Embryo–maternal cross-talk has emerged as a vitally important process for embryo development and implantation, which is driven by secreted factors and extracellular vesicles (EVs). The EV cargo of bioactive molecules significantly influences target cells and primes them for critical stages of reproductive biology, including embryo development, adhesion, and implantation. Recent research has suggested that EVs and their cargo represent a powerful non-invasive tool that can be leveraged to assess embryo and maternal tissue quality during assisted reproduction treatments. Here, we review the current scientific literature regarding the intercellular cross-talk between embryos and maternal tissues from fertilization to implantation, focusing on human biology and signaling mechanisms identified in animal models.

The transmission of DNA through extracellular vesicles (EVs) represents a novel genetic material transfer mechanism that may impact genome evolution and tumorigenesis. We aimed to investigate the potential for vertical mitochondrial (mt)DNA transmission within maternal endometrial EVs to the pre-implantation embryo and describe any effect on embryo bioenergetics. We discovered that the human endometrium secretes all three general subtypes of EV - apoptotic bodies (ABs), microvesicles (MVs), and exosomes (EXOs) - into the human endometrial fluid (EF) within the uterine cavity. EVs become uniformly secreted into the EF during the menstrual cycle, with the proportion of different EV populations remaining constant; however, MVs contain significantly higher levels of DNA than ABs or EXOs. During the window of implantation, MVs contain an eleven-fold higher level of mtDNA when compared to cells-of-origin within the receptive endometrium, which possesses a lower mtDNA content and displays the upregulated expression of mitophagy-related genes. Furthermore, we demonstrate the internalization of EV-derived mtDNA by trophoblast cells of murine embryos, which associated with a reduction in mitochondrial respiration and ATP production. These findings suggest that the maternal endometrium suffers a reduction in mtDNA content during the preconceptional period, that mtDNA becomes packaged into secreted EVs that the embryo uptakes, and that the transfer of mtDNA to the embryo within EVs occurs alongside the modulation of bioenergetics during implantation.

The transmission of DNA through extracellular vesicles (EVs) represents a novel genetic material transfer mechanism that may impact genome evolution and tumorigenesis. We aimed to investigate the potential for vertical mitochondrial (mt)DNA transmission within maternal endometrial EVs to the pre-implantation embryo and describe any effect on embryo bioenergetics. We discovered that the human endometrium secretes all three general subtypes of EV - apoptotic bodies (ABs), microvesicles (MVs), and exosomes (EXOs) - into the human endometrial fluid (EF) within the uterine cavity. EVs become uniformly secreted into the EF during the menstrual cycle, with the proportion of different EV populations remaining constant; however, MVs contain significantly higher levels of DNA than ABs or EXOs. During the window of implantation, MVs contain an eleven-fold higher level of mtDNA when compared to cells-of-origin within the receptive endometrium, which possesses a lower mtDNA content and displays the upregulated expression of mitophagy-related genes. Furthermore, we demonstrate the internalization of EV-derived mtDNA by trophoblast cells of murine embryos, which associated with a reduction in mitochondrial respiration and ATP production. These findings suggest that the maternal endometrium suffers a reduction in mtDNA content during the preconceptional period, that mtDNA becomes packaged into secreted EVs that the embryo uptakes, and that the transfer of mtDNA to the embryo within EVs occurs alongside the modulation of bioenergetics during implantation.

The transmission of DNA through extracellular vesicles (EVs) represents a novel genetic material transfer mechanism that may impact genome evolution and tumorigenesis. We aimed to investigate the potential for vertical mitochondrial (mt)DNA transmission within maternal endometrial EVs to the pre-implantation embryo and describe any effect on embryo bioenergetics. We discovered that the human endometrium secretes all three general subtypes of EV - apoptotic bodies (ABs), microvesicles (MVs), and exosomes (EXOs) - into the human endometrial fluid (EF) within the uterine cavity. EVs become uniformly secreted into the EF during the menstrual cycle, with the proportion of different EV populations remaining constant; however, MVs contain significantly higher levels of DNA than ABs or EXOs. During the window of implantation, MVs contain an eleven-fold higher level of mtDNA when compared to cells-of-origin within the receptive endometrium, which possesses a lower mtDNA content and displays the upregulated expression of mitophagy-related genes. Furthermore, we demonstrate the internalization of EV-derived mtDNA by trophoblast cells of murine embryos, which associated with a reduction in mitochondrial respiration and ATP production. These findings suggest that the maternal endometrium suffers a reduction in mtDNA content during the preconceptional period, that mtDNA becomes packaged into secreted EVs that the embryo uptakes, and that the transfer of mtDNA to the embryo within EVs occurs alongside the modulation of bioenergetics during implantation.

Study question Does maternal endometrial mitochondrial (mt) DNA cargo of EVs modulate embryo bioenergetics during embryo implantation? Summary answer We demonstrate the vertical transmission of maternal mtDNA within endometrial-derived EVs and their uptake by the trophoblast which reduces mitochondrial respiration and ATP production. What is known already The release and uptake of membrane-enclosed compartments with specific cargos, commonly known as EVs, represents a novel cell-to-cell communication mechanism in physiological and pathogenic conditions. EVs are generally classified into three populations based on their biogenetic pathways, composition, and physical characteristics: apoptotic bodies (ABs), microvesicles (MVs), and exosomes (EXOs). Among other contents, EVs contain single- and double-stranded DNA, with their relative abundance varying depending on the cell and vesicle type. The vertical transmission of EV-associated DNA has been proposed as a novel genetic material transfer mechanism that may impact genome evolution and tumorigenesis. Study design, size, duration Prospective observational multicenter analysis in which EVs were obtained from endometrial fluid from healthy donors aged 18–35 years (n = 10) during the receptive phase of their natural cycle and under hormonal replacement therapy in pre-receptive (P + 2), receptive (P + 5), post-receptive (P + 8) stages (n = 13). Participants/materials, setting, methods Endometrial EVs isolated using ultracentrifugation and classified according to `parameters obtained from electron microscopy, Western blotting, and size distribution analysis. DNA copy number identified using high throughput sequencing. EV-associated DNA tagged with 5-ethynyl-2’-deoxyuridine was followed by confocal imaging after co-incubation of EVs with murine embryos (n = 200). ATP levels assessed using the FLASC luciferase reporter system, and the Seahorse XFE96 extracellular flux analyzer used to measure embryo oxygen consumption rate (OCR) (n = 400). Main results and the role of chance The human endometrium secretes all three EV types - ABs, MVs, and EXOs - into the human endometrial fluid. Deep sequencing revealed that EVs encapsulated nuclear and mtDNA. When analyzing endometrial biopsies, we observed the reduced mtDNA content of endometrial cells and the activation of mitochondrial clearance mechanisms, which coincided with the time of embryo implantation together with specific enrichment in endometrial MVs secreted during the periconceptional period. EVs were internalized and DNA was transferred to the cytoplasm and nuclei of trophectoderm of murine embryos. We analyzed ATP concentrations in murine embryos and found a significant reduction in ATP levels following the coculture of embryos with a combination of all EVs types compared to control embryos cultured without endometrial EVs (p < 0.001). Finally, we demonstrated a reduction in the OCR in embryos treated with endometrial EVs obtained during the receptive phase compared with the pre-receptive phase. In conclusion, maternal EVs modulate the bioenergetics of the preimplantation embryo by increasing the embryo's metabolic rate and oxygen consumption during the periconceptional period. Limitations, reasons for caution These results were obtained using a combination of a human endometrial model and a murine embryo model. Wider implications of the findings Our results suggest that the vertical transmission of maternal mtDNA encapsulated within EVs to the trophectoderm might energetically assist the preimplantation embryo through the implantation process. Trial registration number N/A