Generation of human endothelium in pig embryos deficient in ETV2

Microenvironmental determinants of endothelial cell heterogeneity

During development, endothelial cells (ECs) undergo an extraordinary specialization by which generic capillary microcirculatory networks spanning from arteries to veins transform into patterned organotypic zonated blood vessels. These capillary ECs become specialized to support the cellular and metabolic demands of each specific organ, including supplying tissue-specific angiocrine factors that orchestrate organ development, maintenance of organ-specific functions and regeneration of injured adult organs. Here, we illustrate the mechanisms by which microenvironmental signals emanating from non-vascular niche cells induce generic ECs to acquire specific inter-organ and intra-organ functional attributes. We describe how perivascular, parenchymal and immune cells dictate vascular heterogeneity and capillary zonation, and how this system is maintained through tissue-specific signalling activated by vasculogenic and angiogenic factors and deposition of matrix components. We also discuss how perturbation of organotypic vascular niche cues lead to erasure of EC signatures, contributing to the pathogenesis of disease processes. We also describe approaches that use reconstitution of tissue-specific signatures of ECs to promote regeneration of damaged organs.

Allogeneic, Xenogeneic, and Exogenic Hearts for Transplantation

The only curative therapy for end-stage heart failure is orthotopic allogeneic heart transplantation. This therapy has extended the survival of patients worldwide but is limited due to the scarcity of donor organs. Potential alternative donor sources of organs for transplantation include genetically-modified (GM) large animal donors (ie, xenografts) and human organs developed in large animal hosts. These strategies utilize gene editing and somatic cell nuclear transfer technologies to engineer partially or completely humanized organs. Preclinical xenotransplantation studies of GM pig hearts into baboons have already provided an important clinical foundation, as two patients have received cardiac xenografts from GM pigs and have survived for up to 2 months. Additional issues need to be addressed in order for patients to survive more than 1 year, which would make these strategies clinically applicable. Thus, in combination with immunosuppression agents, xenogeneic and exogenic organ sources hold tremendous promise for an unlimited and transformative supply of organs for transplantation.

Interleukin-7 promotes porcine early embryogenesis in vitro and inner cell mass development through PI3K/AKT pathway after parthenogenetic activation

Interleukin-7 (IL-7) plays a crucial role in cell survival and proliferation through the phosphatidylinositol-3-kinase (PI3K)/AKT signaling. While we previously demonstrated the beneficial role of IL-7 in early porcine embryonic development, the underlying molecular mechanisms remained unclear. We hypothesized that IL-7 would enhance early embryogenesis and promote inner cell mass (ICM) formation via PI3K/AKT pathway activation. To test this, embryos were cultured with wortmannin (Wort), a PI3K inhibitor, with or without IL-7 after parthenogenetic activation. IL-7 supplementation significantly increased cleavage and blastocyst formation rates compared to the control (p < 0.05), while mitigating Wort-induced developmental impairment. Moreover, IL-7 significantly reduced blastocyst apoptosis and increased total cell numbers compared to the control (p < 0.05), thereby counteracting pro-apoptotic effects of Wort. Furthermore, IL-7 treatment significantly promoted ICM formation through the PI3K/AKT pathway, as demonstrated by increased SOX2 + cell numbers and ICM-specific gene expression, with elevated phosphorylated AKT levels compared to the control (p < 0.05). Notably, IL-7 significantly improved mitochondrial function and biogenesis-related gene expression compared to the control (p < 0.05) through a PI3K/AKT-independent pathway. These findings suggest that IL-7-mediated PI3K/AKT signaling enhances porcine early embryonic development in vitro, providing insights into mechanisms that regulate early embryonic development in mammals.

Cell therapies and liver organogenesis technologies: Promising strategies for end-stage liver disease

End-stage liver disease represents a critical hepatic condition with high mortality, for which liver transplantation remains the only effective treatment. However, the scarcity of suitable donors results in numerous patients dying while awaiting transplantation. Novel strategies, including cell therapies and technologies mimicking liver organogenesis, offer promising alternatives for treating end-stage liver disease by potentially providing new sources of liver grafts. Recently, significant progress has been made in this field, including stem cell transplantation, hepatocyte transplantation, in vitro liver tissue generation, and liver replacement technologies. Several clinical studies have demonstrated that stem cell transplantation and hepatocyte transplantation can prolong patient survival and serve as a bridge to liver transplantation. Furthermore, in vitro liver tissue generation technologies, such as liver organoids and three-dimensional bioprinting, can generate hepatic tissues with sophisticated structures and functions, making them promising transplantation materials. Notably, liver replacement technologies hold considerable potential for producing biologically functional and transplantable liver grafts. In this review, we discuss the fundamental principles and recent advancements in cell therapies and liver organogenesis technologies while also addressing the challenges and future prospects in this rapidly evolving field.

Interspecies Blastocyst Complementation and the Genesis of Chimeric Solid Human Organs

Solid organ transplantation remains a life-saving treatment for patients worldwide. Unfortunately, the supply of donor organs cannot meet the current need, making the search for alternative sources even more essential. Xenotransplantation using sophisticated genetic engineering techniques to delete and overexpress specific genes in the donor animal has been investigated as a possible option. However, the use of exogenous tissue presents another host of obstacles, particularly regarding organ rejection. Given these limitations, interspecies blastocyst complementation in combination with precise gene knockouts presents a unique, promising pathway for the transplant organ shortage. In recent years, great advancements have been made in the field, with encouraging results in producing a donor-derived organ in a chimeric host. That said, one of the major barriers to successful interspecies chimerism is the mismatch in the developmental stages of the donor and the host cells in the chimeric embryo. Another major barrier to successful chimerism is the mismatch in the developmental speeds between the donor and host cells in the chimeric embryos. This review outlines 19 studies in which blastocyst complementation was used to generate solid organs. In particular, the genesis of the liver, lung, kidney, pancreas, heart, thyroid, thymus and parathyroids was investigated. Of the 19 studies, 7 included an interspecies model. Of the 7, one was completed using human donor cells in a pig host, and all others were rat-mouse chimeras. While very promising results have been demonstrated, with great advancements in the field, several challenges continue to persist. In particular, successful chimerism, organ generation and donor contribution, synchronized donor-host development, as well as ethical concerns regarding human-animal chimeras remain important aspects that will need to be addressed in future research.

Foxa1 disruption enhances human cell integration in human-mouse interspecies chimeras

Blastocyst complementation can potentially generate a rodent model with humanized nasopharyngeal epithelium (NE) that supports sustained Epstein-Barr virus (EBV) infection, enabling comprehensive studies of EBV biology in nasopharyngeal carcinoma. However, during this process, the specific gene knockouts required to establish a developmental niche for NE remain unclear. We performed bioinformatics analyses and generated Foxa1 mutant mice to confirm that Foxa1 disruption could potentially create a developmental niche for NE. Subsequently, MYD88-inactivated human pluripotent stem cells (hPSCs) were constructed and complemented with Foxa1-deficient mouse blastocysts, with Nosip-deficient mouse blastocysts as a control. The chimerism of human cells in mouse embryos was evaluated from E8.5 to E12.5 using genomic DNA PCR and immunohistochemistry. Our bioinformatics analysis indicated that the expression patterns of Foxa1 in E8.5 to E16.5 mouse embryos underscore its critical role in NE development. The generated mice with Foxa1 disordered region mutations displayed morphological abnormality in NE, suggesting Foxa1-knockouts could potentially establish a developmental niche for NE. In chimeric assays, human cells integrated into 80.00% of Foxa1-deficient embryos, compared with the 4.17% in controls. Immunohistochemistry results revealed robust proliferation of human cells in Foxa1-deficient mouse embryos. However, chimeras from Foxa1-deficient mouse embryos did not survive beyond E10.5, hindering the evaluation of human cell integration in mouse NE. Foxa1 disruption in mouse embryos significantly enhances the integration of human cells in human-mouse interspecies chimeras, thereby facilitating the generation of endoderm-derived organs through blastocyst complementation. Overcoming chimeras' embryonic lethality is crucial for successfully generating humanized NE in Foxa1-deficient mouse embryos.

Research Guideline Recommendations for Research on Stem Cells, Human Embryos, and Gene Editing

Recent advances in biomedical technologies have extended the boundaries of previously established regulatory guidelines pertaining to stem cell research. These guidelines constrained the study of human pluripotent stem cells (hPSCs) and their derivatives from use under various conditions, including the introduction of hPSCs into the brains of host animals because of concerns of humanizing the brains of animal species. Other guidelines constrained the use of hPSCs in creating human-animal chimeras because of the potential contribution of human stem cells not only to the brain but also to the germline. Some regulatory guidelines forbid the growing of human embryos ex vivo beyond the stage of primitive streak development because of concerns regarding the creation of human forms of life ex vivo. At the subcellular level, there are guidelines regulating the transfer of mitochondria within human embryos. At the molecular level, there are guidelines regulating genome editing to prevent permanent genetic alterations in germline cells. These and other issues related to stem cells have been reviewed, and new research guidelines established by the International Society for Stem Cell Research (ISSCR) for its membership. Because many of the recommended changes by the ISSCR impact research being conducted by members of the American Society for Neural Therapy and Repair (ASNTR), the ASNTR established a task force to review relevant recommendations by the ISSCR to determine which new guidelines to adopt for research conducted by the ASNTR society membership. The final ASNTR recommendations are presented in this document.

Blastocyst complementation generates exogenous donor-derived liver in ahepatic pigs

Liver diseases are one of the leading causes of morbidity and mortality worldwide. Globally, liver diseases are responsible for approximately 2 million deaths annually (1 of every 25 deaths). Many of the patients with chronic liver diseases can benefit from organ transplantation. However, stringent criteria for placement on organ transplantation waitlist and chronic shortage of organs preclude access to patients. To bridge the shortfall, generation of chimeric human organs in pigs has long been considered as an alternative. Here, we report feasibility of the approach by generating chimeric livers in pigs using a conditional blastocyst complementation approach that creates a vacant niche in chimeric hosts, enabling the initiation of organogenesis through donor-derived pluripotent cells. Porcine fetal fibroblasts were sequentially targeted for knockin of CRE into the endogenous FOXA3 locus (FOXA3CRE) followed by floxing of exon 1 of HHEX (FOXA3CREHHEXloxP/loxP) locus. The conditional HHEX knockout and constitutive GFP donor (COL1ACAG:LACZ 2A EGFP) were used as nuclear donors to generate host embryos by somatic cell nuclear transfer, and complemented and transferred into estrus synchronized surrogates. In the resulting fetuses, donor EGFP blastomeres reconstituted hepatocytes as confirmed by immunohistochemistry. These results potentially pave the way for exogenous donor-derived hepatogenesis in large animal models.

The perspective for next-generation lung replacement therapies: functional whole lung generation by blastocyst complementation

PURPOSE OF REVIEW

Blastocyst complementation represents a promising frontier in next-generation lung replacement therapies. This review aims to elucidate the future prospects of lung blastocyst complementation within clinical settings, summarizing the latest studies on generating functional lungs through this technique. It also explores and discusses host animal selection relevant to interspecific chimera formation, a challenge integral to creating functional human lungs via blastocyst complementation.

RECENT FINDINGS

Various gene mutations have been utilized to create vacant lung niches, enhancing the efficacy of donor cell contribution to the complemented lungs in rodent models. By controlling the lineage to induce gene mutations, chimerism in both the lung epithelium and mesenchyme has been improved. Interspecific blastocyst complementation underscores the complexity of developmental programs across species, with several genes identified that enhance chimera formation between humans and other mammals.

SUMMARY

While functional lungs have been generated via intraspecies blastocyst complementation, the generation of functional interspecific lungs remains unrealized. Addressing the challenges of controlling the host lung niche and selecting host animals relevant to interspecific barriers between donor human and host cells is critical to enabling the generation of functional humanized or entire human lungs in large animals.

Producing Human Livers From Human Stem Cells Via Blastocyst Complementation

The need for organ transplants exceeds donor organ availability. In the quest to solve this shortage, the most remarkable area of advancement is organ production through the use of chimeric embryos, commonly known as blastocyst complementation. This technique involves the combination of different species to generate chimeras, where the extent of donor cell contribution to the desired tissue or organ can be regulated. However, ethical concerns arise with the use of brain tissue in such chimeras. Furthermore, the ratio of contributed cells to host animal cells in the chimeric system is low in the production of chimeras associated with cell apoptosis. This review discusses the latest innovations in blastocyst complementation and highlights the progress made in creating organs for transplant.

Parathyroid Gland Generation from Pluripotent Stem Cells

Patients with permanent hypoparathyroidism require lifelong treatment. Current replacement therapies sometimes have adverse effects (e.g., hypercalciuria and chronic kidney disease). Generating parathyroid glands (PTGs) from the patient's own induced pluripotent stem cells (PSCs), with transplantation of these PTGs, would be an effective treatment option. Multiple methods for generating PTGs from PSCs have been reported. One major trend is in vitro differentiation of PSCs into PTGs. Another is in vivo generation of PSC-derived PTGs by injecting PSCs into PTG-deficient embryos. This review discusses current achievements and challenges in present and future PTG regenerative medicine.

Toward developing human organs via embryo models and chimeras

Developing functional organs from stem cells remains a challenging goal in regenerative medicine. Existing methodologies, such as tissue engineering, bioprinting, and organoids, only offer partial solutions. This perspective focuses on two promising approaches emerging for engineering human organs from stem cells: stem cell-based embryo models and interspecies organogenesis. Both approaches exploit the premise of guiding stem cells to mimic natural development. We begin by summarizing what is known about early human development as a blueprint for recapitulating organogenesis in both embryo models and interspecies chimeras. The latest advances in both fields are discussed before highlighting the technological and knowledge gaps to be addressed before the goal of developing human organs could be achieved using the two approaches. We conclude by discussing challenges facing embryo modeling and interspecies organogenesis and outlining future prospects for advancing both fields toward the generation of human tissues and organs for basic research and translational applications.

A single-cell atlas of pig gastrulation as a resource for comparative embryology

Cell-fate decisions during mammalian gastrulation are poorly understood outside of rodent embryos. The embryonic disc of pig embryos mirrors humans, making them a useful proxy for studying gastrulation. Here we present a single-cell transcriptomic atlas of pig gastrulation, revealing cell-fate emergence dynamics, as well as conserved and divergent gene programs governing early porcine, primate, and murine development. We highlight heterochronicity in extraembryonic cell-types, despite the broad conservation of cell-type-specific transcriptional programs. We apply these findings in combination with functional investigations, to outline conserved spatial, molecular, and temporal events during definitive endoderm specification. We find early FOXA2 + /TBXT- embryonic disc cells directly form definitive endoderm, contrasting later-emerging FOXA2/TBXT+ node/notochord progenitors. Unlike mesoderm, none of these progenitors undergo epithelial-to-mesenchymal transition. Endoderm/Node fate hinges on balanced WNT and hypoblast-derived NODAL, which is extinguished upon endodermal differentiation. These findings emphasise the interplay between temporal and topological signalling in fate determination during gastrulation.

Generation of allogeneic and xenogeneic functional muscle stem cells for intramuscular transplantation

Satellite cells, the stem cells of skeletal muscle tissue, hold a remarkable regeneration capacity and therapeutic potential in regenerative medicine. However, low satellite cell yield from autologous or donor-derived muscles hinders the adoption of satellite cell transplantation for the treatment of muscle diseases, including Duchenne muscular dystrophy (DMD). To address this limitation, here we investigated whether satellite cells can be derived in allogeneic or xenogeneic animal hosts. First, injection of CRISPR/Cas9-corrected Dmdmdx mouse induced pluripotent stem cells (iPSCs) into mouse blastocysts carrying an ablation system of host satellite cells gave rise to intraspecies chimeras exclusively carrying iPSC-derived satellite cells. Furthermore, injection of genetically corrected DMD iPSCs into rat blastocysts resulted in the formation of interspecies rat-mouse chimeras harboring mouse satellite cells. Notably, iPSC-derived satellite cells or derivative myoblasts produced in intraspecies or interspecies chimeras restored dystrophin expression in DMD mice following intramuscular transplantation and contributed to the satellite cell pool. Collectively, this study demonstrates the feasibility of producing therapeutically competent stem cells across divergent animal species, raising the possibility of generating human muscle stem cells in large animals for regenerative medicine purposes.

Chimeric Livers: Interspecies Blastocyst Complementation and Xenotransplantation for End-Stage Liver Disease

Orthotopic liver transplantation (OLT) currently serves as the sole definitive treatment for thousands of patients suffering from end-stage liver disease; and the existing supply of donor livers for OLT is drastically outpaced by the increasing demand. To alleviate this significant gap in treatment, several experimental approaches have been devised with the aim of either offering interim support to patients waiting on the transplant list or bioengineering complete livers for OLT by infusing them with fresh hepatic cells. Recently, interspecies blastocyst complementation has emerged as a promising method for generating complete organs in utero over a short timeframe. When coupled with gene editing technology, it has brought about a potentially revolutionary transformation in regenerative medicine. Blastocyst complementation harbors notable potential for generating complete human livers in large animals, which could be used for xenotransplantation in humans, addressing the scarcity of livers for OLT. Nevertheless, substantial experimental and ethical challenges still need to be overcome to produce human livers in larger domestic animals like pigs. This review compiles the current understanding of interspecies blastocyst complementation and outlines future possibilities for liver xenotransplantation in humans.

Interspecies chimerism with human embryonic stem cells generates functional human dopamine neurons at low efficiency

Interspecies chimeras offer great potential for regenerative medicine and the creation of human disease models. Whether human pluripotent stem cell-derived neurons in an interspecies chimera can differentiate into functional neurons and integrate into host neural circuity is not known. Here, we show, using Engrailed 1 (En1) as a development niche, that human naive-like embryonic stem cells (ESCs) can incorporate into embryonic and adult mouse brains. Human-derived neurons including tyrosine hydroxylase (TH)+ neurons integrate into the mouse brain at low efficiency. These TH+ neurons have electrophysiologic properties consistent with their human origin. In addition, these human-derived neurons in the mouse brain accumulate pathologic phosphorylated α-synuclein in response to α-synuclein preformed fibrils. Optimization of human/mouse chimeras could be used to study human neuronal differentiation and human brain disorders.

Generation of rat-derived lung epithelial cells in Fgfr2b-deficient mice retains species-specific development

Regenerative medicine is a tool to compensate for the shortage of lungs for transplantation, but it remains difficult to construct a lung in vitro due to the complex three-dimensional structures and multiple cell types required. A blastocyst complementation method using interspecies chimeric animals has been attracting attention as a way to create complex organs in animals, although successful lung formation using interspecies chimeric animals has not yet been achieved. Here, we applied a reverse-blastocyst complementation method to clarify the conditions required to form lungs in an Fgfr2b-deficient mouse model. We then successfully formed a rat-derived lung in the mouse model by applying a tetraploid-based organ-complementation method. Importantly, rat lung epithelial cells retained their developmental timing even in the mouse body. These findings provide useful insights to overcome the barrier of species-specific developmental timing to generate functional lungs in interspecies chimeras.

Generation of heart and vascular system in rodents by blastocyst complementation

Generating organs from stem cells through blastocyst complementation is a promising approach to meet the clinical need for transplants. In order to generate rejection-free organs, complementation of both parenchymal and vascular cells must be achieved, as endothelial cells play a key role in graft rejection. Here, we used a lineage-specific cell ablation system to produce mouse embryos unable to form both the cardiac and vascular systems. By mouse intraspecies blastocyst complementation, we rescued heart and vascular system development separately and in combination, obtaining complemented hearts with cardiomyocytes and endothelial cells of exogenous origin. Complemented chimeras were viable and reached adult stage, showing normal cardiac function and no signs of histopathological defects in the heart. Furthermore, we implemented the cell ablation system for rat-to-mouse blastocyst complementation, obtaining xenogeneic hearts whose cardiomyocytes were completely of rat origin. These results represent an advance in the experimentation towards the in vivo generation of transplantable organs.

Live birth of chimeric monkey with high contribution from embryonic stem cells

A formal demonstration that mammalian pluripotent stem cells possess preimplantation embryonic cell-like (naive) pluripotency is the generation of chimeric animals through early embryo complementation with homologous cells. Whereas such naive pluripotency has been well demonstrated in rodents, poor chimerism has been achieved in other species including non-human primates due to the inability of the donor cells to match the developmental state of the host embryos. Here, we have systematically tested various culture conditions for establishing monkey naive embryonic stem cells and optimized the procedures for chimeric embryo culture. This approach generated an aborted fetus and a live chimeric monkey with high donor cell contribution. A stringent characterization pipeline demonstrated that donor cells efficiently (up to 90%) incorporated into various tissues (including the gonads and placenta) of the chimeric monkeys. Our results have major implications for the study of primate naive pluripotency and genetic engineering of non-human primates.

Generation of a humanized mesonephros in pigs from induced pluripotent stem cells via embryo complementation

Heterologous organ transplantation is an effective way of replacing organ function but is limited by severe organ shortage. Although generating human organs in other large mammals through embryo complementation would be a groundbreaking solution, it faces many challenges, especially the poor integration of human cells into the recipient tissues. To produce human cells with superior intra-niche competitiveness, we combined optimized pluripotent stem cell culture conditions with the inducible overexpression of two pro-survival genes (MYCN and BCL2). The resulting cells had substantially enhanced viability in the xeno-environment of interspecies chimeric blastocyst and successfully formed organized human-pig chimeric middle-stage kidney (mesonephros) structures up to embryonic day 28 inside nephric-defective pig embryos lacking SIX1 and SALL1. Our findings demonstrate proof of principle of the possibility of generating a humanized primordial organ in organogenesis-disabled pigs, opening an exciting avenue for regenerative medicine and an artificial window for studying human kidney development.

Human and Pig Pluripotent Stem Cells: From Cellular Products to Organogenesis and Beyond

Pluripotent stem cells (PSCs) are important for studying development and hold great promise in regenerative medicine due to their ability to differentiate into various cell types. In this review, we comprehensively discuss the potential applications of both human and pig PSCs and provide an overview of the current progress and challenges in this field. In addition to exploring the therapeutic uses of PSC-derived cellular products, we also shed light on their significance in the study of interspecies chimeras, which has led to the creation of transplantable human or humanized pig organs. Moreover, we emphasize the importance of pig PSCs as an ideal cell source for genetic engineering, facilitating the development of genetically modified pigs for pig-to-human xenotransplantation. Despite the achievements that have been made, further investigations and refinement of PSC technologies are necessary to unlock their full potential in regenerative medicine and effectively address critical healthcare challenges.

Recapitulating porcine cardiac development in vitro: from expanded potential stem cell to embryo culture models

Domestic pigs (Sus scrofa) share many genetic, anatomical, and physiological traits with humans and therefore constitute an excellent preclinical animal model. Fundamental understanding of the cellular and molecular processes governing early porcine cardiogenesis is critical for developing advanced porcine models used for the study of heart diseases and new regenerative therapies. Here, we provide a detailed characterization of porcine cardiogenesis based on fetal porcine hearts at various developmental stages and cardiac cells derived from porcine expanded pluripotent stem cells (pEPSCs), i.e., stem cells having the potential to give rise to both embryonic and extraembryonic tissue. We notably demonstrate for the first time that pEPSCs can differentiate into cardiovascular progenitor cells (CPCs), functional cardiomyocytes (CMs), epicardial cells and epicardial-derived cells (EPDCs) in vitro. Furthermore, we present an enhanced system for whole-embryo culture which allows continuous ex utero development of porcine post-implantation embryos from the cardiac crescent stage (ED14) up to the cardiac looping (ED17) stage. These new techniques provide a versatile platform for studying porcine cardiac development and disease modeling.

Review: Progress in producing chimeric ungulate livestock for agricultural applications

The progress made in recent years in the derivation and culture of pluripotent stem cells from farm animals opens up the possibility of creating livestock chimeras. Chimeras producing gametes exclusively derived from elite donor stem cells could pass superior genetics on to the next generation and thereby reduce the genetic lag that typically exists between the elite breeding sector and the commercial production sector, especially for industries like beef and sheep where genetics is commonly disseminated through natural service mating. Chimeras carrying germ cells generated from genome-edited or genetically engineered pluripotent stem cells could further disseminate useful genomic alterations such as climate adaptation, animal welfare improvements, the repair of deleterious genetic conditions, and/or the elimination of undesired traits such as disease susceptibility to the next generation. Despite the successful production of chimeras with germ cells generated from pluripotent donor stem cells injected into preimplantation-stage blastocysts in model species, there are no documented cases of this occurring in livestock. Here, we review the literature on the derivation of pluripotent stem cells from ungulates, and progress in the production of chimeric ungulate livestock for agricultural applications, drawing on insights from studies done in model species, and discuss future possibilities of this fast-moving and developing field. Aside from the technical aspects, the consistency of the regulatory approach taken by different jurisdictions towards chimeric ungulate livestock with germ cells generated from pluripotent stem cells and their progeny will be an important determinant of breeding industry uptake and adoption in animal agriculture.

ETV2/ER71, the key factor leading the paths to vascular regeneration and angiogenic reprogramming

Extensive efforts have been made to achieve vascular regeneration accompanying tissue repair for treating vascular dysfunction-associated diseases. Recent advancements in stem cell biology and cell reprogramming have opened unforeseen opportunities to promote angiogenesis in vivo and generate autologous endothelial cells (ECs) for clinical use. We have, for the first time, identified a unique endothelial-specific transcription factor, ETV2/ER71, and revealed its essential role in regulating endothelial cell generation and function, along with vascular regeneration and tissue repair. Furthermore, we and other groups have demonstrated its ability to directly reprogram terminally differentiated non-ECs into functional ECs, proposing ETV2/ER71 as an effective therapeutic target for vascular diseases. In this review, we discuss the up-to-date status of studies on ETV2/ER71, spanning from its molecular mechanism to vasculo-angiogenic role and direct cell reprogramming toward ECs. Furthermore, we discuss future directions to deploy the clinical potential of ETV2/ER71 as a novel and potent target for vascular disorders such as cardiovascular disease, neurovascular impairment and cancer.

RNA Sensing and Innate Immunity Constitutes a Barrier for Interspecies Chimerism

Interspecies chimera formation with human pluripotent stem cells (PSCs) holds great promise to generate humanized animal models and provide donor organs for transplant. However, the approach is currently limited by low levels of human cells ultimately represented in chimeric embryos. Different strategies have been developed to improve chimerism by genetically editing donor human PSCs. To date, however, it remains unexplored if human chimerism can be enhanced in animals through modifying the host embryos. Leveraging the interspecies PSC competition model, here we discovered retinoic acid-inducible gene I (RIG-I)-like receptor (RLR) signaling, an RNA sensor, in "winner" cells plays an important role in the competitive interactions between co-cultured mouse and human PSCs. We found that genetic inactivation of Ddx58/Ifih1-Mavs-Irf7 axis compromised the "winner" status of mouse PSCs and their ability to outcompete PSCs from evolutionarily distant species during co-culture. Furthermore, by using Mavs-deficient mouse embryos we substantially improved unmodified donor human cell survival. Comparative transcriptome analyses based on species-specific sequences suggest contact-dependent human-to-mouse transfer of RNAs likely plays a part in mediating the cross-species interactions. Taken together, these findings establish a previously unrecognized role of RNA sensing and innate immunity in "winner" cells during cell competition and provides a proof-of-concept for modifying host embryos, rather than donor PSCs, to enhance interspecies chimerism.

ETV2 and VEZF1 interaction and regulation of the hematoendothelial lineage during embryogenesis

Ets variant 2 (Etv2), a member of the Ets factor family, has an essential role in the formation of endothelial and hematopoietic cell lineages during embryonic development. The functional role of ETS transcription factors is, in part, dependent on the interacting proteins. There are relatively few studies exploring the coordinated interplay between ETV2 and its interacting proteins that regulate mesodermal lineage determination. In order to identify novel ETV2 interacting partners, a yeast two-hybrid analysis was performed and the C2H2 zinc finger transcription factor VEZF1 (vascular endothelial zinc finger 1) was identified as a binding factor, which was specifically expressed within the endothelium during vascular development. To confirm this interaction, co-immunoprecipitation and GST pull down assays demonstrated the direct interaction between ETV2 and VEZF1. During embryoid body differentiation, Etv2 achieved its peak expression at day 3.0 followed by rapid downregulation, on the other hand Vezf1 expression increased through day 6 of EB differentiation. We have previously shown that ETV2 potently activated Flt1 gene transcription. Using a Flt1 promoter-luciferase reporter assay, we demonstrated that VEZF1 co-activated the Flt1 promoter. Electrophoretic mobility shift assay and Chromatin immunoprecipitation established VEZF1 binding to the Flt1 promoter. Vezf1 knockout embryonic stem cells had downregulation of hematoendothelial marker genes when undergoing embryoid body mediated mesodermal differentiation whereas overexpression of VEZF1 induced the expression of hematoendothelial genes during differentiation. These current studies provide insight into the co-regulation of the hemato-endothelial lineage development via a co-operative interaction between ETV2 and VEZF1.

Towards human organ generation using interspecies blastocyst complementation: Challenges and perspectives for therapy

Millions of people suffer from end-stage refractory diseases. The ideal treatment option for terminally ill patients is organ transplantation. However, donor organs are in absolute shortage, and sadly, most patients die while waiting for a donor organ. To date, no technology has achieved long-term sustainable patient-derived organ generation. In this regard, emerging technologies of chimeric human organ production via blastocyst complementation (BC) holds great promise. To take human organ generation via BC and transplantation to the next step, we reviewed current emerging organ generation technologies and the associated efficiency of chimera formation in human cells from the standpoint of developmental biology.

A 3-Gene Random Forest Model to Diagnose Non-obstructive Azoospermia Based on Transcription Factor-Related Henes

Non-obstructive azoospermia (NOA) is one of the most severe forms of male infertility, but its diagnosis biomarkers with high sensitivity and specificity are largely unknown. Transcription factors (TFs) play essential roles in many pathological processes in different diseases. Herein, we aimed to identify the TFs showing high diagnosis ability for NOA through machine learning algorithms. The transcriptome data of the testicular tissue from 11 control and 47 NOA subjects were set as the training dataset; meanwhile, 1665 TFs were retrieved from the HumanTFDB. Through the feature extraction methods, including genomic difference analysis, Lasso, Boruta, SVM-RFE, and logistic regression, ETV2, TBX2, and ZNF689 were ultimately screened and then were included in the random forest (RF) diagnosis model. The RF model displayed high predictive power in the training (F-measure = 1) and two external validation (n = 31, F-measure = 0.902; n = 20, F-measure = 0.941) cohorts. The seminal plasma and testicular biopsy samples of 20 control and 20 NOA patients were collected from the local hospital, and the expression levels of ETV2, TBX2, and ZNF689 were measured via RT-qPCR and immunohistochemistry. The RF model could also distinguish the NOA samples in the local cohort (F-measure = 0.741). Single-cell RNA sequencing analysis, which was based on the 432 testicular cell samples from an NOA patient, showed that ETV2, TBX2, and ZNF689 were all significantly associated with spermatogenesis. In all, a 3-TF random forest diagnosis model was successfully established, providing novel insights into the latent mechanisms of NOA.

Xenotransplantation and exotransplantation: Strategies to expand the number of donor organs

Cardiovascular disease is common and has a high mortality. Due to the limited number of organs available for orthotopic heart transplantation, alternative therapies have received intense interest. In this commentary we contrast xenotransplantation and blastocyst complementation to produce pigs that will serve as donors for organ transplantation. These strategies hold tremendous promise and have the potential to provide an unlimited number of organs for chronic, terminal diseases.

A Technological and Regulatory Review on Human-Animal Chimera Research: The Current Landscape of Biology, Law, and Public Opinion

Organ transplantation is a highly utilized treatment for many medical conditions, yet the number of patients waiting for organs far exceeds the number available. The challenges and limitations currently associated with organ transplantation and technological advances in gene editing techniques have led scientists to pursue alternate solutions to the donor organ shortage. Growing human organs in animals and harvesting those organs for transplantation into humans is one such solution. These chimeric animals usually have certain genes necessary for a specific organ's development inhibited at an early developmental stage, followed by the addition of cultured pluripotent human cells to fill that developmental niche. The result is a chimeric animal that contains human organs which are available for transplant into a patient, circumventing some of the limitations currently involved in donor organ transplantation. In this review, we will discuss both the current scientific and legal landscape of human-animal chimera (HAC) research. We present an overview of the technological advances that allow for the creation of HACs, the patents that currently exist on these methods, as well as current public attitude and understanding that can influence HAC research policy. We complement our scientific and public attitude discussion with a regulatory overview of chimera research at both the national and state level, while also contrasting current U.S. legislation with regulations in other countries. Overall, we provide a comprehensive analysis of the legal and scientific barriers to conducting research on HACs for the generation of transplantable human organs, as well as provide recommendations for the future.

Blastocyst complementation and interspecies chimeras in gene edited pigs

The only curative therapy for many endstage diseases is allograft organ transplantation. Due to the limited supply of donor organs, relatively few patients are recipients of a transplanted organ. Therefore, new strategies are warranted to address this unmet need. Using gene editing technologies, somatic cell nuclear transfer and human induced pluripotent stem cell technologies, interspecies chimeric organs have been pursued with promising results. In this review, we highlight the overall technical strategy, the successful early results and the hurdles that need to be addressed in order for these approaches to produce a successful organ that could be transplanted in patients with endstage diseases.

Podocalyxin-Like Protein 1 Regulates Pluripotency through the Cholesterol Biosynthesis Pathway

Deciphering signaling mechanisms critical for the extended pluripotent stem cell (EPSC) state and primed pluripotency is necessary for understanding embryonic development. Here, a membrane protein, podocalyxin-like protein 1 (PODXL) as being essential for extended and primed pluripotency, is identified. Alteration of PODXL expression levels affects self-renewal, protein expression of c-MYC and telomerase, and induced pluripotent stem cell (iPSC) and EPSC colony formation. PODXL is the first membrane protein reported to regulate de novo cholesterol biosynthesis, and human pluripotent stem cells (hPSCs) are more sensitive to cholesterol depletion than fibroblasts. The addition of exogenous cholesterol fully restores PODXL knockdown-mediated loss of pluripotency. PODXL affects lipid raft dynamics via the regulation of cholesterol. PODXL recruits the RAC1/CDC42/actin network to regulate SREBP1 and SREBP2 maturation and lipid raft dynamics. Single-cell RNA sequencing reveals PODXL overexpression enhanced chimerism between human cells in mouse host embryos (hEPSCs 57%). Interestingly, in the human-mouse chimeras, laminin and collagen signaling-related pathways are dominant in PODXL overexpressing cells. It is concluded that cholesterol regulation via PODXL signaling is critical for ESC/EPSC.

Mechanisms and strategies to promote cardiac xenotransplantation

End stage heart failure is a terminal disease, and the only curative therapy is orthotopic heart transplantation. Due to limited organ availability, alternative strategies have received intense interest for treatment of patients with advanced heart failure. Recent studies using gene-edited porcine organs suggest that cardiac xenotransplantation may provide a future source of organs. In this review, we highlight the historical milestones for cardiac xenotransplantation and the gene editing strategies designed to overcome immunological barriers, which have culminated in a recent cardiac pig-to-human xenotransplant. We also discuss recent results of studies on the engineering of human-porcine chimeric organs that may provide an alternative and complementary strategy to overcome some of the major immunological barriers to producing a new source of transplantable organs.

From a Single Cell to a Whole Human Liver: Disease Modeling and Transplantation

Although the underlying cause may vary across countries and demographic groups, liver disease is a major cause of morbidity and mortality globally. Orthotopic liver transplantation is the only definitive treatment for liver failure but is limited by the lack of donor livers. The development of drugs that prevent the progression of liver disease and the generation of alternative liver constructs for transplantation could help alleviate the burden of liver disease. Bioengineered livers containing human induced pluripotent stem cell (iPSC)-derived liver cells are being utilized to study liver disease and to identify and test potential therapeutics. Moreover, bioengineered livers containing pig hepatocytes and endothelial cells have been shown to function and survive after transplantation into pig models of liver failure, providing preclinical evidence toward future clinical applications. Finally, bioengineered livers containing human iPSC-derived liver cells have been shown to function and survive after transplantation in rodents but require considerable optimization and testing prior to clinical use. In conclusion, bioengineered livers have emerged as a suitable tool for modeling liver diseases and as a promising alternative graft for clinical transplantation. The integration of novel technologies and techniques for the assembly and analysis of bioengineered livers will undoubtedly expand future applications in basic research and clinical transplantation.

CRISPR/Cas-edited pigs for personalized medicine: more than preclinical test-system

Novel CRISPR-Cas-based genome editing tools made it feasible to introduce a variety of precise genomic modifications in the pig genome, including introducing multiple edits simultaneously, inserting long DNA sequences into specifically targeted loci, and performing nucleotide transitions and transversions. Pigs serve as a vital agricultural resource and animal model in biomedical studies, given their advantages over the other models. Pigs share high similarities to humans regarding body/organ size, anatomy, physiology, and a metabolic profile. The pig genome can be modified to carry the same genetic mutations found in humans to replicate inherited diseases to provide preclinical trials of drugs. Moreover, CRISPR-based modification of pigs antigen profile makes it possible to offer porcine organs for xenotransplantation with minimal transplant rejection responses. This review summarizes recent advances in endonuclease-mediated genome editing tools and research progress of genome-edited pigs as personalized test-systems for preclinical trials and as donors of organs with human-fit antigen profile.

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Ex vivo anchored PD-L1 functionally prevent in vivo renal allograft rejection

Organ transplantation is the optimal treatment for patients with end-stage diseases. T cell activation is a major contributing factor toward the trigger of rejection. Induction therapy with T cell depleting agent is a common option but increases the risk of severe systemic infections. The ideal therapy should precisely target the allograft. Here, we developed a membrane-anchored-protein PD-L1 (map-PD-L1), which effectively anchored onto the surface of rat glomerular endothelial cells (rgEC). The expression of PD-L1 increased directly with map-PD-L1 concentration and incubation time. Moreover, map-PD-L1 was even stably anchored to rgEC at low temperature. Map-PD-L1 could bind to PD-1 and significantly promote T cell apoptosis and inhibited T cell activation. Using kidney transplantation models, we found that ex vivo perfusion of donor kidneys with map-PD-L1 significantly protected grafts against acute injury without using any immunosuppressant. We found map-PD-L1 could reduce T cell graft infiltration and increase intragraft Treg infiltration, suggesting a long-term effect in allograft protection. More importantly, modifying donor organs in vitro was not only safe, but also significantly reduced the side effects of systemic application. Our results suggested that ex vivo perfusion of donor organ with map-PD-L1 might provide a viable clinical option for organ-targeted induction therapy in organ transplantation.

The regulatory role of pioneer factors during cardiovascular lineage specification – A mini review

Cardiovascular disease (CVD) remains the number one cause of death worldwide. Ischemic heart disease contributes to heart failure and has considerable morbidity and mortality. Therefore, alternative therapeutic strategies are urgently needed. One class of epigenetic regulators known as pioneer factors has emerged as an important tool for the development of regenerative therapies for the treatment of CVD. Pioneer factors bind closed chromatin and remodel it to drive lineage specification. Here, we review pioneer factors within the cardiovascular lineage, particularly during development and reprogramming and highlight the implications this field of research has for the future development of cardiac specific regenerative therapies.

Why it is important to study human-monkey embryonic chimeras in a dish

The study of human–animal chimeras is fraught with technical and ethical challenges. In this Comment, we discuss the importance and future of human–monkey chimera research within the context of current scientific and regulatory obstacles.

Exclusive generation of rat spermatozoa in sterile mice utilizing blastocyst complementation with pluripotent stem cells

Blastocyst complementation denotes a technique that aims to generate organs, tissues, or cell types in animal chimeras via injection of pluripotent stem cells (PSCs) into genetically compromised blastocyst-stage embryos. Here, we report on successful complementation of the male germline in adult chimeras following injection of mouse or rat PSCs into mouse blastocysts carrying a mutation in Tsc22d3, an essential gene for spermatozoa production. Injection of mouse PSCs into Tsc22d3-Knockout (KO) blastocysts gave rise to intraspecies chimeras exclusively embodying PSC-derived functional spermatozoa. In addition, injection of rat embryonic stem cells (rESCs) into Tsc22d3-KO embryos produced interspecies mouse-rat chimeras solely harboring rat spermatids and spermatozoa capable of fertilizing oocytes. Furthermore, using single-cell RNA sequencing, we deconstructed rat spermatogenesis occurring in a mouse-rat chimera testis. Collectively, this study details a method for exclusive xenogeneic germ cell production in vivo, with implications that may extend to rat transgenesis, or endangered animal species conservation efforts.

Human muscle in gene edited pigs for treatment of volumetric muscle loss

Focusing on complex extremity trauma and volumetric muscle loss (VML) injuries, this review highlights: 1) the current pathophysiologic limitations of the injury sequela; 2) the gene editing strategy of the pig as a model that provides a novel treatment approach; 3) the notion that human skeletal muscle derived from gene edited, humanized pigs provides a groundbreaking treatment option; and 4) the impact of this technologic platform and how it will advance to far more multifaceted applications. This review seeks to shed insights on a novel treatment option using gene edited pigs as a platform which is necessary to overcome the clinical challenges and limitations in the field.

One-Step In Vitro Generation of ETV2-Null Pig Embryos

Simple Summary

One of the latest goals in regenerative medicine is to use pluripotent stem cells to generate whole organs in vivo through the blastocyst complementation technique. This method consists of the microinjection of pluripotent stem cells into preimplantation embryos that have been genetically modified to ablate the development of a target organ. By taking advantage of the spatiotemporal clues present in the developing embryo, pluripotent stem cells are able to colonize the empty developmental niche and create the missing organ. Combining human pluripotent stem cells with genetically engineered pig embryos, it would be possible to obtain humanized organs that could be used for transplantation, and, therefore, solve the worldwide issue of insufficient availability of transplantable organs. As endothelial cells play a critical role in xenotransplantation rejection in all organs, in this study, we optimized a protocol to generate a vascular-disabled preimplantation pig embryo using the CRISPR/Cas9 system. This protocol could be used to generate avascular embryos for blastocyst complementation experiments and work towards the generation of rejection-free humanized organs in pigs.

Abstract

Each year, tens of thousands of people worldwide die of end-stage organ failure due to the limited availability of organs for use in transplantation. To meet this clinical demand, one of the last frontiers of regenerative medicine is the generation of humanized organs in pigs from pluripotent stem cells (PSCs) via blastocyst complementation. For this, organ-disabled pig models are needed. As endothelial cells (ECs) play a critical role in xenotransplantation rejection in every organ, we aimed to produce hematoendothelial-disabled pig embryos targeting the master transcription factor ETV2 via CRISPR-Cas9-mediated genome modification. In this study, we designed five different guide RNAs (gRNAs) against the DNA-binding domain of the porcine ETV2 gene, which were tested on porcine fibroblasts in vitro. Four out of five guides showed cleavage capacity and, subsequently, these four guides were microinjected individually as ribonucleoprotein complexes (RNPs) into one-cell-stage porcine embryos. Next, we combined the two gRNAs that showed the highest targeting efficiency and microinjected them at higher concentrations. Under these conditions, we significantly improved the rate of biallelic mutation. Hence, here, we describe an efficient one-step method for the generation of hematoendothelial-disabled pig embryos via CRISPR-Cas9 microinjection in zygotes. This model could be used in experimentation related to the in vivo generation of humanized organs.

Cell competition and the regulative nature of early mammalian development

The mammalian embryo exhibits a remarkable plasticity that allows it to correct for the presence of aberrant cells, adjust its growth so that its size is in accordance with its developmental stage, or integrate cells of another species to form fully functional organs. Here, we will discuss the contribution that cell competition, a quality control that eliminates viable cells that are less fit than their neighbors, makes to this plasticity. We will do this by reviewing the roles that cell competition plays in the early mammalian embryo and how they contribute to ensure normal development of the embryo.

Kidney ECM Pregel Nanoarchitectonics for Microarrays to Accelerate Harvesting Gene-Edited Porcine Primary Monoclonal Spheres

One of the key steps of using CRISPR/Cas9 to obtain gene-edited cells used in generating gene-edited animals combined with somatic cell nuclear transplantation (SCNT) is to harvest monoclonal cells with genetic modifications. However, primary cells used as nuclear donors always grow slowly and fragile after a series of gene-editing operations. The extracellular matrix (ECM) formulated directly from different organs comprises complex proteins and growth factors that can improve and regulate the cellular functions of primary cells. Herein, sodium lauryl ether sulfate (SLES) detergent was first used to perfuse porcine kidney ECM, and the biological properties of the kidney ECM were optimized. Then, we used a porcine kidney ECM pregel to pattern the microarray and developed a novel strategy to shorten the time of obtaining gene-edited monoclonal cell spheroids with low damage in batches. Our results showed that the SLES-perfused porcine kidney ECM pregel displayed superior biological activities in releasing growth factors and promoting cell proliferation. Finally, combined with microarray technology, we quickly obtained monoclonal cells in good condition, and the cells used as nuclear donors to construct recombinant embryos showed a significantly higher success rate than those of the traditional method. We further successfully produced genetically edited pigs.

Generating functional cells through enhanced interspecies chimerism with human pluripotent stem cells

Obtaining functional human cells through interspecies chimerism with human pluripotent stem cells (hPSCs) remains unsuccessful due to its extremely low efficiency. Here, we show that hPSCs failed to differentiate and contribute teratoma in the presence of mouse PSCs (mPSCs), while MYCN, a pro-growth factor, dramatically promotes hPSC contributions in teratoma co-formation by hPSCs/mPSCs. MYCN combined with BCL2 (M/B) greatly enhanced conventional hPSCs to integrate into pre-implantation embryos of different species, such as mice, rabbits, and pigs, and substantially contributed to mouse post-implantation chimera in embryonic and extra-embryonic tissues. Strikingly, M/B-hPSCs injected into pre-implantation Flk-1^+/- mouse embryos show further enhanced chimerism that allows for obtaining live human CD34⁺ blood progenitor cells from chimeras through cell sorting. The chimera-derived human CD34⁺ cells further gave rise to various subtype blood cells in a typical colony-forming unit (CFU) assay. Thus, we provide proof of concept to obtain functional human cells through enhanced interspecies chimerism with hPSCs.

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hPSCs undergo severe apoptosis when differentiated together with mESCs

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MYCN overcomes apoptosis of hPSCs in co-differentiation with mESCs

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MYCN plus BCL2 largely enhance interspecies chimera efficiency of hPSCs

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Obtaining functional human HPCs through enhanced interspecies chimerism with hPSCs

Xenotransplantation and interspecies organogenesis: current status and issues

Pancreas (and islet) transplantation is the only curative treatment for type 1 diabetes patients whose β-cell functions have been abolished. However, the lack of donor organs has been the major hurdle to save a large number of patients. Therefore, transplantation of animal organs is expected to be an alternative method to solve the serious shortage of donor organs. More recently, a method to generate organs from pluripotent stem cells inside the body of other species has been developed. This interspecies organ generation using blastocyst complementation (BC) is expected to be the next-generation regenerative medicine. Here, we describe the recent advances and future prospects for these two approaches.

CRISPR/Cas9-Mediated Gene Editing in Porcine Models for Medical Research

Pigs have been extensively used as the research models for human disease pathogenesis and gene therapy. They are also the optimal source of cells, tissues, and organs for xenotransplantation due to anatomical and physiological similarities to humans. Several breakthroughs in gene-editing technologies, including the advent of clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR-associated 9 (Cas9), have greatly improved the efficiency of genetic manipulation and significantly broadened the application of gene-edited large animal models. In this review, we have not only outlined the important applications of the CRISPR/Cas9 system in pigs as a means to study human diseases but also discussed the potential challenges of the use of CRISPR/Cas9 in large animals.

Gene Editing in Pluripotent Stem Cells and Their Derived Organoids

With the rapid rise in gene-editing technology, pluripotent stem cells (PSCs) and their derived organoids have increasingly broader and practical applications in regenerative medicine. Gene-editing technologies, from large-scale nucleic acid endonucleases to CRISPR, have ignited a global research and development boom with significant implications in regenerative medicine. The development of regenerative medicine technologies, regardless of whether it is PSCs or gene editing, is consistently met with controversy. Are the tools for rewriting the code of life a boon to humanity or a Pandora's box? These technologies raise concerns regarding ethical issues, unexpected mutations, viral infection, etc. These concerns remain even as new treatments emerge. However, the potential negatives cannot obscure the virtues of PSC gene editing, which have, and will continue to, benefit mankind at an unprecedented rate. Here, we briefly introduce current gene-editing technology and its application in PSCs and their derived organoids, while addressing ethical concerns and safety risks and discussing the latest progress in PSC gene editing. Gene editing in PSCs creates visualized in vitro models, providing opportunities for examining mechanisms of known and unknown mutations and offering new possibilities for the treatment of cancer, genetic diseases, and other serious or refractory disorders. From model construction to treatment exploration, the important role of PSCs combined with gene editing in basic and clinical medicine studies is illustrated. The applications, characteristics, and existing challenges are summarized in combination with our lab experiences in this field in an effort to help gene-editing technology better serve humans in a regulated manner. Current preclinical and clinical trials have demonstrated initial safety and efficacy of PSC gene editing; however, for better application in clinical settings, additional investigation is warranted.

Fumarylacetoacetate hydrolase gene as a knockout target for hepatic chimerism and donor liver production

A reliable source of human hepatocytes and transplantable livers is needed. Interspecies embryo complementation, which involves implanting donor human stem cells into early morula/blastocyst stage animal embryos, is an emerging solution to the shortage of transplantable livers. We review proposed mutations in the recipient embryo to disable hepatogenesis, and discuss the advantages of using fumarylacetoacetate hydrolase knockouts and other genetic modifications to disable hepatogenesis. Interspecies blastocyst complementation using porcine recipients for primate donors has been achieved, although percentages of chimerism remain persistently low. Recent investigation into the dynamic transcriptomes of pigs and primates have created new opportunities to intimately match the stage of developing animal embryos with one of the many varieties of human induced pluripotent stem cell. We discuss techniques for decreasing donor cell apoptosis, targeting donor tissue to endodermal structures to avoid neural or germline chimerism, and decreasing the immunogenicity of chimeric organs by generating donor endothelium.