 12, 14, 16, and 18 post-fertilization. We defined the cellular composition and gene expression of embryonic disc, hypoblast, and trophoblast lineages in bovine peri-implantation embryos, and identified markers and pathway signaling that represent distinct stages of bovine peri-implantation lineages; the expression of selected markers was validated in peri-implantation embryos. Using detailed time-course transcriptomic analyses, we revealed a previously unrecognized primitive trophoblast cell lineage. We also characterized conserved and divergence peri-implantation lineage programs between bovine and other mammalian species. Finally, we established cell-cell communication signaling underlies embryonic and extraembryonic cell interaction to ensure proper early development. These data provide foundational information to discover essential biological signaling underpinning bovine peri-implantation development.

HH5 Double-Carrier Embryos Fail to Progress through Early Conceptus Elongation

Massive genotyping in cattle has uncovered several deleterious haplotypes that cause pre-term mortality. Holstein Haplotype 5 (HH5) is a deleterious haplotype present in the Holstein Friesian population that involves the ablation of the Transcription Factor B1 mitochondrial (TFB1M) gene. The developmental stage at which HH5 double-carrier (DC, homozygous) embryos or fetuses die remains unknown and this is a relevant information to estimate the economic losses associated to the inadvertent cross between carriers. To determine if HH5 DC survive to maternal recognition of pregnancy, embryonic day (E)14 embryos were flushed from superovulated carrier cows inseminated with a carrier bull. DC E14 conceptuses were recovered at Mendelian rates but they failed to achieve early elongation, as evidenced by a drastic (>26-fold) reduction in the proliferation of extraembryonic membranes compared with carrier or non-carrier embryos. To assess development at earlier stages, TFB1M knockout (KO) embryos -functionally equivalent to DC embryos- were generated by CRISPR technology and cultured to the blastocyst stage -Day (D)8- and to the early embryonic disc stage -D12-. No significant effect of TFB1M ablation was observed on the differentiation and proliferation of embryonic lineages and relative mtDNA content up to D12. In conclusion, HH5 DC embryos are able to develop to early embryonic disc stage but fail to undergo early conceptus elongation, required for pregnancy recognition.

Developmental progression continues during embryonic diapause in the roe deer

Embryonic diapause in mammals is a temporary developmental delay occurring at the blastocyst stage. In contrast to other diapausing species displaying a full arrest, the blastocyst of the European roe deer (Capreolus capreolus) proliferates continuously and displays considerable morphological changes in the inner cell mass. We hypothesised that developmental progression also continues during this period. Here we evaluate the mRNA abundance of developmental marker genes in embryos during diapause and elongation. Our results show that morphological rearrangements of the epiblast during diapause correlate with gene expression patterns and changes in cell polarity. Immunohistochemical staining further supports these findings. Primitive endoderm formation occurs during diapause in embryos composed of around 3,000 cells. Gastrulation coincides with elongation and thus takes place after embryo reactivation. The slow developmental progression makes the roe deer an interesting model for unravelling the link between proliferation and differentiation and requirements for embryo survival.

Understanding bovine embryo elongation: a transcriptomic study of trophoblastic vesicles

Background: During the process of elongation, the embryo increases in size within the uterus, while the extra-embryonic tissues (EETs) develop and differentiate in preparation for implantation. As it grows, the ovoid embryo transforms into a tubular form first and then a filamentous form. This process is directed by numerous genes and pathways, the expression of which may be altered in the case of developmental irregularities such as when the conceptus is shorter than expected or when the embryo develops after splitting. In bovines, efforts to understand the molecular basis of elongation have employed trophoblastic vesicles (TVs)-short tubular EET pieces that lack an embryo-which also elongate in vivo. To date, however, we lack molecular analyses of TVs at the ovoid or filamentous stages that might shed light on the expression changes involved. Methods: Following in vivo development, we collected bovine conceptuses from the ovoid (D12) to filamentous stages (D18), sectioned them into small pieces with or without their embryonic disc (ED), and then, transferred them to a receptive bovine uterus to assess their elongation abilities. We also grew spherical blastocysts in vitro up to D8 and subjected them to the same treatment. Then, we assessed the differences in gene expression between different samples and fully elongating controls at different stages of elongation using a bovine array (10 K) and an extended qPCR array comprising 224 genes across 24 pathways. Results: In vivo, TVs elongated more or less depending on the stage at which they had been created and the time spent in utero. Their daily elongation rates differed from control EET, with the rates of TVs sometimes resembling those of earlier-stage EET. Overall, the molecular signatures of TVs followed a similar developmental trajectory as intact EET from D12-D18. However, within each stage, TVs and intact EET displayed distinct expression dynamics, some of which were shared with other short epithelial models. Conclusion: Differences between TVs and EET likely result from multiple factors, including a reduction in the length and signaling capabilities of TVs, delayed elongation from inadequate uterine signals, and modified crosstalk between the conceptus and the uterus. These findings confirm that close coordination between uterine, embryonic, and extra-embryonic tissues is required to orchestrate proper elongation and, based on the partial differentiation observed, raise questions about the presence/absence of certain developmental cues or even their asynchronies.

Using extended growth of cattle embryos in culture to gain insights into bovine developmental events on embryonic days 8 to 10

We have previously described an extended embryo culture system, based on uterine composition, growth factors and the cell culture additive B27, for growing cattle embryos in vitro beyond embryonic day 7. Here, extended in vitro embryos are compared to embryos developed in the uterus and are used to establish a developmental staging framework useful for understanding developmental events occurring until Day 10. Immunofluorescence or mRNA expression of the ICM/epiblast markers OCT4, SOX2 and NANOG, hypoblast markers GATA6, SOX17 and GATA4 and trophoblast genes CDX2, GATA3, ASCL2 and IFNT revealed the presence of four stages during this period that can be molecularly distinguished. These are expanded blastocyst, hatched blastocyst, hypoblast layering and early hypoblast migration. Interestingly NANOG and SOX17 show reciprocal expression at the expanded blastocyst stage, well before SOX2 and GATA6 expression refines to a similar so-called "salt and pepper" mutually exclusive expression in the ICM at the hatched blastocyst stage. GATA4 expression is only seen from stages when the hypoblast starts migrating around the blastocyst cavity. Intriguingly, trophoblast still expresses GATA6 and OCT4 in all cells during the expanded blastocyst phase, while SOX2 and SOX17 are seen in only some trophoblast cells. By the hypoblast-epiblast layering stage no trophoblast expression remains except for that of OCT4 protein, which starts waning in trophoblast once the hypoblast begins migrating. Lastly, it is shown that cultured embryos exhibit increased expression of the stress marker TP53 in the epiblast and hypoblast at late stages in comparison to embryos produced in the uterine environment.

Transcriptional Factors Mediated Reprogramming to Pluripotency

A unique kind of pluripotent cell, i.e., Induced pluripotent stem cells (iPSCs), now being targeted for iPSC synthesis, are produced by reprogramming animal and human differentiated cells (with no change in genetic makeup for the sake of high efficacy iPSCs formation). The conversion of specific cells to iPSCs has revolutionized stem cell research by making pluripotent cells more controllable for regenerative therapy. For the past 15 years, somatic cell reprogramming to pluripotency with force expression of specified factors has been a fascinating field of biomedical study. For that technological primary viewpoint reprogramming method, a cocktail of four transcription factors (TF) has required: Kruppel-like factor 4 (KLF4), four-octamer binding protein 34 (OCT3/4), MYC and SOX2 (together referred to as OSKM) and host cells. IPS cells have great potential for future tissue replacement treatments because of their ability to self-renew and specialize in all adult cell types, although factor-mediated reprogramming mechanisms are still poorly understood medically. This technique has dramatically improved performance and efficiency, making it more useful in drug discovery, disease remodeling, and regenerative medicine. Moreover, in these four TF cocktails, more than 30 reprogramming combinations were proposed, but for reprogramming effectiveness, only a few numbers have been demonstrated for the somatic cells of humans and mice. Stoichiometry, a combination of reprogramming agents and chromatin remodeling compounds, impacts kinetics, quality, and efficiency in stem cell research.

Transcriptome changes associated with elongation of bovine conceptuses I: Differentially expressed transcripts in the conceptus on day 17 after insemination

The objective was to characterize transcriptome changes associated with elongation in bovine conceptuses during preimplantation stages. Nonlactating Holstein cows were euthanized 17 d after artificial insemination (AI) and the uterine horn ipsilateral to the CL was flushed with saline solution. Recovered conceptuses were classified as small (1.2 to 6.9 cm; n = 9), medium (10.5 to 16.0 cm; n = 9), or large (18.0 to 26.4 cm; n = 10). Total mRNA was extracted and subjected to transcriptome analyses using the Affymetrix Gene Chip Bovine array. Data were normalized using the GCRMA method and analyzed by robust regression using the Linear Models for Microarray library within Bioconductor in R. Transcripts with P ≤ 0.05 after adjustment for false discovery rate and fold change ≥1.5 were considered differentially expressed. Functional analyses were conducted using the Ingenuity Pathway Analysis platform. Comparisons between large versus small (LvsS), large versus medium (LvsM), and medium versus small (MvsS) conceptuses yielded a total of 634, 240, and 63 differentially expressed transcripts, respectively. Top canonical pathways of known involvement with embryo growth that were upregulated in large conceptuses included actin cytoskeleton (LvsS), integrin signaling (LvsS and LvsM), ephrin receptor (LvsS), mesenchymal transition by growth factor (LvsM), and regulation of calpain protease (LvsS). Transcripts involved with lipid metabolism pathways (LXR/RXR, FXR/RXR, hepatic fibrosis) were associated with the LvsS and LvsM, and some transcripts such as APOC2, APOH, APOM, RARA, RBP4, and PPARGC1A, were involved in these pathways. An overall network summary associated biological downstream effects of invasion of cells, proliferation of embryonic cells, and inhibition of organismal death in the LvsS. In conclusion, differently expressed transcripts in the LvsS comparison were associated with the cell growth, adhesion, and organismal development, although part of these findings could be attributed to differences in circulatory concentrations of progesterone of the cows that bore large and small conceptuses. The large and medium conceptuses developed under similar concentrations of progesterone and presented 240 differently expressed transcripts, associated with cell differentiation, metabolite regulation, and other biological processes.

Single-cell transcriptional landscapes of bovine peri-implantation development

Supporting healthy pregnancy outcomes requires a comprehensive understanding of the cellular hierarchy and underlying molecular mechanisms during peri-implantation development. Here, we present a single-cell transcriptome-wide view of the bovine peri-implantation embryo development at day 12, 14, 16 and 18, when most of the pregnancy failure occurs in cattle. We defined the development and dynamic progression of cellular composition and gene expression of embryonic disc, hypoblast, and trophoblast lineages during bovine peri-implantation development. Notably, the comprehensive transcriptomic mapping of trophoblast development revealed a previously unrecognized primitive trophoblast cell lineage that is responsible for pregnancy maintenance in bovine prior to the time when binucleate cells emerge. We analyzed novel markers for the cell lineage development during bovine early development. We also identified cell-cell communication signaling underling embryonic and extraembryonic cell interaction to ensure proper early development. Collectively, our work provides foundational information to discover essential biological pathways underpinning bovine peri-implantation development and the molecular causes of the early pregnancy failure during this critical period.

Significance Statement

Peri-implantation development is essential for successful reproduction in mammalian species, and cattle have a unique process of elongation that proceeds for two weeks prior to implantation and represents a period when many pregnancies fail. Although the bovine embryo elongation has been studied histologically, the essential cellular and molecular factors governing lineage differentiation remain unexplored. This study profiled the transcriptome of single cells in the bovine peri-implantation development throughout day 12, 14, 16, and 18, and identified peri-implantation stage-related features of cell lineages. The candidate regulatory genes, factors, pathways and embryonic and extraembryonic cell interactions were also prioritized to ensure proper embryo elongation in cattle.

Developmental Hurdles That Can Compromise Pregnancy during the First Month of Gestation in Cattle

Several key developmental events are associated with early embryonic pregnancy losses in beef and dairy cows. These developmental problems are observed at a greater frequency in pregnancies generated from in-vitro-produced bovine embryos. This review describes critical problems that arise during oocyte maturation, fertilization, early embryonic development, compaction and blastulation, embryonic cell lineage specification, elongation, gastrulation, and placentation. Additionally, discussed are potential remediation strategies, but unfortunately, corrective actions are not available for several of the problems being discussed. Further research is needed to produce bovine embryos that have a greater likelihood of surviving to term.

Review: Embryonic diapause in the European roe deer - slowed, but not stopped

Embryonic diapause in mammals describes a transient reduction of proliferation and developmental progression occurring at the blastocyst stage. It was first described in the European roe deer (Capreolus capreolus) in the 19th century, and later found to occur in at least over 130 mammalian species across several taxa. Diapause is often displayed as an interruption, a halt, or an arrest of embryonic development. In this review, we explore reduced, but not stopped pace of growth, proliferation and developmental progression during embryonic diapause and revisit early embryonic proliferation and continued slow development as peculiar phenomenon in the roe deer.

The evolution of gastrulation morphologies

During gastrulation, early embryos specify and reorganise the topology of their germ layers. Surprisingly, this fundamental and early process does not appear to be rigidly constrained by evolutionary pressures; instead, the morphology of gastrulation is highly variable throughout the animal kingdom. Recent experimental results demonstrate that it is possible to generate different alternative gastrulation modes in single organisms, such as in early cnidarian, arthropod and vertebrate embryos. Here, we review the mechanisms that underlie the plasticity of vertebrate gastrulation both when experimentally manipulated and during evolution. Using the insights obtained from these experiments we discuss the effects of the increase in yolk volume on the morphology of gastrulation and provide new insights into two crucial innovations during amniote gastrulation: the transition from a ring-shaped mesoderm domain in anamniotes to a crescent-shaped domain in amniotes, and the evolution of the reptilian blastoporal plate/canal into the avian primitive streak.

Transforming growth factor beta (TGFβ) pathway is essential for hypoblast and epiblast development in ovine post-hatching embryos

The developmental failures occurring between blastocyst hatching and implantation in farm ungulates are a major cause of pregnancy losses. At the expanded blastocyst stage, three cell lineages emerge in the embryo: trophoblast, hypoblast and epiblast, the latter being the most vulnerable during post-hatching development. Transforming growth factor beta (TGFβ) signaling pathway is involved in hypoblast and epiblast development; however, previous in vitro functional studies are limited to the expanded blastocyst stage. In this study, we have analyzed the effect of TGFβ inhibition with 10, 20 or 40 μM SB431542 during ovine post-hatching developmental period using a recently developed culture system able to recapitulate major developmental landmarks. We have found a negative effect of TGFβ inhibition on hypoblast and epiblast development that could be partially reverted by Rho-associated protein kinase (ROCK) inhibitor Y-27632. Our findings provide new insights into the molecular networks regulating embryo development beyond the expanded blastocyst and could help to elucidate the causes of early pregnancy losses in farm ungulates.

Alternative mammalian strategies leading towards gastrulation: losing polar trophoblast (Rauber's layer) or gaining an epiblast cavity

Using embryological data from 14 mammalian orders, the hypothesis is presented that in placental mammals, epiblast cavitation and polar trophoblast loss are alternative developmental solutions to shield the central epiblast from extraembryonic signalling. It is argued that such reciprocal signalling between the edge of the epiblast and the adjoining polar trophoblast or edge of the mural trophoblast or with the amniotic ectoderm is necessary for the induction of gastrulation. This article is part of the theme issue 'Extraembryonic tissues: exploring concepts, definitions and functions across the animal kingdom'.

The first week following insemination is the period of major pregnancy failure in pasture-grazed dairy cows

A 60% pregnancy success for inseminations is targeted to optimize production efficiency for dairy cows within a seasonal, pasture-grazed system. Routine measures of pregnancy success are widely available but are limited, in practice, to a gestation stage beyond the first 28 d. Although some historical data exist on embryonic mortality before this stage, productivity of dairy systems and genetics of the cows have advanced significantly in recent decades. Accordingly, the aim was to construct an updated estimate of pregnancy success at key developmental stages during the first 70 d after insemination. Blood samples were collected for progesterone concentrations on d 0 and 7. A temporal series of 4 groups spanning fertilization through d 70 were conducted on 4 seasonal, pasture-grazed dairy farms (n = 1,467 cows) during the first 21 d of the seasonal breeding period. Morphological examination was undertaken on embryos collected on d 7 (group E7) and 15 (group E15), and pregnancy was diagnosed via ultrasonography on approximately d 28 and 35 (group E35) as well as d 70 (group E70). Fertilization, embryo, and fetal evaluation for viability established a pregnancy success pattern. Additionally, cow and on-farm risk factor variables associated with pregnancy success were evaluated. We estimated pregnancy success rates of 70.9%, 59.1%, 63.8%, 62.3%, and 56.7% at d 7, 15, 28, 35, and 70, respectively. Fertilization failure (15.8%) and embryonic arrest before the morula stage (10.3%) were the major developmental events contributing to first-week pregnancy failures. Embryo elongation failure of 7% contributed to pregnancy failure during the second week. The risk factors for pregnancy success that were related to the cows included interval between calving and insemination, and d-7 plasma progesterone concentrations, whereas insemination sire was associated with pregnancy outcome. Most pregnancy failure occurs during the first week among seasonal-calving pasture-grazed dairy cows.

OCT4/POU5F1 is indispensable for the lineage differentiation of the inner cell mass in bovine embryos

The mammalian blastocyst undergoes two lineage segregations, that is, formation of the trophectoderm and subsequently differentiation of the hypoblast (HB) from the inner cell mass, leaving the epiblast (EPI) as the remaining pluripotent lineage. To clarify the expression patterns of markers specific for these lineages in bovine embryos, we analyzed day 7, 9, and 12 blastocysts completely produced in vivo by staining for OCT4, NANOG, SOX2 (EPI), and GATA6, SOX17 (HB) and identified genes specific for these developmental stages in a global transcriptomics approach. To study the role of OCT4, we generated OCT4-deficient (OCT4 KO) embryos via somatic cell nuclear transfer or in vitro fertilization. OCT4 KO embryos reached the expanded blastocyst stage by day 8 but lost NANOG and SOX17 expression, while SOX2 and GATA6 were unaffected. Blastocysts transferred to recipient cows from day 6 to 9 expanded, but the OCT4 KO phenotype was not rescued by the uterine environment. Exposure of OCT4 KO embryos to exogenous FGF4 or chimeric complementation with OCT4 intact embryos did not restore NANOG or SOX17 in OCT4-deficient cells. Our data show that OCT4 is required cell autonomously for the maintenance of pluripotency of the EPI and differentiation of the HB in bovine embryos.

In vitro culture of ovine embryos up to early gastrulating stages

Developmental failures occurring shortly after blastocyst hatching from the zona pellucida constitute a major cause of pregnancy losses in both humans and farm ungulates. The developmental events occurring following hatching in ungulates include the proliferation and maturation of extra-embryonic membranes – trophoblast and hypoblast – and the formation of a flat embryonic disc, similar to that found in humans, which initiates gastrulation prior to implantation. Unfortunately, our understanding of these key processes for embryo survival is limited because current culture systems cannot sustain ungulate embryo development beyond hatching. Here, we report a culture system that recapitulates most developmental landmarks of gastrulating ovine embryos: trophoblast maturation, hypoblast migration, embryonic disc formation, disappearance of the Rauber's layer, epiblast polarization and mesoderm differentiation. Our system represents a highly valuable platform for exploring the cell differentiation, proliferation and migration processes governing gastrulation in a flat embryonic disc and for understanding pregnancy failures during the second week of gestation.

This article has an associated ‘The people behind the papers’ interview.

Molecular Regulation of Paused Pluripotency in Early Mammalian Embryos and Stem Cells

The energetically costly mammalian investment in gestation and lactation requires plentiful nutritional sources and thus links the environmental conditions to reproductive success. Flexibility in adjusting developmental timing enhances chances of survival in adverse conditions. Over 130 mammalian species can reversibly pause early embryonic development by switching to a near dormant state that can be sustained for months, a phenomenon called embryonic diapause. Lineage-specific cells are retained during diapause, and they proliferate and differentiate upon activation. Studying diapause thus reveals principles of pluripotency and dormancy and is not only relevant for development, but also for regeneration and cancer. In this review, we focus on the molecular regulation of diapause in early mammalian embryos and relate it to maintenance of potency in stem cells in vitro. Diapause is established and maintained by active rewiring of the embryonic metabolome, epigenome, and gene expression in communication with maternal tissues. Herein, we particularly discuss factors required at distinct stages of diapause to induce, maintain, and terminate dormancy.

Expression patterns of signalling molecules and transcription factors in the early rabbit embryo and their significance for modelling amniote axis formation

The anterior-posterior axis is a central element of the body plan and, during amniote gastrulation, forms through several transient domains with specific morphogenetic activities. In the chick, experimentally proven activity of signalling molecules and transcription factors lead to the concept of a 'global positioning system' for initial axis formation whereas in the (mammotypical) rabbit embryo, a series of morphological or molecular domains are part of a putative 'three-anchor-point model'. Because circular expression patterns of genes involved in axis formation exist in both amniote groups prior to, and during, gastrulation and may thus be suited to reconcile these models, the expression patterns of selected genes known in the chick, namely the ones coding for the transcription factors eomes and tbx6, the signalling molecule wnt3 and the wnt inhibitor pkdcc, were analysed in the rabbit embryonic disc using in situ hybridisation and placing emphasis on their germ layer location. Peripheral wnt3 and eomes expression in all layers is found initially to be complementary to central pkdcc expression in the hypoblast during early axis formation. Pkdcc then appears - together with a posterior-anterior gradient in wnt3 and eomes domains - in the epiblast posteriorly before the emerging primitive streak is marked by pkdcc and tbx6 at its anterior and posterior extremities, respectively. Conserved circular expression patterns deduced from some of this data may point to shared mechanisms in amniote axis formation while the reshaping of localised gene expression patterns is discussed as part of the 'three-anchor-point model' for establishing the mammalian body plan.

Lineage Differentiation Markers as a Proxy for Embryo Viability in Farm Ungulates

Embryonic losses constitute a major burden for reproductive efficiency of farm animals. Pregnancy losses in ungulate species, which include cattle, pigs, sheep and goats, majorly occur during the second week of gestation, when the embryo experiences a series of cell differentiation, proliferation, and migration processes encompassed under the term conceptus elongation. Conceptus elongation takes place following blastocyst hatching and involves a massive proliferation of the extraembryonic membranes trophoblast and hypoblast, and the formation of flat embryonic disc derived from the epiblast, which ultimately gastrulates generating the three germ layers. This process occurs prior to implantation and it is exclusive from ungulates, as embryos from other mammalian species such as rodents or humans implant right after hatching. The critical differences in embryo development between ungulates and mice, the most studied mammalian model, have precluded the identification of the genes governing lineage differentiation in livestock species. Furthermore, conceptus elongation has not been recapitulated in vitro, hindering the study of these cellular events. Luckily, recent advances on transcriptomics, genome modification and post-hatching in vitro culture are shedding light into this largely unknown developmental window, uncovering possible molecular markers to determine embryo quality. In this review, we summarize the events occurring during ungulate pre-implantation development, highlighting recent findings which reveal that several dogmas in Developmental Biology established by knock-out murine models do not hold true for other mammals, including humans and farm animals. The developmental failures associated to in vitro produced embryos in farm animals are also discussed together with Developmental Biology tools to assess embryo quality, including molecular markers to assess proper lineage commitment and a post-hatching in vitro culture system able to directly determine developmental potential circumventing the need of experimental animals.

A New Toolbox in Experimental Embryology-Alternative Model Organisms for Studying Preimplantation Development

Preimplantation development is well conserved across mammalian species, but major differences in developmental kinetics, regulation of early lineage differentiation and implantation require studies in different model organisms, especially to better understand human development. Large domestic species, such as cattle and pig, resemble human development in many different aspects, i.e., the timing of zygotic genome activation, mechanisms of early lineage differentiations and the period until blastocyst formation. In this article, we give an overview of different assisted reproductive technologies, which are well established in cattle and pig and make them easily accessible to study early embryonic development. We outline the available technologies to create genetically modified models and to modulate lineage differentiation as well as recent methodological developments in genome sequencing and imaging, which form an immense toolbox for research. Finally, we compare the most recent findings in regulation of the first lineage differentiations across species and show how alternative models enhance our understanding of preimplantation development.

Day 7 embryo quality and suboptimal uterine environment influence morphometry of Day 16 conceptus in dairy cows

The objectives of this study were to investigate the effect of Day 7 embryo quality and subclinical endometritis (SCE) in repeat breeder recipient cows on morphometry of Day 16 embryo and to determine the association of %PMN, serum progesterone and Day 16 conceptus length. Holstein dairy cows that failed to conceive at least 3 times, (parity, 3 and 4; body condition score, 3 to 3.5 out of 5) with subclinical endometritis (n = 180; SCE, >6% PMN on endometrial cytology) or without subclinical endometritis (n = 180; No-SCE, ≤ 6% PMN) were selected. Cows in each group received single, frozen-thawed, quality 1 (n = 60), 2 (n = 60) and 3 (n = 60) embryos (compact morula or early blastocyst) on Day 7 post estrus in the uterine horn ipsilateral to the ovary containing a corpus luteum, using standard nonsurgical techniques. Only cows that expressed estrus (Select-Synch protocol) and with acceptable corpus luteum (≥1.5 cm in size) were included. Conceptuses were collected on Day 16 from all recipient cows by standard non-surgical uterine flushing technique, using an 18-g embryo collection catheter with Phosphate Buffered Saline (pH 7.4). Blood samples were collected on Day 16 to determine serum progesterone concentrations. After collection, conceptuses were weighed and measured, and were categorized as tubular (underdeveloped, 10-20 mm) or filamentous (normal, >25 mm). Between cows with SCE and No-SCE, mean (±SEM) width (1.68 ± 0.13 mm vs. 1.84 ± 0.16 mm), length (34.4 ± 9.6 mm vs. 55.8 ± 13.4 mm) and weight (22.3 ± 3.7 vs. 40.6 ± 6.4 mg) of Day 16 conceptuses differed (P < 0.05). The mean width (1.75 ± 0.19 mm vs. 1.81 ± 0.22 mm), length (57.7 ± 11.2 vs. 51.1 ± 13.6 mm) and weight (34.3 ± 6.4 vs. 38.5 ± 8.2 mg) of Day 16 embryo following transfer of Day 7 embryo quality grade 1 and grade 2 embryos were not different (P > 0.1), but both differed from the mean width (1.59 ± 0.11 mm), length (28.9 ± 9.7 mm) and weight (25.3 ± 4.6 mg) of Day 16 embryo from Day 7 embryo quality grade 3 (P < 0.05). Total percentage of embryos recovered differed between SCE and No-SCE groups (P < 0.05; 36.1 vs 48.9%). Total percentage of embryos recovered on Day 16 following transfer of grade 1 (53.3%) and 2 (44.2%) Day 7 embryos were greater (P < 0.05) compared with transfer of grade 3 embryos (29.2%) (P < 0.001). Total percentage of filamentous embryos recovered was lower for SCE cows compared with No-SCE cows (P < 0.01; 15.0 vs. 25.6%). Total percentage of tubular embryos recovered did not differ between SCE and No-SCE cows (P > 0.1; 21.1% vs. 22.8%). Filamentous embryo recovered for grade 3 was lower (P < 0.05) compared with grade 1 in both SCE (8.3 vs. 21.7%) and No-SCE groups (15.0 vs. 33.3%). The mean (±SEM) CL volume (cm³; 11.8 ± 0.29 vs. 15.9 ± 0.31) and progesterone concentrations (ng/mL; 5.17 ± 1.8 vs. 8.2 ± 1.2) on Day 16 differed between SCE and No-SCE groups (P < 0.05) but not among Day 7 embryo grade groups (P > 0.1). The mean (±SEM) CL volume (cm³; 15.6 ± 0.28 vs 12.1 ± 3.9) and serum progesterone concentrations (ng/mL; 8.6 ± 1.4 vs. 4.9 ± 1.9) on Day 16 differed (P < 0.05) between cows yielded filamentous and tubular embryos. When all cows were considered, multiple regression analysis showed that the %PMN (P < 0.0001), progesterone concentrations (P < 0.0001), embryo qulaity (P < 0.05) and %PMN by progesterone interactions (P < 0.0001) influenced the length of Day 16 conceptus. Among cows without subclinical endometritis, only progesterone concentrations (P < 0.0001) and among cows with subclinical endometritis, only %PMN (P < 0.04) influenced the length of Day 16 conceptus. Progesterone concentrations (P < 0.0001) influenced the length of Day 16 conceptus in cows that received embryo quality 1 and 2. Progesterone concentration by %PMN interaction (P < 0.05) also influenced the length of Day 16 conceptus in cows that received embryo quality 2. The %PMN (P = 0.05) influenced the length of Day 16 conceptus in cows that received embryo quality 3. In conclusion, poor quality Day 7 embryo and presence of SCE negatively influenced early embryo development between Days 7 and 16 of gestation probably by dysregulated embryo-maternal interactions due to lower progesterone, prompting loss of the conceptus in sub-optimal uterine environment.

Growing cattle embryos beyond Day 8 - An investigation of media components

The growth of viable cattle embryos in culture to stages beyond the hatching blastocyst is of interest to developmental biologists wishing to understand developmental events beyond the first lineage decision, as well as for commercial applications, because a lengthening of the culturing time allows more time for diagnostic tests on biopsies, whereas extended survival can be used as a better assay system for monitoring developmental potential. We here report on a novel extended culture medium for embryo growth until embryonic day (Day) 12. We used a non-invasive morphological characterisation system that scored viability, inner cell mass (ICM) grade, hatching and embryo and ICM diameter. The basal medium was based on published uterine fluid concentrations of amino acids, carbohydrates and electrolytes. Addition of fetal bovine serum was necessary and the additive ITSX greatly improved culture success. We tested the inclusion of a seven-growth factor cocktail consisting of Activin A, Artemin, BMP4, EGF, FGF4, GM-CSF/CSF2 and LIF, as well as omission of individual components of the cocktail. In the context of the growth factor cocktail, Artemin and BMP4 provided the greatest benefit, while FGF omission had more positive than negative effects on embryo characteristics. Lastly, replacement of ITSX by B27-additive led to the most successful culture of embryos, in all media permutations.

Embryonic disc formation following post-hatching bovine embryo development in vitro

Failures during conceptus elongation are a major cause of pregnancy losses in ungulates, exerting a relevant economic impact on farming. The developmental events occurring during this period are poorly understood, mainly because this process cannot be recapitulated in vitro. Previous studies have established an in vitro post-hatching development (PHD) system that supports bovine embryo development beyond the blastocyst stage, based on agarose gel tunnels and serum- and glucose-enriched medium. Unfortunately, under this system embryonic disc formation is not achieved and embryos show notorious signs of apoptosis and necrosis. The objective of this study has been to develop an in vitro system able to support embryonic disc formation. We first compared post-hatching development inside agarose tunnels or free-floating over an agarose-coated dish in serum- and glucose-enriched medium (PHD medium). Culture inside agarose tunnels shaped embryo morphology by physical constriction, but it restricted embryo growth and did not provide any significant advantage in terms of development of hypoblast and epiblast lineages. In contrast to PHD medium, a chemically defined and enriched medium (N2B27) supported complete hypoblast migration and epiblast survival in vitro, even in the absence of agarose coating. Cells expressing the pluripotency marker SOX2 were observed in ~56% of the embryos and ~25% developed embryonic disc-like structures formed by SOX2+ cells. In summary, here we provide a culture system that supports trophectoderm proliferation, hypoblast migration and epiblast survival after the blastocyst stage.

Identification of reliable reference genes for expression studies in maternal reproductive tissues and foetal tissues of pregnant cows

The relationship between the conceptus and the maternal uterine environment is crucial for the successful establishment and maintenance of pregnancy in cattle. Gene expression analysis of the conceptus and maternal reproductive tissues is a favourable method to assess the embryonic maternal interaction. The reliability of the commonly used method reverse transcription-quantitative polymerase chain reaction (RT-qPCR) depends on proper normalization to stable reference genes (RGs). The objective of this study was to determine the expression stability of 10 potential RGs in maternal reproductive tissues and foetal tissues, and to analyse the effect of RG selection on the calculation of the relative expression of target genes. The expression stability of 10 potential RGs was analysed in eight different tissues from three pregnant dairy cows. Three programs-GeNorm, NormFinder and Bestkeeper-were used to identify the best RGs. According to all three programs, the most stable RG was CNOT11, whereas the least stable RGs were GAPDH and HPRT1. GeNorm analysis showed that a combination of five RGs (SDHA, PPIA, CNOT11, RPS9 and RPL19) was necessary for appropriate data normalization. However, NormFinder analysis indicated that the combination of CNOT11 and PPIA was the most suitable. When target genes were normalized to these RGs, the relative expression of the Radical S-adenosyl methionine domain containing 2 gene was not affected by the choice of RGs, whereas a large difference was observed in the expression profile of the Nuclear erythroid2-related factor 2 gene between the most stable and least stable RGs. The results indicate that careful selection of RGs is crucial under different conditions, especially for target genes with relatively small fold changes. Furthermore, the results provide useful information for the selection of RGs for evaluating genes affecting bovine reproduction.

Bovine endometrium responds differentially to age-matched short and long conceptuses†

This study combined in vitro production of bovine blastocysts, multiple embryo transfer techniques, and a conceptus-endometrial explant co-culture system to test the hypothesis that bovine endometrium exposed to long vs. short day 15 conceptuses would exhibit a different transcriptome profile reflective of potential for successful pregnancy establishment. Bovine endometrial explants collected at the late luteal stage of the estrous cycle were cultured in RPMI medium for 6 h with nothing (control), 100 ng/mL recombinant ovine interferon tau (IFNT), a long day 15 conceptus, or a short day 15 conceptus. Transcriptional profiling of the endometrial explants found that exposure of endometrium to IFNT, long conceptuses, or short conceptuses altered (P < 0.05) expression of 491, 498, and 230 transcripts, respectively, compared to the control. Further analysis revealed three categories of differentially expressed genes (DEG): (i) commonly responsive to exposure to IFNT and conceptuses, irrespective of size (n = 223); (ii) commonly responsive to IFNT and long conceptuses only (n = 168); and genes induced by the presence of a conceptus but independent of IFNT (n = 108). Of those 108 genes, 101 were exclusively induced by long conceptuses and functional analysis revealed that regulation of molecular function, magnesium-ion transmembrane transport, and clathrin coat assembly were the principal gene ontologies associated with these DEG. In conclusion, bovine endometrium responds differently to age-matched conceptuses of varying size in both an IFNT-dependent and -independent manner, which may be reflective of the likelihood of successful pregnancy establishment.

On the enigmatic disappearance of Rauber's layer

The polar trophoblast overlays the epiblast in eutherian mammals and, depending on the species, has one of two different fates. It either remains a single-layered, thinning epithelium called "Rauber's layer," which soon disintegrates, or, alternatively, it keeps proliferating, contributing heavily to the population of differentiating, invasive trophoblast cells and, at least in mice, to the induction of gastrulation. While loss of the persistent polar trophoblast in mice leads to reduced induction of gastrulation, we show here that prevention of the loss of the polar trophoblast in cattle results in ectopic domains of the gastrulation marker, BRACHYURY This phenotype, and increased epiblast proliferation, arose when Rauber's layer was maintained for a day longer by countering apoptosis through BCL2 overexpression. This suggests that the disappearance of Rauber's layer is a necessity, presumably to avoid excessive signaling interactions between this layer and the subjacent epiblast. We note that, in all species in which the polar trophoblast persists, including humans and mice, ectopic polar trophoblast signaling is prevented via epiblast cavitation which leads to the (pro)amniotic cavity, whose function is to distance the central epiblast from such signaling interactions.

A Multidisciplinary Review of the Roles of Cripto in the Scientific Literature Through a Bibliometric Analysis of its Biological Roles

Cripto is a small glycosylphosphatidylinisitol (GPI)-anchored and secreted oncofetal protein that plays important roles in regulating normal physiological processes, including stem cell differentiation, embryonal development, and tissue growth and remodeling, as well as pathological processes such as tumor initiation and progression. Cripto functions as a co-receptor for TGF-β ligands such as Nodal, GDF1, and GDF3. Soluble and secreted forms of Cripto also exhibit growth factor-like activity and activate SRC/MAPK/PI3K/AKT pathways. Glucose-Regulated Protein 78 kDa (GRP78) binds Cripto at the cell surface and has been shown to be required for Cripto signaling via both TGF-β and SRC/MAPK/PI3K/AKT pathways. To provide a comprehensive overview of the scientific literature related to Cripto, we performed, for the first time, a bibliometric analysis of the biological roles of Cripto as reported in the scientific literature covering the last 10 years. We present different fields of knowledge in comprehensive areas of research on Cripto, ranging from basic to translational research, using a keyword-driven approach. Our ultimate aim is to aid the scientific community in conducting targeted research by identifying areas where research has been conducted so far and, perhaps more importantly, where critical knowledge is still missing.

WNT signalling supported by MEK/ERK inhibition is essential to maintain pluripotency in bovine preimplantation embryo

Capturing stable embryonic stem cell (ESC) lines from domesticated animals still remains one of the challenges of non-rodent embryology. The stake is high, as stable ESCs derived from species such as cattle present high economic and scientific value. Understanding of the processes leading to the embryonic lineage segregation is crucial to provide species-orientated molecular environment capable of supporting self-renewal and pluripotency. Therefore, the aim of this study was to validate the action of the two core regulatory pathways (WNT and MEK/ERK) during bovine embryo development. In vitro produced bovine embryos were obtained in the presence of inhibitors (i), which enable activation of the WNT pathway (via GSK3i, CHIR99021) and suppression of MEK signalling by PD0325901 in the 2i system and PD184325 and SU5402 in the 3i system. We have followed the changes in the distribution of the key lineage specific markers both at the transcript and protein level. Our results showed that WNT signalling promotes the expression of key inner cell mass (ICM) specific markers in bovine embryos, regardless of the MEK/ERK inhibitor cocktail used. MEK/ERK downregulation is crucial to maintain OCT4 and NANOG expression within the ICM and to prevent their exclusion from the trophectoderm (TE). At the same time, the classical TE marker (CDX2) was downregulated at the mRNA and protein level. As a follow up for the observed pluripotency stimulating effect of the inhibitors, we have tested the potential of the 2i and the 3i culture conditions (supported by LIF) to derive primary bovine ESC lines. As a result, we propose a model in which all of the primary signalling pathways determining embryonic cell fate are active in bovine embryos, yet the requirement for pluripotency maintenance in cattle may differ from the described standards. WNT activation leads to the formation (and stabilisation of the ICM) and MEK/ERK signalling is maintained at low levels. Unlike in the mouse, GATA6 is expressed in both ICM and TE. MEK/ERK signalling affects HP formation in cattle, but this process is activated at the post-blastocyst stage. With regard to self-renewal, 2i is preferable, as 3i also blocks the FGF receptor, what may prevent PI3K signalling, important for pluripotency and self-renewal.

Challenges in studying preimplantation embryo-maternal interaction in cattle

A comprehensive understanding of the complex embryo-maternal interactions during the preimplantation period requires the analysis of the very early stages of pregnancy encompassing early embryonic development, maternal recognition and the events leading to implantation. Despite the fact that embryo development until blastocyst stage is somewhat autonomous (i.e., does not require contact with the maternal reproductive tract and can be successfully recapitulated in vitro), many studies on ruminant embryo production have focused on the fundamental question of why: (i) only 30%-40% of immature oocytes develop to the blastocyst stage and (ii) the quality of such blastocysts continually lags behind that of blastocysts produced in vivo. Clear evidence indicates that in vitro culture conditions are far from optimal with deficiencies being manifested in short- and long-term effects on the embryo. Thus, enhanced knowledge of mechanisms controlling embryo-maternal interactions would allow the design of novel strategies to improve in vitro embryo conditions and reproductive outcomes in cattle.

Functional characterization of NANOG in goat pre-implantation embryonic development

Nanog as a novel pluripotent cell-specific gene plays important roles in regulation of signaling pathways for maintenance and induction of pluripotency in inner cell mass (ICM) and embryonic stem cells (ESC) in mouse. The molecular features and transcription regulation of NANOG gene in domestic animals are not well defined. In this study, we performed knockdown of NANOG mRNA in goat embryos and examined its effect on early embryonic development. Presumptive zygotes were injected with a volume of 8-10 pl NANOG or scrambled (SCR) siRNA, and subsequently cleavage and blastocyst formation rate were assessed. Furthermore, gene expression analysis was carried out in 6-8 cell and blastocyst derived embryos from non-injected controls, SCR - and siRNA-injected presumptive zygotes. Cleavage and blastocyst rates in siRNA groups were insignificantly lower than the control and SCR groups. Embryos with reduced expression of NANOG showed decrease in number of trophectoderm (TE) and total cells in blastocysts. Analysis of expression of developmentally important genes (SOX2, OCT4 and NANOG), which work as a network, showed that NANOG knockdown results in significant increase in expression of SOX2 and OCT4 and among the possible target genes (CDX2, REX1 and GATA4) of this network, only GATA4 showed increased expression. Our results suggest that NANOG is likely to be required for proliferation of trophoblastic cells.

Pre-implantation Development of Domestic Animals

During the first days following fertilization, cells of mammalian embryo gradually lose totipotency, acquiring distinct identity. The first three lineages specified in the mammalian embryo are pluripotent epiblast, which later gives rise to the embryo proper, and two extraembryonic lineages, hypoblast (also known as primitive endoderm) and trophectoderm, which form tissues supporting development of the fetus in utero. Most of our knowledge regarding the mechanisms of early lineage specification in mammals comes from studies in the mouse. However, the growing body of evidence points to both similarities and species-specific differences. Understanding molecular and cellular mechanisms of early embryonic development in nonrodent mammals expands our understanding of basic mechanisms of differentiation and is essential for the development of effective protocols for assisted reproduction in agriculture, veterinary medicine, and for biomedical research. This review summarizes the current state of knowledge on key events in epiblast, hypoblast, and trophoblast differentiation in domestic mammals.

Prosurvival effect of cumulus prostaglandin G/H synthase 2/prostaglandin2 signaling on bovine blastocyst: impact on in vivo posthatching development

Apoptotic activity is a common physiological process which culminates at the blastocyst stage in the preimplantation embryo of many mammals. The degree of embryonic cell death can be influenced by the oocyte microenvironment. However, the prognostic significance of the incidence of apoptosis remains undefined. Prostaglandin E2 (PGE2) derived from prostaglandin G/H synthase-2 (PTGS2) activity is a well-known prosurvival factor that is mainly studied in oncology. PGE2 is the predominant PTGS2-derived prostaglandin present in the oocyte microenvironment during the periconceptional period. Using an in vitro model of bovine embryo production followed by transfer and collection procedures, we investigated the impact of periconceptional PGE2 on the occurrence of spontaneous apoptosis in embryos and on subsequent in vivo posthatching development. Different periconceptional PGE2 environments were obtained using NS-398, a specific inhibitor of PTGS2 activity, and exogenous PGE2. We assessed the level of embryonic cell death in blastocysts at day 8 postfertilization by counting total cell numbers, by the immunohistochemical staining of active caspase-3, and by quantifying terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling signals and apoptosis regulator (BCL-2/BAX) mRNA expression. Morphometric parameters were used to estimate the developmental stage of the embryonic disk and the extent of trophoblast elongation on day 15 conceptuses. Our findings indicate that periconceptional PGE2 signaling durably impacts oocytes, conferring increased resistance to spontaneous apoptosis in blastocysts and promoting embryonic disk development and the elongation process during preimplantation development.

A mathematical model of the interaction between bovine blastocyst developmental stage and progesterone-stimulated uterine factors on differential embryonic development observed on Day 15 of gestation

A complex interaction between the developing bovine embryo and the growth potential of the uterine milieu it inhabits results in an embryo capable of developing past the maternal recognition stage and on to a successful pregnancy. Previously, we observed variation in the lengths of embryos recovered 8 d after bulk transfer of Day 7 in vitro-produced (IVP) blastocysts into the same uterus. Potential causes of the differential embryonic growth were examined and modeled using 2 rounds of bulk (n = 4-6) IVP transfers and recovery of these embryos 8 d later. Morphological and gene expression measurements of the embryos were determined and the progesterone concentration of the cows was measured throughout the reproductive cycle as a reflection of the status of the uterine environment. These data were used to develop and evaluate a model that describes the interaction between the uterine environment and the growth rate of the developing embryo. Expression of 6 trophectoderm genes (IFNT, TKDP1, PAG11, PTGS2, DKK1, and PDPN) was correlated with conceptus length. The model determined that if the embryo develops to blastocyst stage, the uterine environment, driven by progesterone, is a more important component than blastocyst size in the stimulation of embryonic growth rate to ensure adequate interferon tau (IFNT) for pregnancy recognition. We detected an effect of Day 7 progesterone on the expression of all 6 genes, embryonic disc size, and trophectoderm length on Day 15. We also found effects of embryo transfer size on trophectoderm length and expression of IFNT and PAG11 on Day 15. Lower energy balance over the period from transfer to recovery was associated with reduced embryo growth to Day 15, and this effect was independent of progesterone. Energy balance also affected expression of PDPN and TKDP1 on Day 15. We observed an effect of energy balance from transfer to recovery on embryo survival in cows with partial embryo losses, where embryo factors dominate embryo survival, with cows with greater energy balance having lower embryo losses. This effect was independent of energy balance 40 d before transfer and suggests that energy balance has direct, immediate effects on the embryo and maternal environment during this period. Furthermore, energy balance effects on embryo survival in cows with partial embryo losses were largely mediated by expression of TKDP1, PAG11, and PDPN. These results provide candidate signaling pathways for the effect of progesterone and energy balance on embryo growth and survival.

Gene expression analysis of bovine embryonic disc, trophoblast and parietal hypoblast at the start of gastrulation

In cattle early gastrulation-stage embryos (Stage 5), four tissues can be discerned: (i) the top layer of the embryonic disc consisting of embryonic ectoderm (EmE); (ii) the bottom layer of the disc consisting of mesoderm, endoderm and visceral hypoblast (MEH); (iii) the trophoblast (TB); and (iv) the parietal hypoblast. We performed microsurgery followed by RNA-seq to analyse the transcriptome of these four tissues as well as a developmentally earlier pre-gastrulation embryonic disc. The cattle EmE transcriptome was similar at Stages 4 and 5, characterised by the OCT4/SOX2/NANOG pluripotency network. Expression of genes associated with primordial germ cells suggest their presence in the EmE tissue at these stages. Anterior visceral hypoblast genes were transcribed in the Stage 4 disc, but no longer by Stage 5. The Stage 5 MEH layer was equally similar to mouse embryonic and extraembryonic visceral endoderm. Our data suggest that the first mesoderm to invaginate in cattle embryos is fated to become extraembryonic. TGFβ, FGF, VEGF, PDGFA, IGF2, IHH and WNT signals and receptors were expressed, however the representative members of the FGF families differed from that seen in equivalent tissues of mouse embryos. The TB transcriptome was unique and differed significantly from that of mice. FGF signalling in the TB may be autocrine with both FGFR2 and FGF2 expressed. Our data revealed a range of potential inter-tissue interactions, highlighted significant differences in early development between mice and cattle and yielded insight into the developmental events occurring at the start of gastrulation.

Preattachment Embryos of Domestic Animals: Insights into Development and Paracrine Secretions

In mammalian species, endometrial receptivity is driven by maternal factors independently of embryo signals. When pregnancy initiates, paracrine secretions of the preattachment embryo are essential both for maternal recognition and endometrium preparation for implantation and for coordinating development of embryonic and extraembryonic tissues of the conceptus. This review mainly focuses on domestic large animal species. We first illustrate the major steps of preattachment embryo development, including elongation in bovine, ovine, porcine, and equine species. We next highlight conceptus secretions that are involved in the communication between extraembryonic and embryonic tissues, as well as between the conceptus and the endometrium. Finally, we introduce experimental data demonstrating the intimate connection between conceptus secretions and endometrial activity and how adverse events perturbing this interplay may affect the progression of implantation that will subsequently impact pregnancy outcome, postnatal health, and expression of production traits in livestock offspring.

Global assessment of imprinted gene expression in the bovine conceptus by next generation sequencing

Genomic imprinting is an epigenetic mechanism that leads to parental-allele-specific gene expression. Approximately 150 imprinted genes have been identified in humans and mice but less than 30 have been described as imprinted in cattle. For the purpose of de novo identification of imprinted genes in bovine, we determined global monoallelic gene expression in brain, skeletal muscle, liver, kidney and placenta of day ∼105 Bos taurus indicus × Bos taurus taurus F₁ conceptuses using RNA sequencing. To accomplish this, we developed a bioinformatics pipeline to identify parent-specific single nucleotide polymorphism alleles after filtering adenosine to inosine (A-to-I) RNA editing sites. We identified 53 genes subject to monoallelic expression. Twenty three are genes known to be imprinted in the cow and an additional 7 have previously been characterized as imprinted in human and/or mouse that have not been reported as imprinted in cattle. Of the remaining 23 genes, we found that 10 are uncharacterized or unannotated transcripts located in known imprinted clusters, whereas the other 13 genes are distributed throughout the bovine genome and are not close to any known imprinted clusters. To exclude potential cis-eQTL effects on allele expression, we corroborated the parental specificity of monoallelic expression in day 86 Bos taurus taurus × Bos taurus taurus conceptuses and identified 8 novel bovine imprinted genes. Further, we identified 671 candidate A-to-I RNA editing sites and describe random X-inactivation in day 15 bovine extraembryonic membranes. Our results expand the imprinted gene list in bovine and demonstrate that monoallelic gene expression can be the result of cis-eQTL effects.

Determinant molecular markers for peri-gastrulating bovine embryo development

Peri-gastrulation defines the time frame between blastocyst formation and implantation that also corresponds in cattle to elongation, pregnancy recognition and uterine secretion. Optimally, this developmental window prepares the conceptus for implantation, placenta formation and fetal development. However, this is a highly sensitive period, as evidenced by the incidence of embryo loss or early post-implantation mortality after AI, embryo transfer or somatic cell nuclear transfer. Elongation markers have often been used within this time frame to assess developmental defects or delays, originating either from the embryo, the uterus or the dam. Comparatively, gastrulation markers have not received great attention, although elongation and gastrulation are linked by reciprocal interactions at the molecular and cellular levels. To make this clearer, this peri-gastrulating period is described herein with a focus on its main developmental landmarks, and the resilience of the landmarks in the face of biotechnologies is questioned.