潜在的成体肝干/祖细胞

研究者在50年前提出“成体肝组织内存在肝干细胞”这一假说.肝干/祖细胞研究对干细胞基础研究及其临床治疗研究都有着重要的意义.研究者也已从损伤的人类和啮齿动物的肝组织中分离到激活的肝干/祖细胞,并建立了相应的培养体系.目前的研究表明,肝内存在多个具有干细胞特性的细胞,且它们位于肝内不同的区域.另外,潜在的肝干/祖细胞的分子表达谱也已得到一定的阐述.本文结合肝干/祖细胞的研究,对成体肝干/祖细胞的潜在类型、来源与定位作一回顾.     

Feeder-free Derivation of Neural Crest Progenitor Cells from Human Pluripotent Stem Cells

Human pluripotent stem cells (hPSCs) have great potential for studying human embryonic development, for modeling human diseases in the dish and as a source of transplantable cells for regenerative applications after disease or accidents. Neural crest (NC) cells are the precursors for a large variety of adult somatic cells, such as cells from the peripheral nervous system and glia, melanocytes and mesenchymal cells. They are a valuable source of cells to study aspects of human embryonic development, including cell fate specification and migration. Further differentiation of NC progenitor cells into terminally differentiated cell types offers the possibility to model human diseases in vitro, investigate disease mechanisms and generate cells for regenerative medicine. This article presents the adaptation of a currently available in vitro differentiation protocol for the derivation of NC cells from hPSCs. This new protocol requires 18 days of differentiation, is feeder-free, easily scalable and highly reproducible among human embryonic stem cell (hESC) lines as well as human induced pluripotent stem cell (hiPSC) lines. Both old and new protocols yield NC cells of equal identity.     

Mesenchymal to epithelial transition during tissue homeostasis and regeneration: Patching up the Drosophila midgut epithelium

Stem cells are responsible for preserving morphology and function of adult tissues. Stem cells divide to self-renew and to generate progenitor cells to sustain cell demand from the tissue throughout the organism''s life. Unlike stem cells, the progenitor cells have limited proliferation potential but have the capacity to terminally differentiate and thereby to substitute older or damaged mature cells. Recent findings indicate that adult stem cells can adapt their division kinetics dynamically to match changes in tissue demand during homeostasis and regeneration. However, cell turnover not only requires stem cell division but also needs timed differentiation of the progenitor cells, which has been much less explored. In this Extra View article, we discuss the ability of progenitor cells to actively postpone terminal differentiation in the absence of a local demand and how tissue demand activates terminal differentiation via a conserved mesenchymal-epithelial transition program revealed in our recent EMBO J paper and other published and unpublished data. The extent of the significance of these results is discussed for models of tissue dynamics during both homeostasis and regeneration.     

Mesenchymal to epithelial transition during tissue homeostasis and regeneration: Patching up the Drosophila midgut epithelium

Stem cells are responsible for preserving morphology and function of adult tissues. Stem cells divide to self-renew and to generate progenitor cells to sustain cell demand from the tissue throughout the organism's life. Unlike stem cells, the progenitor cells have limited proliferation potential but have the capacity to terminally differentiate and thereby to substitute older or damaged mature cells. Recent findings indicate that adult stem cells can adapt their division kinetics dynamically to match changes in tissue demand during homeostasis and regeneration. However, cell turnover not only requires stem cell division but also needs timed differentiation of the progenitor cells, which has been much less explored. In this Extra View article, we discuss the ability of progenitor cells to actively postpone terminal differentiation in the absence of a local demand and how tissue demand activates terminal differentiation via a conserved mesenchymal-epithelial transition program revealed in our recent EMBO J paper and other published and unpublished data. The extent of the significance of these results is discussed for models of tissue dynamics during both homeostasis and regeneration.     

Autophagy in hematopoietic stem/progenitor cells exposed to heavy metals: Biological implications and toxicological relevance

The inherent toxicity of many metal compounds, together with their widespread environmental distribution, raises concerns of potential health hazards. Little is known about the impact of these important environmental toxicants on adult stem/progenitor cells, necessary for tissue homeostasis and repair. We recently reported that autophagy is implicated in the response of hematopoietic stem/progenitor cells to toxic concentrations of hexavalent chromium (Cr[VI]) and cadmium (Cd), two well known carcinogenic heavy metal cations. Autophagy may lead to cell death if carried out too extensively, but also acts as a survival pathway in cells under stress. In stem/progenitor cells, an autophagic phenotype could mitigate metal-induced toxicity, contributing to the conservation of tissue renewal capability. Given the key role of toxic damage to adult stem/progenitor cells in cancer, it is necessary to investigate whether autophagic responses modulate the carcinogenic potential of exposure to heavy metals during stem/progenitor cell differentiation.     

Adult-derived stem cells and their potential for use in tissue repair and molecular medicine

This report reviews three categories of precursor cells present within adults. The first category of precursor cell, the epiblast-like stem cell, has the potential of forming cells from all three embryonic germ layer lineages, e.g., ectoderm, mesoderm, and endoderm. The second category of precursor cell, the germ layer lineage stem cell, consists of three separate cells. Each of the three cells is committed to form cells limited to a specific embryonic germ layer lineage. Thus the second category consists of germ layer lineage ectodermal stem cells, germ layer lineage mesodermal stem cells, and germ layer lineage endodermal stem cells. The third category of precursor cells, progenitor cells, contains a multitude of cells. These cells are committed to form specific cell and tissue types and are the immediate precursors to the differentiated cells and tissues of the adult. The three categories of precursor cells can be readily isolated from adult tissues. They can be distinguished from each other based on their size, growth in cell culture, expressed genes, cell surface markers, and potential for differentiation. This report also discusses new findings. These findings include the karyotypic analysis of germ layer lineage stem cells; the appearance of dopaminergic neurons after implantation of naive adult pluripotent stem cells into a 6-hydroxydopamine-lesioned Parkinson's model; and the use of adult stem cells as transport mechanisms for exogenous genetic material. We conclude by discussing the potential roles of adult-derived precursor cells as building blocks for tissue repair and as delivery vehicles for molecular medicine.     

Stem-cell-based approaches for regenerative medicine

Recent success in transplantation of islets raises the hopes of diabetic patients that replacement therapies may be a feasible treatment of their disease. Although several lines of evidence suggest that stem cells exist in the pancreas, it is still technically hard for us to isolate or maintain the stem cells in vitro. The establishment of human embryonic stem (ES) cells has excited scientists regarding their potential medical use in tissue replacement therapy. When applied with appropriate signals, ES cells can be directed to differentiate into a specific cell lineage. Therefore, ES cells are no doubt an excellent source not only for regenerative medicine but also for studies of early events of pancreatic development, and to portray the pancreatic progenitor cells. Despite many attempts that have been tried, the efficiency of differentiation of ES cells into islets is still very low. This low efficiency reflects our lack of understanding of the intrinsic and extrinsic signals which regulate the developmental processes of the pancreas. In this review, I present a summary of recent works on ES cells, the identification of pancreatic progenitor cells from the adult pancreas, and refer to the possibilities of transdifferentiation from adult stem cells derived from other tissues.     

Skin-derived human adult stem cells surprisingly share many features with human pancreatic stem cells

Multiple tissue niches in the human body are now recognised to harbour stem cells. Here, we have asked how different adult stem cell populations, isolated from two ontogenetically distinct human organs (skin, pancreas), actually are with respect to a panel of standard markers/characteristics. Here we show that an easily accessible adult human tissue such as skin may serve as a convenient source of adult stem cell-like populations that share markers with stem cells derived from an internal, exocrine organ. Surprisingly, both, human pancreas- and skin-derived stem/progenitor cells demonstrate differentiation patterns across lineage boundaries into cell types of ectoderm (e.g. PGP 9.5+ and GFAP+), mesoderm (e.g. alpha-SMA+) and entoderm (e.g. amylase+ and albumin+). This intriguing differentiation capability warrants systemic follow-up, since it raises the theoretical possibility that an adult human skin-derived progenitor cell population could be envisioned for possible application in cell replacement therapies.     

Progenitor cells as remote "bioreactors": neuroprotection via modulation of the systemic inflammatory response

Acute central nervous system(CNS)injuries such as spinal cord injury,traumatic brain injury,autoimmune encephalomyelitis,and ischemic stroke are associ- ated with significant morbidity,mortality,and health care costs worldwide.Preliminary research has shown potential neuroprotection associated with adult tissue derived stem/progenitor cell based therapies.While initial research indicated that engraftment and transdif- ferentiation into neural cells could explain the observed benefit,the exact mechanism remains controversial.A second hypothesis details localized stem/progenitor cell engraftment with alteration of the loco-regional milieu;however,the limited rate of cell engraftment makes this theory less likely.There is a growing amount of pre-clinical data supporting the idea that,after intravenous injection,stem/progenitor cells interact with immuno- logic cells located in organ systems distant to the CNS,thereby altering the systemic immunologic/inflammatory response.Such distant cell"bioreactors"could modulate the observed post-injury pro-inflammatory environment and lead to neuroprotection.In this review,we discuss the current literature detailing the above mechanisms of action for adult stem/progenitor cell based therapies in the CNS.     

Isolation and therapeutic potential of human haemopoietic stem cells

The haemopoietic stem cell (HSC) has long been regarded as an archetypal, tissue specific, stem cell, capable of completely regenerating haemopoiesis after myeloablation. It has proved relatively easy to harvest HSC, from bone marrow or peripheral blood. In turn, isolation of these cells has allowed therapeutic stem cell transplantation protocols to be developed, that capitalise on their prodigious self renewal and proliferative capabilities. Ex vivo approaches have been described to isolate, genetically manipulateand expand pluripotent stem cell subsets. These techniques have been crucial to the development of gene therapy, and may allow adults to enjoy the potential advantages of cord blood transplantation. Recently, huge conceptual changes have occurred in stem cell biology. In particular, the dogma that, in adults, stem cells are exclusively tissue restricted has been questioned and there is great excitement surrounding the potential plasticity of these cells, with the profound implications that this has, for developing novel cellular therapies. Mesenchymal stem cells, multipotent adult progenitor cells and embryonic stem cells are potential sources of cells for transplantation purposes. These cells may be directed toproduce HSC, in vitro and in the future may be used for therapeutic, or drug development, purposes. This revised version was published online in July 2006 with corrections to the Cover Date.     

Nestin-positive progenitor cells derived from adult human pancreatic islets of Langerhans contain side population (SP) cells defined by expression of the ABCG2 (BCRP1) ATP-binding cassette transporter

The disease diabetes mellitus arises as a consequence of a failure of the beta-cells in the islets of Langerhans of the pancreas to produce insulin in the amounts required to meet the needs of the body. Whole pancreas or islet transplants in patients with severe diabetes effectively restore insulin production. A lack of availability of donor pancreata requires the development of alternative sources of islets such as the ex vivo culture and differentiation of stem/progenitor cells. Earlier we discovered multipotential progenitor cells in islets isolated from adult human pancreata that express the neural stem cell marker nestin: nestin-positive islet-derived progenitor cells (NIPs). Recently it was shown that the exclusion of the Hoechst 33342 dye, which defines the pluripotential side population (SP) of hematopoietic stem cells, is mediated by the ATP-binding cassette transporter, ABCG2. Here we report that the human islet-derived NIPs contain a substantial subpopulation of SP cells that co-express ABCG2, MDR1, and nestin. Thus NIPs may be a potential source of adult pluripotential stem/progenitor cells useful for the production of islet tissue for transplantation into diabetic subjects.     

Adult reserve stem cells and their potential for tissue engineering

Tissue restoration is the process whereby multiple damaged cell types are replaced to restore the histoarchitecture and function to the tissue. Several theories, have been proposed to explain the phenomenon of tissue restoration in amphibians and in animals belonging to higher order. These theories include dedifferentiation of damaged tissues, transdifferentiation of lineage-committed progenitor cells, and activation of reserve, precursor cells. Studies by Young et al. and others demonstrated that connective tissue compartments throughout postnatal individuals contain reserve precursor cells. Subsequent repetitive single cell-cloning and cell-sorting studies revealed that these reserve precursor cells consisted of multiple populations of cells, including, tissue-specific progenitor cells, germ-layer lineage stem cells, and pluripotent stem cells. Tissue-specific progenitor cells display various capacities for differentiation, ranging from unipotency (forming a single cell type) to multipotency (forming multiple cell types). However, all progenitor cells demonstrate a finite life span of 50 to 70 population doublings before programmed cell senescence and cell death occurs. Germ-layer lineage stem cells can form a wider range of cell types than a progenitor cell. An individual germ-layer lineage stem cell can form all cells types within its respective germ-layer lineage (i.e., ectoderm, mesoderm, or endoderm). Pluripotent stem cells can form a wider range of cell types than a single germ-layer lineage stem cell. A single pluripotent stem cell can form cells belonging to all three germ layer lineages. Both germ-layer lineage stem cells and pluripotent stem cells exhibit extended capabilities for self-renewal, far surpassing the limited life span of progenitor cells (50–70 population doublings). The authors propose that the activation of quiescent tissue-specific progenitor cells, germ-layer lineage stem cells, and/or pluripotent stem cells may be a potential explanation, along with dedifferentiation and transdifferentiation, for the process of tissue restoration. Several model systems are currently being investigated to determine the possibilities of using these adult quiescent reserve precursor cells for tissue engineering.     

Progenitor cell therapy for heart disease

Many cell types are currently being studied as potential sources of cardiomyocytes for cell transplantation therapy to repair and regenerate damaged myocardium. The question remains as to which progenitor cell represents the best candidate. Bone marrow-derived cells and endothelial progenitor cells have been tested in clinical studies. These cells are safe, but their cardiogenic potential is controversial. The functional benefits observed are probably due to enhanced angiogenesis, reduced ventricular remodeling, or to cytokine-mediated effects that promote the survival of endogenous cells. Human embryonic stem cells represent an unlimited source of cardiomyocytes due to their great differentiation potential, but each step of differentiation must be tightly controlled due to the high risk of teratoma formation. These cells, however, confront ethical barriers and there is a risk of graft rejection. These last two problems can be avoided by using induced pluripotent stem cells (iPS), which can be autologously derived, but the high risk of teratoma formation remains. Cardiac progenitor cells have the advantage of being cardiac committed, but important questions remain unanswered, such as what is the best marker to identify and isolate these cells? To date the different markers used to identify adult cardiac progenitor cells also recognize progenitor cells that are outside the heart. Thus, it cannot be determined whether the cardiac progenitor cells identified in the adult heart represent resident cells present since fetal life or extracardiac cells that colonized the heart after cardiac injury. Developmental studies have identified markers of multipotent progenitors, but it is unknown whether these markers are specific for adult progenitors when expressed in the adult myocardium. Cardiac regeneration is dependent on the stability of the cells transplanted into the host myocardium and on the electromechanical coupling with the endogenous cells. Finally, the promotion of endogenous regenerative processes by mobilizing endogenous progenitors represents a complementary approach to cell transplantation therapy.     

Myocardial regeneration with bone-marrow-derived stem cells

Despite significant therapeutic advances, heart failure remains the predominant cause of mortality in the Western world. Ischaemic cardiomyopathy and myocardial infarction are typified by the irreversible loss of cardiac muscle (cardiomyocytes) and vasculature composed of endothelial cells and smooth muscle cells, which are essential for maintaining cardiac integrity and function. The recent identification of adult and embryonic stem cells has triggered attempts to directly repopulate these tissues by stem cell transplantation as a novel therapeutic option. Reports describing provocative and hopeful examples of myocardial regeneration with adult bone-marrow-derived stem and progenitor cells have increased the enthusiasm for the use of these cells, yet many questions remain regarding their therapeutic potential and the mechanisms responsible for the observed therapeutic effects. In this review article we discuss the current preclinical and clinical advances in bone-marrow-derived stem or progenitor cell therapies for regeneration or repair of the ischaemic myocardium and their multiple related mechanisms involved in myocardial repair and regeneration.     

Informing tendon tissue engineering with embryonic development

Tendon is a strong connective tissue that transduces muscle-generated forces into skeletal motion. In fulfilling this role, tendons are subjected to repeated mechanical loading and high stress, which may result in injury. Tissue engineering with stem cells offers the potential to replace injured/damaged tissue with healthy, new living tissue. Critical to tendon tissue engineering is the induction and guidance of stem cells towards the tendon phenotype. Typical strategies have relied on adult tissue homeostatic and healing factors to influence stem cell differentiation, but have yet to achieve tissue regeneration. A novel paradigm is to use embryonic developmental factors as cues to promote tendon regeneration. Embryonic tendon progenitor cell differentiation in vivo is regulated by a combination of mechanical and chemical factors. We propose that these cues will guide stem cells to recapitulate critical aspects of tenogenesis and effectively direct the cells to differentiate and regenerate new tendon. Here, we review recent efforts to identify mechanical and chemical factors of embryonic tendon development to guide stem/progenitor cell differentiation toward new tendon formation, and discuss the role this work may have in the future of tendon tissue engineering.     

Cell differentiation lineage in the prostate

Prostatic epithelium consists mainly of luminal and basal cells, which are presumed to differentiate from common progenitor/stem cells. We hypothesize that progenitor/stem cells are highly concentrated in the embryonic urogenital sinus epithelium from which prostatic epithelial buds develop. We further hypothesize that these epithelial progenitor/stem cells are also present within the basal compartment of adult prostatic epithelium and that the spectrum of differentiation markers of embryonic and adult progenitor/stem cells will be similar. The present study demonstrates that the majority of cells in embryonic urogenital sinus epithelium and developing prostatic epithelium (rat, mouse, and human) co-expressed luminal cytokeratins 8 and 18 (CK8, CK18), the basal cell cytokeratins (CK14, CK5), p63, and the so-called transitional or intermediate cell markers, cytokeratin 19 (CK19) and glutathione-S-transferase-pi (GSTpi). The majority of luminal cells in adult rodent and human prostates only expressed luminal markers (CK8, CK18), while the basal epithelial cell compartment contained several distinct subpopulations. In the adult prostate, the predominant basal epithelial subpopulation expressed the classical basal cell markers (CK5, CK14, p63) as well as CK19 and GSTpi. However, a small fraction of adult prostatic basal epithelial cells co-expressed the full spectrum of basal and luminal epithelial cell markers (CK5, CK14, CK8, CK18, CK19, p63, GSTpi). This adult prostatic basal epithelial cell subpopulation, thus, exhibited a cell differentiation marker profile similar to that expressed in embryonic urogenital sinus epithelium. These rare adult prostatic basal epithelial cells are proposed to be the progenitor/stem cell population. Thus, we propose that at all stages (embryonic to adult) prostatic epithelial progenitor/stem cells maintain a differentiation marker profile similar to that of the original embryonic progenitor of the prostate, namely urogenital sinus epithelium. Adult progenitor/stem cells co-express both luminal cell, basal cell, and intermediate cell markers. These progenitor/stem cells differentiate into mature luminal cells by maintaining CK8 and CK18, and losing all other makers. Progenitor/stem cells also give rise to mature basal cells by maintaining CK5, CK14, p63, CK19, and GSTpi and losing K8 and K18. Thus, adult prostate basal and luminal cells are proposed to be derived from a common pleuripotent progenitor/stem cell in the basal compartment that maintains its embryonic profile of differentiation markers from embryonic to adult stages.     

Cancer stem cells – the current status of an old concept: literature review and clinical approaches

As regards their morphology and biology, tumours consist of heterogeneous cell populations. The cancer stem cell (CSC) hypothesis assumes that a tumour is hierarchically organized and not all of the cells are equally capable of generating descendants, similarly to normal tissue. The only cells being able to self-renew and produce a heterogeneous tumour cell population are cancer stem cells. CSCs probably derive from normal stem cells, although progenitor cells may be taken into consideration as the source of cancer stem cells. CSCs reside in the niche defined as the microenvironment formed by stromal cells, vasculature and extracellular matrix. The CSC assays include FACS sorting, xenotransplantation to immunodeficient mice (SCID), incubation with Hoechst 33342 dye, cell culture in non-adherent conditions, cell culture with bromodeoxyuridine. CSCs have certain properties that make them resistant to anticancer therapy, which suggests they may be the target for potential therapeutic strategies.     

Common origins of blood and blood vessels in adults?

After embryonic development, the vast majority of cells are differentiated and all organs are in place. Growth of the organism then ensues and continues until adulthood, whereupon cell division largely ceases. In some tissues, notably the bone marrow, skin, and gut, cell proliferation continues throughout life to replace cells lost by attrition. This regeneration is fueled by rare, long-lived, and largely quiescent stem cells that give rise to committed progenitors, which in turn generate large numbers of fully differentiated cells. Mounting evidence suggests that such cells can significantly contribute to tissue repair and regeneration in adults and may therefore prove beneficial for autologous cell and gene therapies. This review focuses on the potential of adult stem cells to give rise to hematopoietic and vascular cells. We discuss evidence that a highly purified population of adult stem cells, termed SP cells, serves as a hematopoietic progenitor and can contribute to vascular regeneration after injury. We also discuss the potential relationship of these cells to the embryonic hemangioblast.