Self-organization of stem cells into embryos: A window on early mammalian development

Recent progress in the identification and in vitro culture of skin organoids

An organoid is a cell-based structure that shows organ-specific properties and shares a similar spatial organization as the corresponding organ. Organoids possess powerful capability to reproduce the key functions of the associated organ structures, and their similarity to the organs makes them physiologically relevant systems. The primary challenge associated with the development of skin organoids is the complexity of the human skin architecture, which encompasses the epidermis and the dermis as well as accessory structures, including hair follicles, sweat glands, and sebaceous glands, that perform various functions such as thermoregulation. The ultimate objectives of developing skin organoids are to regenerate the complete skin structure in vitro and reconstruct the skin in vivo. Consequently, safety, reliability, and the fidelity of the tissue interfaces are key considerations in this process. For this purpose, the present article reviews the most recent advances in this field, focusing on the cell sources, culture methods, culture conditions, and biomarkers for identifying the structure and function of skin organoids developed in vitro or in vivo. The subsequent sections summarize the recent applications of skin organoids in related disease diagnosis and treatments, and discuss the future prospects of these organoids in terms of clinical applications. This review of skin organoids can provide an important foundation for studies on human skin development, disease modeling, and reconstructive surgery, with broad utility for promising future opportunities in both biomedical research and clinical practice.

Novel drug discovery strategies for chronic obstructive pulmonary disease: the latest developments

INTRODUCTION

The journey from initial drug discovery to approval for respiratory diseases typically spans approximately 10.4 years and cost over $2.8 billion. This intricate process involves five stages: target identification, therapeutic molecule discovery, preclinical testing, clinical trials, and regulatory approval.

AREAS COVERED

This review examines novel drug discovery strategies for chronic obstructive pulmonary disease (COPD), focusing on advanced in vitro models that replicate human lung conditions for accurate drug testing according to the following search string: discovery AND strategy AND COPD. It explores targeted molecular therapies, structure-based drug design, and drug repurposing approaches facilitated by computational analysis. The significance of personalized medicine in tailoring treatments for diverse COPDs is emphasized, highlighting the complexity of the disease and the necessity of these innovative methodologies to improve therapeutic outcomes.

EXPERT OPINION

COPD remains a challenging area, with a significant unmet medical need. Despite previous efforts, few effective therapies exist. Innovative in vitro models, targeted molecular therapies, and drug repurposing strategies are showing promise. Emphasizing advanced preclinical models and repurposing existing drugs could transform treatment paradigms, promoting more effective therapies for complex diseases like COPD. These innovations hold potential for enhancing drug discovery efficiency, leading to personalized and precision medicine approaches.

Differential Elasticity Affects Lineage Segregation of Embryonic Stem Cells

The question of what guides lineage segregation is central to development, where cellular differentiation leads to segregated cell populations destined for specialized functions. Here, using optical tweezers measurements of mouse embryonic stem cells, we reveal a mechanical mechanism based on differential elasticity in the second lineage segregation of the embryonic inner cell mass into epiblast (EPI) cells, which will develop into the fetus, and primitive endoderm (PrE), which will form extraembryonic structures such as the yolk sac. Remarkably, we find that these mechanical differences already occur during priming, not just after a cell has committed to differentiation. Specifically, we show that PrE-primed cells exhibit significantly higher elasticity than EPI-primed cells, characterized by lower power spectrum scaling exponents, higher Young's modulus, and lower loss tangent. Using a model of two cell types differing only in elasticity, we show that differential elasticity alone is sufficient to lead to segregation between cell types, suggesting that the mechanical attributes of the cells contribute to the segregation process. Importantly, we find that this process relies on cellular activity. Our findings present differential elasticity as a previously unknown mechanical contributor to lineage segregation during embryo morphogenesis.

The building blocks of embryo models: embryonic and extraembryonic stem cells

The process of a single-celled zygote developing into a complex multicellular organism is precisely regulated at spatial and temporal levels in vivo. However, understanding the mechanisms underlying development, particularly in humans, has been constrained by technical and ethical limitations associated with studying natural embryos. Harnessing the intrinsic ability of embryonic stem cells (ESCs) to self-organize when induced and assembled, researchers have established several embryo models as alternative approaches to studying early development in vitro. Recent studies have revealed the critical role of extraembryonic cells in early development; and many groups have created more sophisticated and precise ESC-derived embryo models by incorporating extraembryonic stem cell lines, such as trophoblast stem cells (TSCs), extraembryonic mesoderm cells (EXMCs), extraembryonic endoderm cells (XENs, in rodents), and hypoblast stem cells (in primates). Here, we summarize the characteristics of existing mouse and human embryonic and extraembryonic stem cells and review recent advancements in developing mouse and human embryo models.

Self-organization of mouse embryonic stem cells into reproducible pre-gastrulation embryo models via CRISPRa programming

Embryonic stem cells (ESCs) can self-organize into structures with spatial and molecular similarities to natural embryos. During development, embryonic and extraembryonic cells differentiate through activation of endogenous regulatory elements while co-developing via cell-cell interactions. However, engineering regulatory elements to self-organize ESCs into embryo models remains underexplored. Here, we demonstrate that CRISPR activation (CRISPRa) of two regulatory elements near Gata6 and Cdx2 generates embryonic patterns resembling pre-gastrulation mouse embryos. Live single-cell imaging revealed that self-patterning occurs through orchestrated collective movement driven by cell-intrinsic fate induction. In 3D, CRISPRa-programmed embryo models (CPEMs) exhibit morphological and transcriptomic similarity to pre-gastrulation mouse embryos. CPEMs allow versatile perturbations, including dual Cdx2-Elf5 activation to enhance trophoblast differentiation and lineage-specific activation of laminin and matrix metalloproteinases, uncovering their roles in basement membrane remodeling and embryo model morphology. Our findings demonstrate that minimal intrinsic epigenome editing can self-organize ESCs into programmable pre-gastrulation embryo models with robust lineage-specific perturbation capabilities.

Synthetic organizer cells guide development via spatial and biochemical instructions

In vitro development relies primarily on treating progenitor cells with media-borne morphogens and thus lacks native-like spatial information. Here, we engineer morphogen-secreting organizer cells programmed to self-assemble, via cell adhesion, around mouse embryonic stem (ES) cells in defined architectures. By inducing the morphogen WNT3A and its antagonist DKK1 from organizer cells, we generated diverse morphogen gradients, varying in range and steepness. These gradients were strongly correlated with morphogenetic outcomes: the range of minimum-maximum WNT activity determined the resulting range of anterior-to-posterior (A-P) axis cell lineages. Strikingly, shallow WNT activity gradients, despite showing truncated A-P lineages, yielded higher-resolution tissue morphologies, such as a beating, chambered cardiac-like structure associated with an endothelial network. Thus, synthetic organizer cells, which integrate spatial, temporal, and biochemical information, provide a powerful way to systematically and flexibly direct the development of ES or other progenitor cells in different directions within the morphogenetic landscape.

Synthetic embryology of the human heart

The evolution of stem cell-based heart models from cells and tissues to organoids and assembloids and recently synthetic embryology gastruloids, is poised to revolutionize our understanding of cardiac development, congenital to adult diseases, and patient customized therapies. Human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) have already been integrated into transplantable patches and are in preclinical efforts to reverse fibrotic scarring from myocardial infarctions. To inform on the complexity of heart diseases, multi-tissue morphogenic heart models are needed that replicate fundamental components of heart function to heart organogenesis in vitro and which require a deep understanding of heart development. Organoid and assembloid models capture selected multicellular cardiac processes, such as chamber formation and priming events for vascularization. Gastruloid heart models offer deeper insights as synthetic embryology to mimic multi-staged developmental events of in vivo heart organogenesis including established heart fields, crescent formation and heart tube development along with vascular systemic foundation and even further steps. The human Elongating Multi-Lineage Organized Cardiac (EMLOC) gastruloid model captures these stages and additional events including chamber genesis, patterned vascularization, and extrinsic central and intrinsic cardiac nervous system (CNS-ICNS) integration guided by spatiotemporal and morphogenic processes with neural crest cells. Gastruloid synthetic embryology heart models offer new insights into previously hidden processes of development and provide powerful platforms for addressing heart disease that extends beyond cardiomyocytes, such as arrhythmogenic diseases, congenital defects, and systemic injury interactions, as in spinal cord injuries. The holistic view that is emerging will reveal heart development and disease in unprecedented detail to drive transformative state-of-the-art innovative applications for heart health.

Harnessing the power of artificial intelligence for human living organoid research

As a powerful paradigm, artificial intelligence (AI) is rapidly impacting every aspect of our day-to-day life and scientific research through interdisciplinary transformations. Living human organoids (LOs) have a great potential for in vitro reshaping many aspects of in vivo true human organs, including organ development, disease occurrence, and drug responses. To date, AI has driven the revolutionary advances of human organoids in life science, precision medicine and pharmaceutical science in an unprecedented way. Herein, we provide a forward-looking review, the frontiers of LOs, covering the engineered construction strategies and multidisciplinary technologies for developing LOs, highlighting the cutting-edge achievements and the prospective applications of AI in LOs, particularly in biological study, disease occurrence, disease diagnosis and prediction and drug screening in preclinical assay. Moreover, we shed light on the new research trends harnessing the power of AI for LO research in the context of multidisciplinary technologies. The aim of this paper is to motivate researchers to explore organ function throughout the human life cycle, narrow the gap between in vitro microphysiological models and the real human body, accurately predict human-related responses to external stimuli (cues and drugs), accelerate the preclinical-to-clinical transformation, and ultimately enhance the health and well-being of patients.

Early autonomous patterning of the anteroposterior axis in gastruloids

Minimal in vitro systems composed of embryonic stem cells (ESCs) have been shown to recapitulate the establishment of the anteroposterior (AP) axis. In contrast to the native embryo, ESC aggregates - such as gastruloids - can break symmetry, which is demarcated by polarization of the mesodermal marker T, autonomously without any localized external cues. However, associated earliest patterning events, such as the spatial restriction of cell fates and concomitant transcriptional changes, remain poorly understood. Here, we dissect the dynamics of AP axis establishment in mouse gastruloids, particularly before external Wnt stimulation. Through single-cell RNA sequencing, we identify key cell state transitions and the molecular signatures of T+ and T- populations underpinning AP polarization. We also show that this process is robust to modifications of aggregate size. Finally, transcriptomic comparison with the mouse embryo indicates that gastruloids develop similar mesendodermal cell types, despite initial differences in their primed pluripotent populations, which adopt a more mesenchymal state in lieu of an epiblast-like transcriptome. Hence, our findings suggest the possibility of alternate ESC states in vivo and in vitro that can converge onto similar cell fates.

Genome-wide screening reveals essential roles for HOX genes and imprinted genes during caudal neurogenesis of human embryonic stem cells

Mapping the essential pathways for neuronal differentiation can uncover new therapeutics and models for neurodevelopmental disorders. We thus utilized a genome-wide loss-of-function library in haploid human embryonic stem cells, differentiated into caudal neuronal cells. We show that essential genes for caudal neurogenesis are enriched for secreted and membrane proteins and that a large group of neurological conditions, including neurodegenerative disorders, manifest early neuronal phenotypes. Furthermore, essential transcription factors are enriched with homeobox (HOX) genes demonstrating synergistic regulation and surprising non-redundant functions between HOXA6 and HOXB6 paralogs. Moreover, we establish the essentialome of imprinted genes during neurogenesis, demonstrating that maternally expressed genes are non-essential in pluripotent cells and their differentiated germ layers, yet several are essential for neuronal development. These include Beckwith-Wiedemann syndrome- and Angelman syndrome-related genes, for which we suggest a novel regulatory pathway. Overall, our work identifies essential pathways for caudal neuronal differentiation and stage-specific phenotypes of neurological disorders.

Stem cell therapies for chronic obstructive pulmonary disease: mesenchymal stem cells as a promising treatment option

Chronic obstructive pulmonary disease(COPD) is an inflammatory disease characterized by the progressive and irreversible structural and functional damage of lung tissue. Although COPD is a significant global disease burden, the available treatments only ameliorate the symptoms, but cannot reverse lung damage. Researchers in regenerative medicine have examined the use of stem cell transplantation for treatment of COPD and other diseases because these cells have the potential for unlimited self-renewal and the ability to undergo directed differentiation. Stem cells are typically classified as embryonic stem cells, induced pluripotent stem cells, and adult stem cells (which includes mesenchymal stem cells [MSCs]), each with its own advantages and disadvantages regarding applications in regenerative medicine. Although the heterogeneity and susceptibility to senescence of MSCs make them require careful consideration for clinical applications. However, the low tumourigenicity and minimal ethical concerns of MSCs make them appear to be excellent candidates. This review summarizes the characteristics of various stem cell types and describes their therapeutic potential in the treatment of COPD, with a particular emphasis on MSCs. We aim to facilitate subsequent in-depth research and preclinical applications of MSCs by providing a comprehensive overview.

Correspondence between multiple signaling and developmental cellular patterns: a computational perspective

The spatial arrangement of variant phenotypes during stem cell division plays a crucial role in the self-organization of cell tissues. The patterns observed in these cellular assemblies, where multiple phenotypes vie for space and resources, are largely influenced by a mixture of different diffusible chemical signals. This complex process is carried out within a chronological framework of interplaying intracellular and intercellular events. This includes receiving external stimulants, whether secreted by other individuals or provided by the environment, interpreting these environmental signals, and incorporating the information to designate cell fate. Here, given two distinct signaling patterns generated by Turing systems, we investigated the spatial distribution of differentiating cells that use these signals as external cues for modifying the production rates. By proposing a computational map, we show that there is a correspondence between the multiple signaling and developmental cellular patterns. In other words, the model provides an appropriate prediction for the final structure of the differentiated cells in a multi-signal, multi-cell environment. Conversely, when a final snapshot of cellular patterns is given, our algorithm can partially identify the signaling patterns that influenced the formation of the cellular structure, provided that the governing dynamic of the signaling patterns is already known.

scHyper: reconstructing cell-cell communication through hypergraph neural networks

Cell-cell communications is crucial for the regulation of cellular life and the establishment of cellular relationships. Most approaches of inferring intercellular communications from single-cell RNA sequencing (scRNA-seq) data lack a comprehensive global network view of multilayered communications. In this context, we propose scHyper, a new method that can infer intercellular communications from a global network perspective and identify the potential impact of all cells, ligand, and receptor expression on the communication score. scHyper designed a new way to represent tripartite relationships, by extracting a heterogeneous hypergraph that includes the source (ligand expression), the target (receptor expression), and the relevant ligand-receptor (L-R) pairs. scHyper is based on hypergraph representation learning, which measures the degree of match between the intrinsic attributes (static embeddings) of nodes and their observed behaviors (dynamic embeddings) in the context (hyperedges), quantifies the probability of forming hyperedges, and thus reconstructs the cell-cell communication score. Additionally, to effectively mine the key mechanisms of signal transmission, we collect a rich dataset of multisubunit complex L-R pairs and propose a nonparametric test to determine significant intercellular communications. Comparing with other tools indicates that scHyper exhibits superior performance and functionality. Experimental results on the human tumor microenvironment and immune cells demonstrate that scHyper offers reliable and unique capabilities for analyzing intercellular communication networks. Therefore, we introduced an effective strategy that can build high-order interaction patterns, surpassing the limitations of most methods that can only handle low-order interactions, thus more accurately interpreting the complexity of intercellular communications.

Novel high throughput screen reports that benzo(a)pyrene overrides mouse trophoblast stem cell multipotency, inducing SAPK activity, HAND1 and differentiated trophoblast giant cells

INTRODUCTION

Cultured mouse trophoblast stem cells (mTSC) maintain proliferation/normal stemness (NS) under FGF4, which when removed, causes normal differentiation (ND). Hypoxic, or hyperosmotic stress forces trophoblast giant cells (TGC) differentiate. Hypoxic, hyperosmotic, and genotoxic benzo(a)pyrene (BaP), which is found in tobacco smoke, force down-regulation of inhibitor of differentiation (Id)2, enabling TGC differentiation. Hypoxic and hyperosmotic stress induce TGC by SAPK-dependent HAND1 increase. Here we test whether BaP forces mTSC-to-TGC while inducing SAPK and HAND1.

METHODS

Hand1 and SAPK activity were assayed by immunoblot, mTSC-to-TGC growth and differentiation were assayed at Tfinal after 72hr exposure of BaP, NS, ND, Retinoic acid (RA), or sorbitol. Nuclear-stained cells were micrographed automatically by a live imager, and assayed by ImageJ/FIJI, Biotek Gen 5, AIVIA proprietary artificial intelligence (AI) software or open source, CellPose artificial intelligence/AI software.

RESULTS

BaP (0.05-1μM) activated SAPK and HAND1 without diminishing growth. TSC-to-TGC differentiation was assayed with increasingly accuracy for 2-4 N cycling nuclei and >4 N differentiating TGC nuclei, using ImageJ/FIJI, Gen 5, AIVIA, or CellPose AI software. The AIVIA and Cellpose AI software matches human accuracy. The lowest BaP effects on SAPK activation/HAND1 increase are >10-fold more sensitive than similar effects for mESC. RA induces 44-47% 1st lineage TGC differentiation, but the same RA dose induces only 1% 1st lineage mESC differentiation.

DISCUSSION

First, these pilot data suggest that mTSC can be used in high throughput screens (HTS) to predict toxicant exposures that force TGC differentiation. Second, mTSC differentiated more cells than mESC for similar stress exposures, Third, open source AI can replace human micrograph quantitation and enable a miscarriage-predicting HTS.

Reconstructing axial progenitor field dynamics in mouse stem cell-derived embryoids

Embryogenesis requires substantial coordination to translate genetic programs to the collective behavior of differentiating cells, but understanding how cellular decisions control tissue morphology remains conceptually and technically challenging. Here, we combine continuous Cas9-based molecular recording with a mouse embryonic stem cell-based model of the embryonic trunk to build single-cell phylogenies that describe the behavior of transient, multipotent neuro-mesodermal progenitors (NMPs) as they commit into neural and somitic cell types. We find that NMPs show subtle transcriptional signatures related to their recent differentiation and contribute to downstream lineages through a surprisingly broad distribution of individual fate outcomes. Although decision-making can be heavily influenced by environmental cues to induce morphological phenotypes, axial progenitors intrinsically mature over developmental time to favor the neural lineage. Using these data, we present an experimental and analytical framework for exploring the non-homeostatic dynamics of transient progenitor populations as they shape complex tissues during critical developmental windows.

Axis formation in annual killifish: Nodal and β-catenin regulate morphogenesis without Huluwa prepatterning

Axis formation in fish and amphibians typically begins with a prepattern of maternal gene products. Annual killifish embryogenesis, however, challenges prepatterning models as blastomeres disperse and then aggregate to form the germ layers and body axes. We show that huluwa, a prepatterning factor thought to break symmetry by stabilizing β-catenin, is truncated and inactive in Nothobranchius furzeri. Nuclear β-catenin is not selectively stabilized on one side of the blastula but accumulates in cells forming the aggregate. Blocking β-catenin activity or Nodal signaling disrupts aggregate formation and germ layer specification. Nodal signaling coordinates cell migration, establishing an early role for this signaling pathway. These results reveal a surprising departure from established mechanisms of axis formation: Huluwa-mediated prepatterning is dispensable, and β-catenin and Nodal regulate morphogenesis.

Oct4 controls basement membrane development during human embryogenesis

Basement membranes (BMs) are sheet-like structures of extracellular matrix (ECM) that provide structural support for many tissues and play a central role in signaling. They are key regulators of cell behavior and tissue functions, and defects in their assembly or composition are involved in numerous human diseases. Due to the differences between human and animal embryogenesis, ethical concerns, legal constraints, the scarcity of human tissue material, and the inaccessibility of the in vivo condition, BM regulation during human embryo development has remained elusive. Using the post-implantation amniotic sac embryoid (PASE), we delineate BM assembly upon post-implantation development and BM disassembly during primitive streak (PS) cell dissemination. Further, we show that the transcription factor Oct4 regulates the expression of BM structural components and receptors and controls BM development by regulating Akt signaling and the small GTPase Rac1. These results represent a relevant step toward a more comprehensive understanding of early human development.

Basal delamination during mouse gastrulation primes pluripotent cells for differentiation

The blueprint of the mammalian body plan is laid out during gastrulation, when a trilaminar embryo is formed. This process entails a burst of proliferation, the ingression of embryonic epiblast cells at the primitive streak, and their priming toward primitive streak fates. How these different events are coordinated remains unknown. Here, we developed and characterized a 3D culture of self-renewing mouse embryonic cells that captures the main transcriptional and architectural features of the early gastrulating mouse epiblast. Using this system in combination with microfabrication and in vivo experiments, we found that proliferation-induced crowding triggers delamination of cells that express high levels of the apical polarity protein aPKC. Upon delamination, cells become more sensitive to Wnt signaling and upregulate the expression of primitive streak markers such as Brachyury. This mechanistic coupling between ingression and differentiation ensures that the right cell types become specified at the right place during embryonic development.

Combinatorial interpretation of BMP and WNT controls the decision between primitive streak and extraembryonic fates

BMP signaling is essential for mammalian gastrulation, as it initiates a cascade of signals that control self-organized patterning. As development is highly dynamic, it is crucial to understand how time-dependent combinatorial signaling affects cellular differentiation. Here, we show that BMP signaling duration is a crucial control parameter that determines cell fates upon the exit from pluripotency through its interplay with the induced secondary signal WNT. BMP signaling directly converts cells from pluripotent to extraembryonic fates while simultaneously upregulating Wnt signaling, which promotes primitive streak and mesodermal specification. Using live-cell imaging of signaling and cell fate reporters together with a simple mathematical model, we show that this circuit produces a temporal morphogen effect where, once BMP signal duration is above a threshold for differentiation, intermediate and long pulses of BMP signaling produce specification of mesoderm and extraembryonic fates, respectively. Our results provide a systems-level picture of how these signaling pathways control the landscape of early human development.

Emerging Contributions of Pluripotent Stem Cells to Reproductive Technologies in Veterinary Medicine

The generation of mature gametes and competent embryos in vitro from pluripotent stem cells has been successfully achieved in a few species, mainly in mice, with recent advances in humans and scarce preliminary reports in other domestic species. These biotechnologies are very attractive as they facilitate the understanding of developmental mechanisms and stages that are generally inaccessible during early embryogenesis, thus enabling advanced reproductive technologies and contributing to the generation of animals of high genetic merit in a short period. Studies on the production of in vitro embryos in pigs and cattle are currently used as study models for humans since they present more similar characteristics when compared to rodents in both the initial embryo development and adult life. This review discusses the most relevant biotechnologies used in veterinary medicine, focusing on the generation of germ-cell-like cells in vitro through the acquisition of totipotent status and the production of embryos in vitro from pluripotent stem cells, thus highlighting the main uses of pluripotent stem cells in livestock species and reproductive medicine.

Responses to commentaries on "The gene: An appraisal"

The central conclusions of "The Gene: An Appraisal" are that genetic variance does not underpin biological evolution, and, therefore, that genes are not Mendel's units of inheritance. In this response, I will address the criticisms I have received via commentaries on that paper by defending the following statements: 1. Epistasis does not explain the power-law fitness profile of the Long-Term Evolution Experiment (LTEE). The data from the evolution of natural systems displays the power-law form ubiquitously. Epistasis plays no role in evolution. 2. The common characteristics of living things (natural systems) are described by the principle of least action in de Maupertuis's original form, which is synonymous with the 2nd law of thermodynamics and Newton's 2nd law of motion in its complete form, i.e., F = dp/dt. Organisms strive to achieve free energy balance with their environments. 3. Based on an appraisal of the scientific environment between 1880 and 1911, I conclude that Johannsen's genotype conception was perhaps, the only option available to him. 4. The power-law fitness profile of the LTEE falsifies Fisher's Genetical Theory of Natural Selection, Johannsen's genotype conception, and the idea that 'Darwinian evolution' is an exception to the generic thermodynamic process of evolution in natural systems. 5. The use of the technique of genome-wide association to identify the causes and the likelihoods of inherited common diseases and behavioural traits is a 'wild goose chase' because genes are not the units of inheritance.

Using Embryo Models to Understand the Development and Progression of Embryonic Lineages: A Focus on Primordial Germ Cell Development

BACKGROUND

Recapitulating mammalian cell type differentiation in vitro promises to improve our understanding of how these processes happen in vivo, while bringing additional prospects for biomedical applications. The establishment of stem cell-derived embryo models and embryonic organoids, which have experienced explosive growth over the last few years, opens new avenues for research due to their scale, reproducibility, and accessibility. Embryo models mimic various developmental stages, exhibit different degrees of complexity, and can be established across species. Since embryo models exhibit multiple lineages organized spatially and temporally, they are likely to provide cellular niches that, to some degree, recapitulate the embryonic setting and enable "co-development" between cell types and neighbouring populations. One example where this is already apparent is in the case of primordial germ cell-like cells (PGCLCs).

SUMMARY

While directed differentiation protocols enable the efficient generation of high PGCLC numbers, embryo models provide an attractive alternative as they enable the study of interactions of PGCLCs with neighbouring cells, alongside the regulatory molecular and biophysical mechanisms of PGC competency. Additionally, some embryo models can recapitulate post-specification stages of PGC development (including migration or gametogenesis), mimicking the inductive signals pushing PGCLCs to mature and differentiate and enabling the study of PGCLC development across stages. Therefore, in vitro models may allow us to address questions of cell type differentiation, and PGC development specifically, that have hitherto been out of reach with existing systems.

KEY MESSAGE

This review evaluates the current advances in stem cell-based embryo models, with a focus on their potential to model cell type-specific differentiation in general and in particular to address open questions in PGC development and gametogenesis.

Self-organization of embryonic stem cells into a reproducible embryo model through epigenome editing

Embryonic stem cells (ESCs) can self-organize in vitro into developmental patterns with spatial organization and molecular similarity to that of early embryonic stages. This self-organization of ESCs requires transmission of signaling cues, via addition of small molecule chemicals or recombinant proteins, to induce distinct embryonic cellular fates and subsequent assembly into structures that can mimic aspects of early embryonic development. During natural embryonic development, different embryonic cell types co-develop together, where each cell type expresses specific fate-inducing transcription factors through activation of non-coding regulatory elements and interactions with neighboring cells. However, previous studies have not fully explored the possibility of engineering endogenous regulatory elements to shape self-organization of ESCs into spatially-ordered embryo models. Here, we hypothesized that cell-intrinsic activation of a minimum number of such endogenous regulatory elements is sufficient to self-organize ESCs into early embryonic models. Our results show that CRISPR-based activation (CRISPRa) of only two endogenous regulatory elements in the genome of pluripotent stem cells is sufficient to generate embryonic patterns that show spatial and molecular resemblance to that of pre-gastrulation mouse embryonic development. Quantitative single-cell live fluorescent imaging showed that the emergence of spatially-ordered embryonic patterns happens through the intrinsic induction of cell fate that leads to an orchestrated collective cellular motion. Based on these results, we propose a straightforward approach to efficiently form 3D embryo models through intrinsic CRISPRa-based epigenome editing and independent of external signaling cues. CRISPRa-Programmed Embryo Models (CPEMs) show highly consistent composition of major embryonic cell types that are spatially-organized, with nearly 80% of the structures forming an embryonic cavity. Single cell transcriptomics confirmed the presence of main embryonic cell types in CPEMs with transcriptional similarity to pre-gastrulation mouse embryos and revealed novel signaling communication links between different embryonic cell types. Our findings offer a programmable embryo model and demonstrate that minimum intrinsic epigenome editing is sufficient to self-organize ESCs into highly consistent pre-gastrulation embryo models.

Human blastoid as an in vitro model of human blastocysts

Human development is a highly coordinated process, with any abnormalities during the early embryonic stages that can often have detrimental consequences. The complexity and nuances of human development underpin its significance in embryo research. However, this research is often hindered by limited availability and ethical considerations associated with the use of donated blastocysts from in vitro fertilization (IVF) surplus. Human blastoids offer promising alternatives as they can be easily generated and manipulated in the laboratory while preserving key characteristics of human blastocysts. In this way, they hold the potential to serve as a scalable and ethically permissible resource in embryology research. By utilizing such human embryo models, we can establish a transformative platform that complements the study with IVF embryos, ultimately enhancing our understanding of human embryogenesis.

The Gene: An appraisal

The gene can be described as the foundational concept of modern biology. As such, it has spilled over into daily discourse, yet it is acknowledged among biologists to be ill-defined. Here, following a short history of the gene, I analyse critically its role in inheritance, evolution, development, and morphogenesis. Wilhelm Johannsen's genotype-conception, formulated in 1910, has been adopted as the foundation stone of genetics, giving the gene a higher degree of prominence than is justified by the evidence. An analysis of the results of the Long-Term Evolution Experiment (LTEE) with E. coli bacteria, grown over 60,000 generations, does not support spontaneous gene mutation as the source of variance for natural selection. From this it follows that the gene is not Mendel's unit of inheritance: that must be Johannsen's transmission-conception at the gamete phenotype level, a form of inheritance that Johannsen did not consider. Alternatively, I contend that biology viewed on the bases of thermodynamics, complex system dynamics, and self-organisation, provides a new framework for the foundations of biology. In this framework, the gene plays a passive role as a vital information store: it is the phenotype that plays the active role in inheritance, evolution, development, and morphogenesis.

EpiTyping: analysis of epigenetic aberrations in parental imprinting and X-chromosome inactivation using RNA-seq

Human pluripotent stem cells (hPSCs) hold a central role in studying human development, in disease modeling and in regenerative medicine. These cells not only acquire genetic modifications when kept in culture, but they may also harbor epigenetic aberrations, mainly involving parental imprinting and X-chromosome inactivation. Here we present a detailed bioinformatic protocol for detecting such aberrations using RNA sequencing data. We provide a pipeline designed to process and analyze RNA sequencing data for the identification of abnormal biallelic expression of imprinted genes, and thus detect loss of imprinting. Furthermore, we show how to differentiate among X-chromosome inactivation, full activation and aberrant erosion of X chromosome in female hPSCs. In addition to providing bioinformatic tools, we discuss the impact of such epigenetic variations in hPSCs on their utility for various purposes. This pipeline can be used by any user with basic understanding of the Linux command line. It is available on GitHub as a software container ( https://github.com/Gal-Keshet/EpiTyping ) and produces reliable results in 1-4 d.

[Some present and future ethical dilemmas surrounding advancements in in vitro fertilization.]

The growing field of assisted human reproduction has achieved unimaginable milestones. Its continuous development and the innovations it generates at times pose both ethical and legal dilemmas. This essay aims to elucidate the progressive changes occurring in the realm of the origin of life due to the development of new options and strategies in assisted human reproduction. First, it constructs an interdisciplinary reflection on human nature and the changes society faces from the perspectives of science, ethics, and law, particularly from the perspective of Spain. Second, it provides a brief overview of current or future biomedical techniques in the field of human reproduction. It concludes with a discussion of the need to reflect on the rapid advancement of science in assisted human reproduction.

A new era of stem cell and developmental biology: from blastoids to synthetic embryos and beyond

Recent discoveries in stem cell and developmental biology have introduced a new era marked by the generation of in vitro models that recapitulate early mammalian development, providing unprecedented opportunities for extensive research in embryogenesis. Here, we present an overview of current techniques that model early mammalian embryogenesis, specifically noting models created from stem cells derived from two significant species: Homo sapiens, for its high relevance, and Mus musculus, a historically common and technically advanced model organism. We aim to provide a holistic understanding of these in vitro models by tracing the historical background of the progress made in stem cell biology and discussing the fundamental underlying principles. At each developmental stage, we present corresponding in vitro models that recapitulate the in vivo embryo and further discuss how these models may be used to model diseases. Through a discussion of these models as well as their potential applications and future challenges, we hope to demonstrate how these innovative advances in stem cell research may be further developed to actualize a model to be used in clinical practice.

Modeling specific aneuploidies: from karyotype manipulations to biological insights

An abnormal chromosome number, or aneuploidy, underlies developmental disorders and is a common feature of cancer, with different cancer types exhibiting distinct patterns of chromosomal gains and losses. To understand how specific aneuploidies emerge in certain tissues and how they contribute to disease development, various methods have been developed to alter the karyotype of mammalian cells and mice. In this review, we provide an overview of both classic and novel strategies for inducing or selecting specific chromosomal gains and losses in human and murine cell systems. We highlight how these customized aneuploidy models helped expanding our knowledge of the consequences of specific aneuploidies to (cancer) cell physiology.

Ethical, legal, regulatory, and policy issues concerning embryoids: a systematic review of the literature

Recent advances in methods to culture pluripotent stem cells to model human development have resulted in entities that increasingly have recapitulated advanced stages of early embryo development. These entities, referred to by numerous terms such as embryoids, are becoming more sophisticated and could resemble human embryos ever more closely as research progresses. This paper reports a systematic review of the ethical, legal, regulatory, and policy questions and concerns found in the literature concerning human embryoid research published from 2016 to 2022. We identified 56 papers that use 53 distinct names or terms to refer to embryoids and four broad categories of ethical, legal, regulatory, or policy considerations in the literature: research justifications/benefits, ethical significance or moral status, permissible use, and regulatory and oversight challenges. Analyzing the full range of issues is a critical step toward fostering more robust ethical, legal, and social implications research in this emerging area and toward developing appropriate oversight.

Geometric Confinement-Mediated Mechanical Tension Directs Patterned Differentiation of Mouse ESCs into Organized Germ Layers

The self-organization of embryonic stem cells (ESCs) into organized tissues with three distinct germ layers is critical to morphogenesis and early development. While the regulation of this self-organization by soluble signals is well established, the involvement of mechanical force gradients in this process remains unclear due to the lack of a suitable platform to study this process. In this study, we developed a 3D microenvironment to examine the influence of mechanical tension gradients on ESC-patterned differentiation during morphogenesis by controlling the geometrical signals (shape and size) of ESC colonies. We found that changes in colony geometry impacted the germ layer pattern, with Cdx2-positive cells being more abundant at edges and in areas with high curvatures. The differentiation patterns were determined by geometry-mediated cell tension gradients, with an extraembryonic mesoderm-like layer forming in high-tension regions and ectodermal-like lineages at low-tension regions in the center. Suppression of cytoskeletal tension hindered ESC self-organization. These results indicate that geometric confinement-mediated mechanical tension plays a crucial role in linking multicellular organization to cell differentiation and impacting tissue patterning.

Human embryoids: A new strategy of recreating the first steps of embryonic development in vitro

Molecular mechanisms surrounding early human embryonic events such as blastocyst formation, implantation, and the specification of the body axes are some of the most attractive research questions of developmental biology today. A knowledge on the detailed signaling landscape underlying these critical events in the human could impact the way we treat early pregnancy disorders and infertility, and considerably advance our abilities to make precise human tissues in a lab. However, owing to ethical, technical, and policy restrictions, research on early human embryo development historically stalled behind animal models. The rapid progress in 3D culture of human embryonic stem cells over the past years created an opportunity to overcome this critical challenge. We review recently developed strategies of making 3D models of the human embryo built from embryonic stem cells, which we refer to as embryoids. We focus on models aimed at reconstituting the 3D epithelial characteristics of the early human embryo, namely the intra/extraembryonic signaling crosstalk, tissue polarity, and embryonic cavities. We identify distinct classes of embryoids based on whether they explicitly include extraembryonic tissues and we argue for the merit of compromising on certain aspects of embryo mimicry in balancing the experimental feasibility with ethical considerations. Human embryoids open gates toward a new field of synthetic human embryology, allowing to study the long inaccessible stages of early human development at unprecedented detail.

Cell state transitions: catch them if you can

The Company of Biologists' 2022 workshop on 'Cell State Transitions: Approaches, Experimental Systems and Models' brought together an international and interdisciplinary team of investigators spanning the fields of cell and developmental biology, stem cell biology, physics, mathematics and engineering to tackle the question of how cells precisely navigate between distinct identities and do so in a dynamic manner. This second edition of the workshop was organized after a successful virtual workshop on the same topic that took place in 2021.

Mechanobiological approaches to synthetic morphogenesis: learning by building

Tissue morphogenesis occurs in a complex physicochemical microenvironment with limited experimental accessibility. This often prevents a clear identification of the processes that govern the formation of a given functional shape. By applying state-of-the-art methods to minimal tissue systems, synthetic morphogenesis aims to engineer the discrete events that are necessary and sufficient to build specific tissue shapes. Here, we review recent advances in synthetic morphogenesis, highlighting how a combination of microfabrication and mechanobiology is fostering our understanding of how tissues are built.

Harnessing synthetic biology to engineer organoids and tissues

The development of an organism depends on intrinsic genetic programs of progenitor cells and their spatiotemporally complex extrinsic environment. Ex vivo generation of organoids from progenitor cells provides a platform for recapitulating and exploring development. Current approaches rely largely on soluble morphogens or engineered biomaterials to manipulate the physical environment, but the emerging field of synthetic biology provides a powerful toolbox to genetically manipulate cell communication, adhesion, and even cell fate. Applying these modular tools to organoids should lead to a deeper understanding of developmental principles, improved organoid models, and an enhanced capability to design tissues for regenerative purposes.

Automated detection and growth tracking of 3D bio-printed organoid clusters using optical coherence tomography with deep convolutional neural networks

Organoids are advancing the development of accurate prediction of drug efficacy and toxicity in vitro. These advancements are attributed to the ability of organoids to recapitulate key structural and functional features of organs and parent tumor. Specifically, organoids are self-organized assembly with a multi-scale structure of 30-800 μm, which exacerbates the difficulty of non-destructive three-dimensional (3D) imaging, tracking and classification analysis for organoid clusters by traditional microscopy techniques. Here, we devise a 3D imaging, segmentation and analysis method based on Optical coherence tomography (OCT) technology and deep convolutional neural networks (CNNs) for printed organoid clusters (Organoid Printing and optical coherence tomography-based analysis, OPO). The results demonstrate that the organoid scale influences the segmentation effect of the neural network. The multi-scale information-guided optimized EGO-Net we designed achieves the best results, especially showing better recognition workout for the biologically significant organoid with diameter ≥50 μm than other neural networks. Moreover, OPO achieves to reconstruct the multiscale structure of organoid clusters within printed microbeads and calibrate the printing errors by segmenting the printed microbeads edges. Overall, the classification, tracking and quantitative analysis based on image reveal that the growth process of organoid undergoes morphological changes such as volume growth, cavity creation and fusion, and quantitative calculation of the volume demonstrates that the growth rate of organoid is associated with the initial scale. The new method we proposed enable the study of growth, structural evolution and heterogeneity for the organoid cluster, which is valuable for drug screening and tumor drug sensitivity detection based on organoids.

Dynamics of DNA hydroxymethylation and methylation during mouse embryonic and germline development

In mammals, DNA 5-hydroxymethylcytosine (5hmC) is involved in methylation reprogramming during early embryonic development. Yet, to what extent 5hmC participates in genome-wide methylation reprogramming remains largely unknown. Here, we characterize the 5hmC landscapes in mouse early embryos and germ cells with parental allele specificity. DNA hydroxymethylation was most strongly correlated with DNA demethylation as compared with de novo or maintenance methylation in zygotes, while 5hmC was targeted to particular de novo methylated sites in postimplantation epiblasts. Surprisingly, DNA replication was also required for 5hmC generation, especially in the female pronucleus. More strikingly, aberrant nuclear localization of Dnmt1/Uhrf1 in mouse zygotes due to maternal deficiency of Nlrp14 led to defects in DNA-replication-coupled passive demethylation and impaired 5hmC deposition, revealing the divergency between genome-wide 5-methylcytosine (5mC) maintenance and Tet-mediated oxidation. In summary, our work provides insights and a valuable resource for the study of epigenetic regulation in early embryo development.

Regulation, functions and transmission of bivalent chromatin during mammalian development

Cells differentiate and progress through development guided by a dynamic chromatin landscape that mediates gene expression programmes. During development, mammalian cells display a paradoxical chromatin state: histone modifications associated with gene activation (trimethylated histone H3 Lys4 (H3K4me3)) and with gene repression (trimethylated H3 Lys27 (H3K27me3)) co-occur at promoters of developmental genes. This bivalent chromatin modification state is thought to poise important regulatory genes for expression or repression during cell-lineage specification. In this Review, we discuss recent work that has expanded our understanding of the molecular basis of bivalent chromatin and its contributions to mammalian development. We describe the factors that establish bivalency, especially histone-lysine N-methyltransferase 2B (KMT2B) and Polycomb repressive complex 2 (PRC2), and consider evidence indicating that PRC1 shapes bivalency and may contribute to its transmission between generations. We posit that bivalency is a key feature of germline and embryonic stem cells, as well as other types of stem and progenitor cells. Finally, we discuss the relevance of bivalent chromtin to human development and cancer, and outline avenues of future research.

Establishment of fishing cat cell biobanking for sustainable conservation

The fishing cat (Prionailurus viverrinus) is a vulnerable wild felid that is currently under threat from habitat destruction and other human activities. The zoo provides insurance to ensure the survival of the fishing cat population. Creating a biobank of fishing cats is a critical component of recent zoo strategies for securely stocking cell samples for long-term survival. Here, our goal was to compare cell biobanking techniques (tissue collection, primary culture, and reprogramming) and tissue sources (ear skin, abdominal skin, testis) from captive (n = 6)/natural (n = 6) vs. living (n = 8)/postmortem (n = 4) fishing cats. First, we show that dermal fibroblasts from the medial border of the helix of the ear pinna and abdominal tissues of living fishing cats can be obtained, whereas postmortem animals provided far fewer fibroblasts from the ears than from the testes. Furthermore, we can extract putative adult spermatogonial stem cells from the postmortem fishing cat's testes. The main barrier to expanding adult fibroblasts was early senescence, which can be overcome by overexpressing reprogramming factors through felid-specific transfection programs, though we demonstrated that reaching iPSC state from adult fibroblasts of fishing cats was ineffective with current virus-free mammal-based induction approaches. Taken together, the success of isolating and expanding primary cells is dependent on a number of factors, including tissue sources, tissue handling, and nature of limited replicative lifespan of the adult fibroblasts. This study provides recommendations for tissue collection and culture procedures for zoological research to facilitate the preservation of cells from both postmortem and living felids.

Exploring standards for multicellular mammalian synthetic biology

Synthetic biology is moving towards bioengineering multicellular mammalian systems that are poised to advance tissue engineering, biomedicine, and the food industry. Despite progress, the field lacks a framework of standards that could greatly accelerate further development. Here, we explore the landscape of standards for multicellular mammalian synthetic biology. We discuss the limits of current technical standards and categorise unaddressed parameters into an abstraction hierarchy. We then define the concept of a 'synthetic multicellular mammalian system' and apply our standard hierarchy framework to illustrate how it could aid bioengineering endeavours. We conclude with promising areas that could shape the future of the field, flagging the need for a critical and holistic consideration of standards that requires cross-disciplinary dialogue.

New insights into the epitranscriptomic control of pluripotent stem cell fate

Each cell in the human body has a distinguishable fate. Pluripotent stem cells are challenged with a myriad of lineage differentiation options. Defects are more likely to be fatal to stem cells than to somatic cells due to the broad impact of the former on early development. Hence, a detailed understanding of the mechanisms that determine the fate of stem cells is needed. The mechanisms by which human pluripotent stem cells, although not fully equipped with complex chromatin structures or epigenetic regulatory mechanisms, accurately control gene expression and are important to the stem cell field. In this review, we examine the events driving pluripotent stem cell fate and the underlying changes in gene expression during early development. In addition, we highlight the role played by the epitranscriptome in the regulation of gene expression that is necessary for each fate-related event.

Stem-cell-based human and mouse embryo models

Synthetic embryology aims to develop embryo-like structures from stem cells to provide new insight into early stages of mammalian development. Recent advances in synthetic embryology have highlighted the remarkable capacity of stem cells to self-organize under certain biochemical or biophysical stimulations, generating structures that recapitulate the fate and form of early mouse/human embryos, in which symmetry breaking, pattern formation, or proper morphogenesis can be observed spontaneously. Here we review recent progress on the design principles for different types of embryoids and discuss the impact of different biochemical and biophysical factors on the process of stem-cell self-organization. We also offer our thoughts about the principal future challenges.

Using high throughput screens to predict miscarriages with placental stem cells and long-term stress effects with embryonic stem cells

A problem in developmental toxicology is the massive loss of life from fertilization through gastrulation, and the surprising lack of knowledge of causes of miscarriage. Half to two-thirds of embryos are lost, and environmental and genetic causes are nearly equal. Simply put, it can be inferred that this is a difficult period for normal embryos, but that environmental stresses may cause homeostatic responses that move from adaptive to maladaptive with increasing exposures. At the lower 50% estimate, miscarriage causes greater loss-of-life than all cancers combined or of all cardio- and cerebral-vascular accidents combined. Surprisingly, we do not know if miscarriage rates are increasing or decreasing. Overshadowed by the magnitude of miscarriages, are insufficient data on teratogenic or epigenetic imbalances in surviving embryos and their stem cells. Superimposed on the difficult normal trajectory for peri-gastrulation embryos are added malnutrition, hormonal, and environmental stresses. An overarching hypothesis is that high throughput screens (HTS) using cultured viable reporter embryonic and placental stem cells (e.g., embryonic stem cells [ESC] and trophoblast stem cells [TSC] that report status using fluorescent reporters in living cells) from the pre-gastrulation embryo will most rapidly test a range of hormonal, environmental, nutritional, drug, and diet supplement stresses that decrease stem cell proliferation and imbalance stemness/differentiation. A second hypothesis is that TSC respond with greater sensitivity in magnitude to stress that would cause miscarriage, but ESC are stress-resistant to irreversible stemness loss and are best used to predict long-term health defects. DevTox testing needs more ESC and TSC HTS to model environmental stresses leading to miscarriage or teratogenesis and more research on epidemiology of stress and miscarriage. This endeavor also requires a shift in emphasis on pre- and early gastrulation events during the difficult period of maximum loss by miscarriage.

Stem cell-derived synthetic embryos self-assemble by exploiting cadherin codes and cortical tension

Mammalian embryos sequentially differentiate into trophectoderm and an inner cell mass, the latter of which differentiates into primitive endoderm and epiblast. Trophoblast stem (TS), extraembryonic endoderm (XEN) and embryonic stem (ES) cells derived from these three lineages can self-assemble into synthetic embryos, but the mechanisms remain unknown. Here, we show that a stem cell-specific cadherin code drives synthetic embryogenesis. The XEN cell cadherin code enables XEN cell sorting into a layer below ES cells, recapitulating the sorting of epiblast and primitive endoderm before implantation. The TS cell cadherin code enables TS cell sorting above ES cells, resembling extraembryonic ectoderm clustering above epiblast following implantation. Whereas differential cadherin expression drives initial cell sorting, cortical tension consolidates tissue organization. By optimizing cadherin code expression in different stem cell lines, we tripled the frequency of correctly formed synthetic embryos. Thus, by exploiting cadherin codes from different stages of development, lineage-specific stem cells bypass the preimplantation structure to directly assemble a postimplantation embryo.

A quantitative biophysical principle to explain the 3D cellular connectivity in curved epithelia

Epithelial cell organization and the mechanical stability of tissues are closely related. In this context, it has been recently shown that packing optimization in bended or folded epithelia is achieved by an energy minimization mechanism that leads to a complex cellular shape: the "scutoid". Here, we focus on the relationship between this shape and the connectivity between cells. We use a combination of computational, experimental, and biophysical approaches to examine how energy drivers affect the three-dimensional (3D) packing of tubular epithelia. We propose an energy-based stochastic model that explains the 3D cellular connectivity. Then, we challenge it by experimentally reducing the cell adhesion. As a result, we observed an increment in the appearance of scutoids that correlated with a decrease in the energy barrier necessary to connect with new cells. We conclude that tubular epithelia satisfy a quantitative biophysical principle that links tissue geometry and energetics with the average cellular connectivity.

Gastruloids: Pluripotent stem cell models of mammalian gastrulation and embryo engineering

Gastrulation is a fundamental and critical process of animal development whereby the mass of cells that results from the proliferation of the zygote transforms itself into a recognizable outline of an organism. The last few years have seen the emergence of a number of experimental models of early mammalian embryogenesis based on Embryonic Stem (ES) cells. One of this is the Gastruloid model. Gastruloids are aggregates of defined numbers of ES cells that, under defined culture conditions, undergo controlled proliferation, symmetry breaking, and the specification of all three germ layers characteristic of vertebrate embryos, and their derivatives. However, they lack brain structures and, surprisingly, reveal a disconnect between cell type specific gene expression and tissue morphogenesis, for example during somitogenesis. Gastruloids have been derived from mouse and human ES cells and several variations of the original model have emerged that reveal a hereto unknown modularity of mammalian embryos. We discuss the organization and development of gastruloids in the context of the embryonic stages that they represent, pointing out similarities and differences between the two. We also point out their potential as a reproducible, scalable and searchable experimental system and highlight some questions posed by the current menagerie of gastruloids.

Human fibroblasts facilitate the generation of iPSCs-derived mammary-like organoids

Background

Breast cancer is the most common malignancy in women worldwide, and its treatment largely depends on mastectomy. Patients after mastectomy suffer from crippled body image, self-esteem, and quality of life. Post-mastectomy breast reconstruction can improve patients’ psychosocial health. Although silicone and fat have been widely used for breast reconstruction, they have remarkable limitations. Our study aimed to establish an improved method for breast reconstruction from human-induced pluripotent stem cells (iPSCs).

Methods

We used a two-step procedure to induce mammary-like organoids (MLOs) from iPSCs and applied transcriptome sequencing to analyze the gene expression profiles during the development process from embryoid bodies (mEBs) to MLOs. Moreover, we evaluated the in vitro effect of fibroblasts cell line HFF (human foreskin fibroblasts) on the size and morphology of MLOs and explored the in vivo effect of HFF on regeneration rate of MLOs.

Results

MLOs had a similar gene expression profile and morphogenesis as the normal mammary glands. Furthermore, the addition of HFF increases the branching ratio and organoid diameters and facilitates the formation of multiple cell layers duct-like structures in MLOs in vitro. Finally, orthotopical transplantation of the MLOs to cleared mammary gland fad pad of NSG mice showed that HFF increases the formation of mammary gland-like structures.

Conclusions

Fibroblasts facilitate iPSC-derived MLOs to generate mammary gland-like structures in both in vitro and in vivo conditions. Our findings lay a foundation for breast reconstruction by using iPSCs.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-022-03023-7.