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.

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.

Paternal effect does not affect in vitro embryo morphokinetics but modulates molecular profile

The use of different sires influences in vitro embryo production (IVP) outcome. Paternal effects are observed from the first cleavages until after embryonic genome activation (EGA). Little is known about the mechanisms that promote in vitro fertility differences, even less about the consequences on embryo development. Therefore, this study aimed to evaluate the paternal effect at fertilization, embryo developmental kinetics, gene expression and quality from high and low in vitro fertility bulls. A retrospective analysis for bull selection was performed using the In vitro Brazil company database from 2012 to 2015. The dataset was edited employing cleavage and blastocyst rates ranking a total of 140 bulls. Subsequently, the dataset was restricted by embryo development rate (blastocyst/cleaved rate) and ten bulls were selected as high (HF; n = 5) and low (LF; n = 5) in vitro fertility groups. IVP embryos derived from high and low fertility bulls were classified according to their stage of development (2 cells, 3-4 cells, 6 cells, 8-16 cells), at 24, 36, 48, 60, 72 hpi, respectively, to evaluate embryo kinetics. Pronuclei formation (24 hpi), cleavage rate (Day 3), development rate, and blastocyst morphology (Grade I and II - Day 7) were also assessed, as well as the abundance of 96 transcripts at 8-16 cell stage and blastocysts. There was no difference in early embryo kinetics (P > 0.05), and cleavage rate (HF = 86.7%; LF = 84.9%; P = 0.25). Nevertheless, the fertilization rate was higher on HF (72%) than LF (62%) and the polyspermy rate was lower on HF compared to LF (HF:16.2% LF:29.2%). As expected, blastocyst rate (HF = 29.4%; LF = 16.0%; P < 0.0001) and development rate (HF = 33.9% LF = 18.9%; P < 0.0001) were higher in HF than LF. At the 8-16 cell stage, 22 transcripts were differentially represented (P ≤ 0.05) between the two groups. Only PGK1 and TFAM levels were higher in HF while transcripts related to stress (6/22, ∼27%), cell proliferation (6/22, ∼27%), lipid metabolism genes (5/22, ∼23%), and other cellular functions (5/22, ∼23%) were higher on LF embryos. Blastocysts had 9 differentially represented transcripts (P ≤ 0.05); being only ACSL3 and ELOV1 higher in the HF group. Lipid metabolism genes (3/9, 33%) and other cellular functions (6/9, 67%) were higher in the LF group. In conclusion, the timing of the first cleavages is not affected by in vitro bull fertility. However, low in vitro fertility bulls presented higher polyspermy rates and produced 8-16 cells embryos with higher levels of transcripts related to apoptosis and cell damage pathways compared to high in vitro fertility ones. Evidence such as polyspermy and increase in apoptotic and oxidative stress genes at the EGA stage suggest that embryo development is impaired in the LF group leading to the reduction of blastocyst rate.

Exosomes derived from placental trophoblast cells regulate endometrial epithelial receptivity in dairy cows during pregnancy

Inadequate fetomaternal interactions could directly lead to pregnancy failure in dairy cows. Exosomes are widely involved in endometrial matrix remodeling, immune function changes, placental development, and other processes of embryo implantation and pregnancy in dairy cows. However, the role of exosomes derived from placental trophoblast cells in regulating the receptivity of endometrial cells and facilitating fetomaternal interaction remains unclear. In this study, bovine trophoblast cells (BTCs) were obtained from bovine placenta and immortalized by transfection with telomerase reverse transcriptase (TERT). Immortalized BTCs still possess the basic and key properties of primary BTCs without exhibiting any neoplastic transformation signs. Subsequently, the effect of trophoblast-derived exosomes (TDEs) on endometrial receptivity in endometrial epithelial cells (EECs) was determined, and the mechanism whereby TDEs and their proteins participate in the fetomaternal interaction during bovine pregnancy were explored. EECs were co-cultured with the exosomes derived from BTCs treated with progesterone (P4). Such treatment enhanced the expression of the endometrial receptivity factors, integrin αv, β3, Wnt7a, and MUC1 by changing the extracellular environment, metabolism, and redox balance in EECs via proteome alignment, compared with no treatment according to the DIA quantitation analysis. Our study demonstrated that trophoblast-derived exosome proteins are one of the most critical elements in fetomaternal interaction, and their changes may act as a key signal in altering endometrial receptivity and provide a potential target for improving fertility.

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.

Bovine embryo elongation is altered due to maternal fatty acid supplementation

The pre-implantation period is prone to embryonic losses in bovine. Embryo-maternal communication is crucial to support embryo development. Thereby, factors of the uterine fluid (UF) are of specific importance. The maternal diet can affect the UF composition. Since omega 3 fatty acids (omega 3 FA) are considered to be beneficial for reproduction, we investigated if dietary omega 3 FA affected factors in the UF related to embryo elongation. Angus heifers (n = 37) were supplemented with either 450 g of rumen-protected fish oil (omega 3 FA) or sunflower oil (omega 6 FA) for a period of 8 weeks. Following cycle synchronization and artificial insemination, the uteri were flushed post mortem to recover the embryos on day 15 of pregnancy. The UF and tissue samples of endometrium and corpus luteum (CL) were collected. Strikingly, the embryo elongation in the omega 3 group was enhanced compared to the omega 6 group. No differences were observed in uterine prostaglandins, even though the endometrial concentration of their precursor arachidonic acid was reduced in omega 3 compared to omega 6 heifers. The dietary FA neither led to differential expression of target genes in endometrium nor CL nor to a differential abundance of low-density lipoprotein cholesterol, cortisol or amino acids in the UF. Interestingly, the omega 3 group displayed a higher plasma progesterone concentration during luteal growth than the omega 6 group, possibly promoting embryo elongation. Further research should include an ovarian perspective to understand the functional link between dietary omega 3 FA and reproductive outcome.

Embryonic maternal interaction in cattle and its relationship with fertility

Embryo mortality is a major contributor to poor reproductive efficiency and profitability in cattle production systems. While conception is achieved (i.e., the oocyte is fertilized) in the vast majority of cases if insemination is carried out correctly, a significant proportion of the resulting embryos fail to develop to term. Appropriate communication between the developing conceptus and the maternal endometrium is essential for the establishment and maintenance of pregnancy in all mammals. Up to the blastocyst stage, around Days 7-9, contact worth the female reproductive system is not required. However, the process of conceptus elongation after hatching and prior to implantation is entirely maternally driven and is essential to ensure that sufficient quantities of interferon-tau (IFNT) are secreted by the developing conceptus to abrogate the mechanisms that bring about luteolysis. While the importance of conceptus-derived IFNT in maternal recognition of pregnancy and prevention of luteolysis in cattle is unequivocal, many questions, such as the threshold level of IFNT required for pregnancy maintenance, remain unanswered. Furthermore, the precise role of IFNT-independent mechanisms in pregnancy establishment remains to be elucidated. Irrespective of this, failure of the conceptus to elongate undoubtedly results in embryonic loss and is thus believed to contribute greatly to reproductive failure in cattle. This review will address some of these answered questions and try to shed some light on those gaps in knowledge that could potentially contribute to improved embryo survival and reproductive efficiency.

Maternal metabolism affects endometrial expression of oxidative stress and FOXL2 genes in cattle

Intensive selection for milk production has led to reduced reproductive efficiency in high-producing dairy cattle. The impact of intensive milk production on oocyte quality as well as early embryo development has been established but few analyses have addressed this question at the initiation of implantation, a critical milestone ensuring a successful pregnancy and normal post-natal development. Our study aimed to determine if contrasted maternal metabolism affects the previously described sensory properties of the endometrium to the conceptus in cattle. Following embryo transfer at Day 7 post-oestrus, endometrial caruncular (CAR) and intercaruncular (ICAR) areas were collected at Day 19 from primiparous postpartum Holstein-Friesian cows that were dried-off immediately after parturition (i.e., never milked; DRY) or milked twice daily (LACT). Gene quantification indicated no significant impact of lactation on endometrial expression of transcripts previously reported as conceptus-regulated (PLET1, PTGS2, SOCS6) and interferon-tau stimulated (RSAD2, SOCS1, SOCS3, STAT1) factors or known as female hormone-regulated genes (FOXL2, SCARA5, PTGS2). Compared with LACT cows, DRY cows exhibited mRNA levels with increased expression for FOXL2 transcription factor and decreased expression for oxidative stress-related genes (CAT, SOD1, SOD2). In vivo and in vitro experiments highlighted that neither interferon-tau nor FOXL2 were involved in transcriptional regulation of CAT, SOD1 and SOD2. In addition, our data showed that variations in maternal metabolism had a higher impact on gene expression in ICAR areas. Collectively, our findings prompt the need to fully understand the extent to which modifications in endometrial physiology drive the trajectory of conceptus development from implantation onwards when maternal metabolism is altered.

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.