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Totipotent stem cells (TSCs), can develop into complete organisms, are used in biological fields such as regenerative medicine, mammalian breeding, and conservation. However, it is difficult to maintain the developmental totipotency and self-renewal capacity of cells cultured from early-stage embryos, which becomes a key factor limiting the research of TSCs. Fortunately, a breakthrough in the study of induced pluripotent stem cells returning to their totipotent state has been made, resulting in the establishment of multiple TSCs and igniting a new wave of stem cell research. Furthermore, the blastocyst-like structures can be generated by the established TSCs, which lays a foundation for synthetic embryos in vitro. In this review, we summarize the totipotent stage of early embryos, the establishment and cultivation of TSCs, and the developmental ability exploration of TSCs to promote further research of TSCs.

Protoplasts are single cells isolated from tissues or organs and are considered a suitable system for cell studies in plants. Embryogenic cells are totipotent stem cells, but their regeneration ability decreases or becomes lost altogether with extension of the culture period. In this study, we isolated and cultured EC-derived protoplasts (EC-pts) from carrots and compared them with non-EC-derived protoplasts (NEC-pts) with respect to their totipotency. The protoplast isolation conditions were optimized, and the EC-pts and NEC-pts were characterized by their cell size and types. Both types of protoplasts were then embedded using the alginate layer (TAL) method, and the resulting EC-pt-TALs and NEC-pt-TALs were cultured for further regeneration. The expression of the EC-specific genes

Embryonic stem cells possess fascinating capacity of self-renewal and developmental potential, leading to significant progress in understanding the molecular basis of pluripotency, disease modeling and reprogramming technology. Recently, 2-cell (2C)-like embryonic stem cells (ESCs) and expanded potential stem cells or extended pluripotent stem cells (EPSCs) generated form early cleavage embryos display some features of totipotent embryos. These cell lines provide valuable in vitro models to study underlying principles of totipotency, cell plasticity and lineage segregation. In this review, we summarize the current progress in this filed, and highlight the application potentials of these cells in the future.

It is challenging to derive totipotent stem cells in vitro that functionally and molecularly resemble cells from totipotent embryos. Here, we report that a chemical cocktail enables the derivation of totipotent-like stem cells, designated as totipotent potential stem (TPS) cells, from 2-cell mouse embryos and extended pluripotent stem cells, and that these TPS cells can be stably maintained long term in vitro. TPS cells shared features with 2-cell mouse embryos in terms of totipotency markers, transcriptome, chromatin accessibility and DNA methylation patterns. In vivo chimera formation assays show that these cells have embryonic and extraembryonic developmental potentials at the single-cell level. Moreover, TPS cells can be induced into blastocyst-like structures resembling preimplantation mouse blastocysts. Mechanistically, inhibition of HDAC1/2 and DOT1L activity and activation of RARγ signaling are important for inducing and maintaining totipotent features of TPS cells. Our study opens up a new path toward fully capturing totipotent stem cells in vitro.

Understanding human development is of fundamental biological and clinical importance. Despite its significance, mechanisms behind human embryogenesis remain largely unknown. Here, we attempt to model human early embryo development with expanded pluripotent stem cells (EPSCs) in 3-dimensions. We define a protocol that allows us to generate self-organizing cystic structures from human EPSCs that display some hallmarks of human early embryogenesis. These structures mimic polarization and cavitation characteristic of pre-implantation development leading to blastocyst morphology formation and the transition to post-implantation-like organization upon extended culture. Single-cell RNA sequencing of these structures reveals subsets of cells bearing some resemblance to epiblast, hypoblast and trophectoderm lineages. Nevertheless, significant divergences from natural blastocysts persist in some key markers, and signalling pathways point towards ways in which morphology and transcriptional-level cell identities may diverge in stem cell models of the embryo. Thus, this stem cell platform provides insights into the design of stem cell models of embryogenesis., Human early development remains largely inaccessible, owing to technical and ethical limitations of working with natural embryos. Here the authors assess the extent to which human expanded pluripotent stem cells can specify distinct cell lineages and capture aspects of early human embryogenesis.

Key message In vitro embryo development is highly plastic; embryo cell fate can be re-established in tissue culture through different pathways. Abstract In most angiosperms, embryo development from the single-celled zygote follows a defined pattern of cell divisions in which apical (embryo proper) and basal (root and suspensor) cell fates are established within the first cell divisions. By contrast, embryos that are induced in vitro in the absence of fertilization show a less regular initial cell division pattern yet develop into histodifferentiated embryos that can be converted into seedlings. We used the Brassica napus microspore embryogenesis system, in which the male gametophyte is reprogrammed in vitro to form haploid embryos, to identify the developmental fates of the different types of embryogenic structures found in culture. Using time-lapse imaging of LEAFY COTYLEDON1-expressing cells, we show that embryogenic cell clusters with very different morphologies are able to form haploid embryos. The timing of surrounding pollen wall (exine) rupture is a major determinant of cell fate in these clusters, with early exine rupture leading to the formation of suspensor-bearing embryos and late rupture to suspensorless embryos. In addition, we show that embryogenic callus, which develops into suspensor-bearing embryos, initially expresses transcripts associated with both basal- and apical-embryo cell fates, suggesting that these two cell fates are fixed later in development. This study reveals the inherent plasticity of in vitro embryo development and identifies new pathways by which embryo cell fate can be established.

In contrast to embryo donation, the permissibility of 2PN cell donation is highly controversial in Germany. This article is based on there being a legal loophole with respect to 2PN cell donation, which results from an inconsistency within the Embryo Protection Act on the normative status of 2PN cells. Following that thesis, the article argues that, on the basis of the normative criterion totipotency (i.e. the capacity to develop into a born human being), 2PN cells should also be considered human embryos within the meaning of the Act and thereby be protected by that Act in the same way as embryos. However, the normative assumption that 2PN cells should already be endowed with human dignity and the right to life has absurd consequences. Moreover, the consistent continuation of the Embryo Protection Act, as well as of the underlying ethical position or argumentation (i.e. the potentiality argument), leads to the even more absurd consequence of having to place every human somatic cell under the protection of human dignity and the right to life. As totipotency or the developmental potential therefore cannot delimit entities considered worthy of protection (i.e. human embryos) from entities considered not worthy of protection (i.e. 2PN cells, gametes, hESC, hiPSC and human somatic cells), it is not a suitable normative criterion. As a paradigmatic case, 2PN cell donation demonstrates that by retaining this normative criterion the now obsolete German Embryo Protection (Act) ultimately undermines itself.

Totipotent cells have the ability to generate embryonic and extra-embryonic tissues. Interestingly, a rare population of cells with totipotent-like potential, known as 2 cell (2C)-like cells, has been identified within ESC cultures. They arise from ESC and display similar features to those found in the 2C embryo. However, the molecular determinants of 2C-like conversion have not been completely elucidated. Here, we show that the CCCTC-binding factor (CTCF) is a barrier for 2C-like reprogramming. Indeed, forced conversion to a 2C-like state by the transcription factor DUX is associated with DNA damage at a subset of CTCF binding sites. Depletion of CTCF in ESC efficiently promotes spontaneous and asynchronous conversion to a 2C-like state and is reversible upon restoration of CTCF levels. This phenotypic reprogramming is specific to pluripotent cells as neural progenitor cells do not show 2C-like conversion upon CTCF-depletion. Furthermore, we show that transcriptional activation of the ZSCAN4 cluster is necessary for successful 2C-like reprogramming. In summary, we reveal an unexpected relationship between CTCF and 2C-like reprogramming., Embryos at the 2-cell (2C) stage are totipotent, and overexpression of Dux transcription factor convert embryonic stem cells (ESCs) to a 2C-like state. Here the authors show that DUX-mediated 2C-like reprogramming is associated with DNA damage at CTCF sites and CTCF depletion promotes 2Clike conversion.

Totipotency refers to single cells' developmental capacity to form an entire organism. Understanding how totipotent stem cells form has implications for chimera generation. In a recent Cell study, Shen et al. (2021) report that inhibition of spliceosomes resets conventional pluripotent stem cells to a cellular state with totipotency features.

Totipotent cells hold enormous potential for regenerative medicine. Thus, the development of cellular models recapitulating totipotent-like features is of paramount importance. Cells resembling the totipotent cells of early embryos arise spontaneously in mouse embryonic stem (ES) cell cultures. Such ‘2-cell-like-cells’ (2CLCs) recapitulate 2-cell-stage features and display expanded cell potential. Here, we used 2CLCs to perform a small-molecule screen to identify new pathways regulating the 2-cell-stage program. We identified retinoids as robust inducers of 2CLCs and the retinoic acid (RA)-signaling pathway as a key component of the regulatory circuitry of totipotent cells in embryos. Using single-cell RNA-seq, we reveal the transcriptional dynamics of 2CLC reprogramming and show that ES cells undergo distinct cellular trajectories in response to RA. Importantly, endogenous RA activity in early embryos is essential for zygotic genome activation and developmental progression. Overall, our data shed light on the gene regulatory networks controlling cellular plasticity and the totipotency program., High-throughput chemical screening identifies retinoic acid signaling as a regulatory pathway of 2-cell-like cell reprogramming and early mouse development.

In mammals, telomere protection is mediated by the essential protein TRF2, which binds chromosome ends and ensures genome integrity(1,2). TRF2 depletion results in end-to-end chromosome fusions in all cell types that have been tested so far. Here we find that TRF2 is dispensable for the proliferation and survival of mouse embryonic stem (ES) cells. Trf2(−/−) (also known as Terf2) ES cells do not exhibit telomere fusions and can be expanded indefinitely. In response to the deletion of TRF2, ES cells exhibit a muted DNA damage response that is characterized by the recruitment of γH2AX—but not 53BP1—to telomeres. To define the mechanisms that control this unique DNA damage response in ES cells, we performed a CRISPR–Cas9-knockout screen. We found a strong dependency of TRF2-null ES cells on the telomere-associated protein POT1B and on the chromatin remodelling factor BRD2. Co-depletion of POT1B or BRD2 with TRF2 restores a canonical DNA damage response at telomeres, resulting in frequent telomere fusions. We found that TRF2 depletion in ES cells activates a totipotent-like two-cell-stage transcriptional program that includes high levels of ZSCAN4. We show that the upregulation of ZSCAN4 contributes to telomere protection in the absence of TRF2. Together, our results uncover a unique response to telomere deprotection during early development.

Embryonic stem cells (ESCs) are derived from the pre-implantation mammalian blastocyst. At this point in time, the newly formed embryo is concerned with the generation and expansion of both the embryonic lineages required to build the embryo and the extra-embryonic lineages that support development. When used in grafting experiments, embryonic cells from early developmental stages can contribute to both embryonic and extra-embryonic lineages, but it is generally accepted that ESCs can give rise to only embryonic lineages. As a result, they are referred to as pluripotent, rather than totipotent. Here, we consider the experimental potential of various ESC populations and a number of recently identified in vitro culture systems producing states beyond pluripotency and reminiscent of those observed during pre-implantation development. We also consider the nature of totipotency and the extent to which cell populations in these culture systems exhibit this property.

Summary Since establishment of the first embryonic stem cells (ESCs), in vitro culture of totipotent cells functionally and molecularly comparable with in vivo blastomeres with embryonic and extraembryonic developmental potential has been a challenge. Here we report that spliceosomal repression in mouse ESCs drives a pluripotent-to-totipotent state transition. Using the splicing inhibitor pladienolide B, we achieve stable in vitro culture of totipotent ESCs comparable at molecular levels with 2- and 4-cell blastomeres, which we call totipotent blastomere-like cells (TBLCs). Mouse chimeric assays combined with single-cell RNA sequencing (scRNA-seq) demonstrate that TBLCs have a robust bidirectional developmental capability to generate multiple embryonic and extraembryonic cell lineages. Mechanically, spliceosomal repression causes widespread splicing inhibition of pluripotent genes, whereas totipotent genes, which contain few short introns, are efficiently spliced and transcriptionally activated. Our study provides a means for capturing and maintaining totipotent stem cells.

Totipotency is the ability of a single cell to give rise to all of the differentiated cell types that build the conceptus, yet how to capture this property in vitro remains incompletely understood. Defining totipotency relies on a variety of assays of variable stringency. Here, we describe criteria to define totipotency. We explain how distinct criteria of increasing stringency can be used to judge totipotency by evaluating candidate totipotent cell types in mice, including early blastomeres and expanded or extended pluripotent stem cells. Our data challenge the notion that expanded or extended pluripotent states harbour increased totipotent potential relative to conventional embryonic stem cells under in vitro and in vivo conditions.

Totipotent cells have more robust developmental potency than any other cell types, giving rise to both embryonic and extraembryonic tissues. Stable totipotent cell cultures and deciphering the principles of totipotency regulation would be invaluable to understand cell plasticity and lineage segregation in early development. Our approach of remodeling the pericentromeric heterochromatin and re-establishing the totipotency-specific broad H3K4me3 domains promotes the pluri-to-totipotency transition. Our protocol establishes a closer match of mouse 2-cell (2C) embryos than any other 2C-like cells. These totipotent-like stem cells (TLSCs) are stable in culture and possess unique molecular features of the mouse 2C embryo. Functionally, TLSCs are competent for germline transmission and give rise to both embryonic and extraembryonic lineages at high frequency. Therefore, TLSCs represent a highly valuable cell type for studies of totipotency and embryology.

Minority subpopulations within embryonic stem cell cultures display an expanded developmental potential similar to that of early embryo blastomeres or the early inner cell mass. The ability to isolate and culture totipotent cells capable of giving rise to the entire conceptus would enhance our capacity to study early embryo development, and might enable more efficient generation of chimeric animals for research and organ production for transplantation. Here we review the biological and molecular characterization of cultured cells with developmental potential similar to totipotent blastomeres, and assess recent progress toward the capture and stabilization of the totipotent state in vitro.

Chromatin is reprogrammed after fertilization to produce a totipotent zygote with the potential to generate a new organism. The maternal genome inherited from the oocyte and the paternal genome provided by sperm coexist as separate haploid nuclei in the zygote. How these two epigenetically distinct genomes are spatially organized is poorly understood. Existing chromosome conformation capture-based methods are not applicable to oocytes and zygotes owing to a paucity of material. To study three-dimensional chromatin organization in rare cell types, we developed a single-nucleus Hi-C (high-resolution chromosome conformation capture) protocol that provides greater than tenfold more contacts per cell than the previous method. Here we show that chromatin architecture is uniquely reorganized during the oocyte-to-zygote transition in mice and is distinct in paternal and maternal nuclei within single-cell zygotes. Features of genomic organization including compartments, topologically associating domains (TADs) and loops are present in individual oocytes when averaged over the genome, but the presence of each feature at a locus varies between cells. At the sub-megabase level, we observed stochastic clusters of contacts that can occur across TAD boundaries but average into TADs. Notably, we found that TADs and loops, but not compartments, are present in zygotic maternal chromatin, suggesting that these are generated by different mechanisms. Our results demonstrate that the global chromatin organization of zygote nuclei is fundamentally different from that of other interphase cells. An understanding of this zygotic chromatin 'ground state' could potentially provide insights into reprogramming cells to a state of totipotency.

Our perception of the role of the previously considered ‘selfish’ or ‘junk’ DNA has been dramatically altered in the past 20 years or so. A large proportion of this non-coding part of mammalian genomes is repetitive in nature, classified as either satellites or transposons. While repetitive elements can be termed selfish in terms of their amplification, such events have surely been co-opted by the host, suggesting by itself a likely altruistic function for the organism at the subject of such natural selection. Indeed numerous examples of transposons regulating the functional output of the host genome have been documented. Transposons provide a powerful framework for large-scale relatively rapid concerted regulatory activities with the ability to drive evolution. Mammalian totipotency has emerged as one key stage of development in which transposon-mediated regulation of gene expression has taken centre stage in the past few years. During this period, large-scale (epigenetic) reprogramming must be accomplished in order to activate the host genome. In mice and men, one particular element murine endogenous retrovirus with leucine tRNA primer (MERVL) (and its counterpart human ERVL (HERVL)) appears to have acquired roles as a key driving force in this process. Here, I will discuss and interpret the current knowledge and its implications regarding the role of transposons, particularly of long interspersed nuclear elements (LINE-1s) and endogenous retroviruses (ERVs), in the regulation of totipotency. This article is part of a discussion meeting issue ‘Crossroads between transposons and gene regulation’.

The murine developing epicardium heterogeneously expresses the transcription factors TCF21 and WT1. Here, we show that this cell heterogeneity is conserved in human epicardium, regulated by BNC1 and associated with cell fate and function. Single cell RNA sequencing of epicardium derived from human pluripotent stem cells (hPSC-epi) revealed that distinct epicardial subpopulations are defined by high levels of expression for the transcription factors BNC1 or TCF21. WT1+ cells are included in the BNC1+ population, which was confirmed in human foetal hearts. THY1 emerged as a membrane marker of the TCF21 population. We show that THY1+ cells can differentiate into cardiac fibroblasts (CFs) and smooth muscle cells (SMCs), whereas THY1− cells were predominantly restricted to SMCs. Knocking down BNC1 during the establishment of the epicardial populations resulted in a homogeneous, predominantly TCF21high population. Network inference methods using transcriptomic data from the different cell lineages derived from the hPSC-epi delivered a core transcriptional network organised around WT1, TCF21 and BNC1. This study unveils a list of epicardial regulators and is a step towards engineering subpopulations of epicardial cells with selective biological activities., Highlighted Article: Single cell analysis of human pluripotent stem cell-derived epicardium revealed that epicardial heterogeneity is regulated by the transcription factor BNC1 and separates the cells into two populations with different properties.

Plants are sessile organisms that have a remarkable developmental plasticity, which ensures their optimal adaptation to environmental stresses. Plant cell totipotency is an extreme example of such plasticity, whereby somatic cells have the potential to form plants via direct shoot organogenesis or somatic embryogenesis in response to various exogenous and/or endogenous signals. Protoplasts provide one of the most suitable systems for investigating molecular mechanisms of totipotency, because they are effectively single cell populations. In this review, we consider the current state of knowledge of the mechanisms that induce cell proliferation from individual, differentiated somatic plant cells. We highlight initial explant metabolic status, ploidy level and isolation procedure as determinants of successful cell reprogramming. We also discuss the importance of auxin signalling and its interaction with stress-regulated pathways in governing cell cycle induction and further stages of plant cell totipotency.

We recently derived mouse expanded potential stem cells (EPSCs) from individual blastomeres by inhibiting the critical molecular pathways that predispose their differentiation. EPSCs had enriched molecular signatures of blastomeres and possessed developmental potency for all embryonic and extra-embryonic cell lineages. Here, we report the derivation of porcine EPSCs, which express key pluripotency genes, are genetically stable, permit genome editing, differentiate to derivatives of the three germ layers in chimeras and produce primordial germ cell-like cells in vitro. Under similar conditions, human embryonic stem cells and induced pluripotent stem cells can be converted, or somatic cells directly reprogrammed, to EPSCs that display the molecular and functional attributes reminiscent of porcine EPSCs. Importantly, trophoblast stem-cell-like cells can be generated from both human and porcine EPSCs. Our pathway-inhibition paradigm thus opens an avenue for generating mammalian pluripotent stem cells, and EPSCs present a unique cellular platform for translational research in biotechnology and regenerative medicine.

Summary: Known as a histone H3K9 methyltransferase, SETDB1 is essential for embryonic development and pluripotent inner cell mass (ICM) establishment. However, its function in pluripotency regulation remains elusive. In this study, we find that under the “ground state” of pluripotency with two inhibitors (2i) of the MEK and GSK3 pathways, Setdb1-knockout fails to induce trophectoderm (TE) differentiation as in serum/LIF (SL), indicating that TE fate restriction is not the direct target of SETDB1. In both conditions, Setdb1-knockout activates a group of genes targeted by SETDB1-mediated H3K9 methylation, including Dux. Notably, Dux is indispensable for the reactivation of 2C-like state genes upon Setdb1 deficiency, delineating the mechanistic role of SETDB1 in totipotency restriction. Furthermore, Setdb1-null ESCs maintain pluripotent marker (e.g., Nanog) expression in the 2i condition. This “ground state” Setdb1-null population undergoes rapid cell death by activating Ripk3 and, subsequently, RIPK1/RIPK3-dependent necroptosis. These results reveal the essential role of Setdb1 between totipotency and pluripotency transition. : Wu et al. report the essential role of Setdb1 in the cell fate transition between totipotency and pluripotency. Setdb1-KO ESCs activate a group of genes targeted by SETDB1-mediated H3K9 methylation, including Dux, then initiate 2C-like totipotency transition. Moreover, Setdb1-KO triggers ESC necroptosis in the “ground state” by activating Ripk3. Keywords: Setdb1, H3K9 methylation, 2C-like totipotency, necroptosis

Totipotency refers to the ability of a cell to generate all of the cell types of an organism. Unlike pluripotency, the establishment of totipotency is poorly understood. In mouse embryonic stem cells, Dux drives a small percentage of cells into a totipotent state by expressing 2-cell-embryo-specific transcripts. To understand how this transition takes place, we performed single-cell RNA-seq, which revealed a two-step transcriptional reprogramming process characterized by downregulation of pluripotent genes in the first step and upregulation of the 2-cell-embryo-specific elements in the second step. To identify factors controlling the transition, we performed a CRISPR-Cas9-mediated screen, which revealed Myc and Dnmt1 as two factors preventing the transition. Mechanistic studies demonstrate that Myc prevents downregulation of pluripotent genes in the first step, while Dnmt1 impedes 2-cell-embryo-specific gene activation in the second step. Collectively, the findings of our study reveal insights into the establishment and regulation of the totipotent state in mouse embryonic stem cells.

The acoel worm Symsagittifera roscoffensis, an early offshoot of the Bilateria and the only well-studied marine acoel that lives in a photosymbiotic relationship, exhibits a centralized nervous system, brain regeneration, and a wide repertoire of complex behaviors such as circatidal rhythmicity, photo/geotaxis, and social interactions. While this animal can be collected by the thousands and is studied historically, significant progress is made over the last decade to develop it as an emerging marine model. The authors here present the feasibility of culturing it in the laboratory and describe the progress made on different areas, including genomic and tissue architectures, highlighting the associated challenges. In light of these developments, and on the ability to access abundant synchronized embryos, the authors put forward S. roscoffensis as a marine system to revisit questions in the areas of photosymbiosis, regeneration, chronobiology, and the study of complex behaviors from a molecular and evolutionary perspective.

Understanding the past is to understand the present. Mammalian life, with all its complexity comes from a humble beginning of a single fertilized egg cell. Achieving this requires an enormous diversification of cellular function, the majority of which is generated through a series of cellular decisions during embryogenesis. The first decisions are made as the embryo prepares for implantation, a process that will require specialization of extra-embryonic lineages while preserving an embryonic one. In this mini-review, we will focus on the mouse as a mammalian model and discuss recent advances in the decision making process of the early embryo.

In order to ensure successful development, cells of the early mammalian embryo must differentiate to either trophectoderm (TE) or inner cell mass (ICM), followed by epiblast (EPI) or primitive endoderm (PE) specification within the ICM. Here, we deciphered the mechanism that assures the correct order of these sequential cell fate decisions. We revealed that TE-deprived ICMs derived from 32-cell blastocysts are still able to reconstruct TE during in vitro culture, confirming totipotency of ICM cells at this stage. ICMs isolated from more advanced blastocysts no longer retain totipotency, failing to form TE and generating PE on their surface. We demonstrated that the transition from full potency to lineage priming is prevented by inhibition of the FGF/MAPK signalling pathway. Moreover, we found that after this first restriction step, ICM cells still retain fate flexibility, manifested by ability to convert their fate into an alternative lineage (PE towards EPI and vice versa), until peri-implantation stage.

Objectives The aim of this study was to identify an epithelial cell line isolated from the spontaneous differentiation of totipotent pig epiblast cells. Methods PICM-31 and its colony-cloned derivative cell line, PICM-31A, were established from the culture and differentiation of an epiblast mass isolated from an 8-day-old pig blastocyst. The cell lines were analyzed by transmission electron microscopy, marker gene expression, and mass spectroscopy-based proteomics. Results The PICM-31 cell lines were continuously cultured and could be successively colony cloned. They spontaneously self-organized into acinarlike structures. Transmission electron microscopy indicated that the cell lines' cells were epithelial and filled with secretory granules. Candidate gene expression analysis of the cells showed an exocrine pancreatic profile that included digestive enzyme expression, for example, carboxypeptidase A1, and expression of the fetal marker, α-fetoprotein. Pancreatic progenitor marker expression included pancreatic and duodenal homeobox 1, NK6 homeobox 1, and pancreas-specific transcription factor 1a, but not neurogenin 3. Proteomic analysis of cellular proteins confirmed the cells' production of digestive enzymes and showed that the cells expressed cytokeratins 8 and 18. Conclusions The PICM-31 cell lines provide in vitro models of fetal pig pancreatic exocrine cells. They are the first demonstration of continuous cultures, that is, cell lines, of nontransformed pig pancreas cells.