Contribution of human embryonic stem cells to mouse blastocysts

Pluripotency: toward a gold standard for human ES and iPS cells

With the advent of technologies for the derivation of embryonic stem cells and reprogrammed stem cells, use of the term "pluripotent" has become widespread. Despite its increased scientific and political importance, there are ambiguities with this designation and a common standard for experimental approaches that precisely define this state in human cells remains elusive. Recent studies have revealed that reprogramming may occur via many pathways which do not always lead to pluripotency. In addition, the pluripotent state itself appears to be highly dynamic, leading to significant variability in the results of molecular studies. Establishment of a stringent set of criteria for defining pluripotency will be vital for biological studies and potential clinical applications in this rapidly evolving field. In this review, we explore the various definitions of pluripotency, examine the current status of pluripotency testing in the field and provide an analysis of how these assays have been used to establish pluripotency in the scientific literature.

Human immature dental pulp stem cells' contribution to developing mouse embryos: production of human/mouse preterm chimaeras

OBJECTIVES

In this study, we aimed at determining whether human immature dental pulp stem cells (hIDPSC) would be able to contribute to different cell types in mouse blastocysts without damaging them. Also, we analysed whether these blastocysts would progress further into embryogenesis when implanted to the uterus of foster mice, and develop human/mouse chimaera with retention of hIDPSC derivates and their differentiation.

MATERIALS AND METHODS

hIDPSC and mouse blastocysts were used in this study. Fluorescence staining of hIDPSC and injection into mouse blastocysts, was performed. Histology, immunohistochemistry, fluorescence in situ hybridization and confocal microscopy were carried out.

RESULTS AND CONCLUSION

hIDPSC showed biological compatibility with the mouse host environment and could survive, proliferate and contribute to the inner cell mass as well as to the trophoblast cell layer after introduction into early mouse embryos (n = 28), which achieved the hatching stage following 24 and 48 h in culture. When transferred to foster mice (n = 5), these blastocysts with hIDPSC (n = 57) yielded embryos (n = 3) and foetuses (n = 6); demonstrating presence of human cells in various organs, such as brain, liver, intestine and hearts, of the human/mouse chimaeras. We verified whether hIDPSC would also be able to differentiate into specific cell types in the mouse environment. Contribution of hIDPSC in at least two types of tissues (muscles and epithelial), was confirmed. We showed that hIDPSC survived, proliferated and differentiated in mouse developing blastocysts and were capable of producing human/mouse chimaeras.

Transplantation of human embryonic stem cells and derivatives to the chick embryo

Traditional methods of studying the differentiation of human embryonic stem cells (hESCs) include generation of embryoid bodies, induced differentiation in vitro, and transplantation to immune-deficient mice. The chick embryo is a well-studied and accessible experimental system that has been used for many years as a xenograft host for mammalian cells. Several years ago, we performed experiments transplanting colonies of hESC into organogenesis-stage chick embryos to establish a novel system for studying the developmental programs and decisions of pluripotent human cells. Fluorescent hESC were used, in order to permit identification of the hESC in living embryos. We transplanted hESC into the trunk of chick embryos, both into and instead of developing somites. Our results showed that hESC survive, migrate, and integrate into the tissues of the chick embryo. Some of the hESC differentiated and the type of embryonic microenvironment that the implanted cells were exposed to modified their differentiation. Several other laboratories have subsequently xenografted hESC-derived cells to chick embryos for evaluating their differentiation in vivo. Therefore, the hESC-chick embryo system is a useful xenograft system complementing studies in rodents and in vitro, as well as uniquely shedding light on early processes in the development of human cells in the embryonic context.

Human ES cell-derived neural rosettes reveal a functionally distinct early neural stem cell stage

Neural stem cells (NSCs) yield both neuronal and glial progeny, but their differentiation potential toward multiple region-specific neuron types remains remarkably poor. In contrast, embryonic stem cell (ESC) progeny readily yield region-specific neuronal fates in response to appropriate developmental signals. Here we demonstrate prospective and clonal isolation of neural rosette cells (termed R-NSCs), a novel NSC type with broad differentiation potential toward CNS and PNS fates and capable of in vivo engraftment. R-NSCs can be derived from human and mouse ESCs or from neural plate stage embryos. While R-NSCs express markers classically associated with NSC fate, we identified a set of genes that specifically mark the R-NSC state. Maintenance of R-NSCs is promoted by activation of SHH and Notch pathways. In the absence of these signals, R-NSCs rapidly lose rosette organization and progress to a more restricted NSC stage. We propose that R-NSCs represent the first characterized NSC stage capable of responding to patterning cues that direct differentiation toward region-specific neuronal fates. In addition, the R-NSC-specific genetic markers presented here offer new tools for harnessing the differentiation potential of human ESCs.

Isolation and directed differentiation of neural crest stem cells derived from human embryonic stem cells

Vertebrate neural crest development depends on pluripotent, migratory precursor cells. Although avian and murine neural crest stem (NCS) cells have been identified, the isolation of human NCS cells has remained elusive. Here we report the derivation of NCS cells from human embryonic stem cells at the neural rosette stage. We show that NCS cells plated at clonal density give rise to multiple neural crest lineages. The human NCS cells can be propagated in vitro and directed toward peripheral nervous system lineages (peripheral neurons, Schwann cells) and mesenchymal lineages (smooth muscle, adipogenic, osteogenic and chondrogenic cells). Transplantation of human NCS cells into the developing chick embryo and adult mouse hosts demonstrates survival, migration and differentiation compatible with neural crest identity. The availability of unlimited numbers of human NCS cells offers new opportunities for studies of neural crest development and for efforts to model and treat neural crest-related disorders.

Extensive contribution of embryonic stem cells to the development of an evolutionarily divergent host

The full potential of embryonic stem (ES) cells to generate precise cell lineages and complex tissues can be best realized when they are differentiated in vivo-i.e. in developing blastocysts. Owing to various practical and ethical constraints, however, it is impossible to introduce ES cells of certain species into blastocysts of the same species. One solution is to introduce ES cells into blastocysts of a different species. However, it is not known whether ES cells can contribute extensively to chimerism when placed into blastocysts of a distantly related species. Here, we address this question using two divergent species, Apodemus sylvaticus and Mus musculus, whose genome sequence differs by approximately 18% from each other. Despite this considerable evolutionary distance, injection of Apodemus ES cells into Mus blastocysts led to viable chimeras bearing extensive Apodemus contributions to all major organs, including the germline, with Apodemus contribution reaching approximately 40% in some tissues. Immunostaining showed that Apodemus ES cells have differentiated into a wide range of cell types in the chimeras. Our results thus provide a proof of principle for the feasibility of differentiating ES cells into a wide range of cell types and perhaps even complex tissues by allowing them to develop in vivo in an evolutionarily divergent host-a strategy that may have important applications in research and therapy. Furthermore, our study demonstrates that mammalian evolution can proceed at two starkly contrasting levels: significant divergence in genome and proteome sequence, yet striking conservation in developmental programs.

Human-animal chimeras in biomedical research

Chimeras are individuals with tissues derived from more than one zygote. Interspecific chimeras have tissues derived from different species. The biological consequences of human-animal chimeras have become an issue of ethical debate. Ironically, human-animal chimeras with human blood, neurons, germ cells, and other tissues have been generated for decades. This has facilitated human biological studies and therapeutic strategies for disease.

Derivation of pluripotent epiblast stem cells from mammalian embryos

Although the first mouse embryonic stem (ES) cell lines were derived 25 years ago using feeder-layer-based blastocyst cultures, subsequent efforts to extend the approach to other mammals, including both laboratory and domestic species, have been relatively unsuccessful. The most notable exceptions were the derivation of non-human primate ES cell lines followed shortly thereafter by their derivation of human ES cells. Despite the apparent common origin and the similar pluripotency of mouse and human embryonic stem cells, recent studies have revealed that they use different signalling pathways to maintain their pluripotent status. Mouse ES cells depend on leukaemia inhibitory factor and bone morphogenetic protein, whereas their human counterparts rely on activin (INHBA)/nodal (NODAL) and fibroblast growth factor (FGF). Here we show that pluripotent stem cells can be derived from the late epiblast layer of post-implantation mouse and rat embryos using chemically defined, activin-containing culture medium that is sufficient for long-term maintenance of human embryonic stem cells. Our results demonstrate that activin/Nodal signalling has an evolutionarily conserved role in the derivation and the maintenance of pluripotency in these novel stem cells. Epiblast stem cells provide a valuable experimental system for determining whether distinctions between mouse and human embryonic stem cells reflect species differences or diverse temporal origins.

Directed differentiation and transplantation of human embryonic stem cell-derived motoneurons

Motoneurons represent a specialized class of neurons essential for the control of body movement. Motoneuron loss is the cause of a wide range of neurological disorders including amyotrophic lateral sclerosis and spinal muscular atrophy. Embryonic stem cells are a promising cell source for the study and potential treatment of motoneuron diseases. Here, we present a novel in vitro protocol of the directed differentiation of human embryonic stem cells (hESCs) into engraftable motoneurons. Neural induction of hESCs was induced on MS5 stromal feeders, resulting in the formation of neural rosettes. In response to sonic hedgehog and retinoic acid, neural rosettes were efficiently directed into spinal motoneurons with appropriate in vitro morphological, physiological, and biochemical properties. Global gene expression analysis was used as an unbiased measure to confirm motoneuron identity and type. Transplantation of motoneuron progeny into the developing chick embryo resulted in robust engraftment, maintenance of motoneuron phenotype, and long-distance axonal projections into peripheral host tissues. Transplantation into the adult rat spinal cord yielded neural grafts comprising a large number of human motoneurons with outgrowth of choline acetyltransferase positive fibers. These data provide evidence for in vivo survival of hESC-derived motoneurons, a key requirement in the development of hESC-based cell therapy in motoneuron disease. Disclosure of potential conflicts of interest is found at the end of this article.

Stem cells and regenerative medicine: principles, prospects and problems

Stem cells have been used routinely for more than three decades to repair tissues and organs damaged by injury or disease, most notably haematopoietic stem cells taken from bone marrow, umbilical cord or, increasingly, from peripheral blood. Other examples, such as grafts of skin to treat severe burns, entail transplantation of stem cells within organized tissue rather than following isolation. The prospect of exploiting stem cells more widely in regenerative medicine was encouraged both by the development of human assisted conception and growing evidence that various adult cells retained greater versatility than had been suspected hitherto. The aim is to employ stem cells as a source of appropriately differentiated cells to replace those lost through physical, chemical or ischaemic injury, or as a result of degenerative disease. This may entail transplantation of just a single type of cell or, more challengingly, require a complex of several different types of cells possessing a defined architecture. Cardiomyocytes, hepatocytes or neuronal cells producing specific transmitters offer promising examples of the former, although how transplanted healthy cells will function in a perturbed tissue environment remains to be established. Recent success in repairing urinary bladder defects with grafts of urothelial and muscle cells seeded on a biodegradable collagen scaffold is an encouraging step towards assembling organs in vitro. Nevertheless, this is still far removed from the level of sophistication required to counter the ever increasing shortfall in supply of kidneys for transplantation. Various problems must be addressed if recent advances in the laboratory are to be translated into clinical practice. In many cases, it has yet to be established that cells derived from adults that retain plasticity are actually stem cells. There is also a pressing need for appropriate assays to ensure that, regardless of source, stem cells maintained in vitro are safe to transplant. Assays currently available for human ES cells are far from ideal. It is, in addition, important to ensure that differentiated cultures are pure and, depending on whether cell renewal is required or to be avoided, retain or lack appropriate stem cells. Neither autografts nor those obtained by so-called 'therapeutic cloning' are options for treating condition with an obvious genetic basis. Moreover, claims that some stem cells are more likely than others to yield successful allografts have yet to be confirmed and explained.

Stem Cells, Regenerative Medicine, and Animal Models of Disease

The field of stem cell biology and regenerative medicine is rapidly moving toward translation to clinical practice, and in doing so has become even more dependent on animal donors and hosts for generating cellular reagents and assaying their potential therapeutic efficacy in models of human disease. Advances in cell culture technologies have revealed a remarkable plasticity of stem cells from embryonic and adult tissues, and transplantation models are now needed to test the ability of these cells to protect at-risk cells and replace cells lost to injury or disease. With such a mandate, issues related to acceptable sources and controversial (e.g., chimeric) models have challenged the field to provide justification of their potential efficacy before the passage of new restrictions that may curb anticipated breakthroughs. Progress from the use of both in vitro and in vivo regenerative medicine models already offers hope both for the facilitation of stem cell phenotyping in recursive gene expression profile models and for the use of stem cells as powerful new therapeutic reagents for cancer, stroke, Parkinson's, and other challenging human diseases that result in movement disorders. This article describes research in support of the following three objectives: (1) To discover the best stem or progenitor cell in vitro protocols for isolating, expanding, and priming these cells to facilitate their massive propagation into just the right type of neuronal precursor cell for protection or replacement protocols for brain injury or disease, including those that affect movement such as Parkinson's disease and stroke; (2) To discover biogenic factors--compounds that affect stem/progenitor cells (e.g., from high-throughput screening and other bioassay approaches)--that will encourage reactive cell genesis, survival, selected differentiation, and restoration of connectivity in central nervous system movement and other disorders; and (3) To establish the best animal models of human disease and injury, using both small and large animals, for testing new regenerative medicine therapeutics.

Guiding embryonic stem cells towards differentiation: lessons from molecular embryology

Embryonic stem cells are uniquely endowed with the capacity of self-renewal and the potential to give rise to all possible cell types, including germ cells. These qualities have made mouse embryonic stem cells a valuable resource for genetic manipulation of the mouse genome. In addition, they present a powerful system for the in vitro dissection of mammalian embryonic development. The recent isolation of human embryonic stem cells has raised a lot of interest for the potential of transposing our knowledge of lineage-specific differentiation of embryonic stem cells to cell-based therapy of human disease. Recent reports have provided insights into the specific differentiation of embryonic stem cells to different cell types of the embryo. However, progress in this direction seems to depend on the knowledge of the mechanisms controlling lineage decisions during embryogenesis.