Tools for automated genome editing of stem cells.

Global Challenges in Stem Cell Research and the Many Roads Ahead

Stem Cell Pioneer Will Direct UM Center for Stem Cell Biology and Regenerative MedicineCurt I. Civin, MD, a pioneer in cancer research who is known for developing a way to isolate stem cells from other blood cells, has joined the University of Maryland School of Medicine. Dr. Civin will become a professor of pediatrics in the Division of Hematology/Oncology, as well as associate dean for research and the founding director of the new University of Maryland School of Medicine Center for Stem Cell Biology and Regenerative Medicine.Dr. Civin comes to the University of Maryland School of Medicine from the Johns Hopkins University School of Medicine, where he has served as a faculty member since 1979. Dr. Civin currently leads projects totaling $21.5 million in extramural research funding. He will bring to the School of Medicine his entire research team, including 15 postdoctoral fellows, graduate students and research technicians.Dean E. Albert Reece, MD, PhD, MBA, who appointed Dr. Civin to his positions, says, “With the recruitment of Dr. Civin and the founding of the new Center for Stem Cell Biology and Regenerative Medicine, the University of Maryland steps into a leadership position in the burgeoning field of stem cell research and regenerative medicine.” Dr. Reece is also Vice President for Medical Affairs, University of Maryland, and John Z. and Akiko K. Bowers Distinguished Professor of the School of Medicine.“Research into stem cells and regenerative medicine will be a key frontier in medicine in the next two decades,” Dean Reece says. “Adding Dr. Civin to our top-tier faculty and establishing the Center that he will direct will give us a tremendously influential position in the field of stem cell science.”Dr. Civin became well known and earned the 1999 National Inventor of the Year Award for his groundbreaking scientific discovery in 1984 of a method for isolating stem cells from other blood cells, a critical step in studying them and for transplanting these cells into patients. Discoveries from his laboratory are used today in both clinical bone marrow stem cell transplantation and leukemia diagnosis. Dr. Civin’s studies now focus on the genes expressed in stem cells. By understanding the inner mechanics of how stem cells work, he hopes to learn how to modify the key properties of stem cells in order to increase their therapeutic potential. In addition, his research includes learning how normal stem cells become cancerous.“Dr. Civin is a nationally renowned physician-scientist who has made significant contributions both in terms of groundbreaking scientific research and the development of new treatments for patients,” says Dean Reece. “He is the perfect fit to lead our new Center for Stem Cell Biology and Regenerative Medicine, where the dual goals are to advance scientific research and, ultimately, to apply those discoveries to patient care.”Dr. Civin envisions creation of a stem cell research initiative that will foster a broad range of interdisciplinary studies designed to understand and to directly affect human health and disease. Developing novel diagnostic methods, treatments and/or prevention for major human diseases can be a key, immediate part of each significant project.His goals for the Center for Stem Cell Biology and Regenerative Medicine and for the field of stem cell biology are twofold. The Center will explore how to manipulate stem cells to allow for much better transplantation and transfusion therapies. Its scientists also will work to understand how stem cells contribute to diseases in order to develop ways to improve conventional treatment and prevention of these disorders.“Our dream for the new Center is to make a significant impact on curing disease,” says Dr. Civin. “That’s really what biomedical research is all about — providing better diagnoses, treatments and preventions.”Partnerships with other researchers within the University of Maryland School of Medicine will be critical to achieving the goals of the scientists at the Center for Stem Cell Biology and Regenerative Medicine. Those scientists will include the School of Medicine researchers who already are studying stem cell biology.The School of Medicine’s stem cell research encompasses more than $2 million in extramural funding annually, including several grants from the Maryland Stem Cell Research Fund.“As I complete 30 years of wonderful experience at Johns Hopkins, I’m confident I will maintain my friendships and working relationships with colleagues there,” says Dr. Civin. “I want this new Center and its work to have a global impact. To that end, I look forward to collaborating with colleagues at Johns Hopkins and at other prestigious Maryland institutions such as the National Cancer Institute and the rest of the National Institutes of Health, as well as other scientists around the globe.”The founding of the University of Maryland Center for Stem Cell Biology and Regenerative Medicine comes at a potential turning point for the science of stem cell research. It is anticipated that President-Elect Barack Obama will repeal federal restrictions on stem cell research and open up NIH funding for the science. “We hope our new center will flourish under these anticipated less restrictive regulations for use of NIH funding in stem cell research,” says Bruce E. Jarrell, MD, executive vice dean at the School of Medicine.Dr. Civin agrees: “Repeal of the current funding restrictions not only will likely free up money for more stem cell projects. It also will allow researchers to spend more time concentrating on the science at hand, rather than concerning themselves with ensuring that projects funded by non-federal sources do not overlap with their currently restricted federal studies. It also will foster a global stem cell research community by removing boundaries between stem cell researchers in the U.S. and in countries where no such restrictions exist.”The quality of the University of Maryland School of Medicine’s faculty was a draw for Dr. Civin as he pondered joining the school. “I was so impressed by the recruitments the School of Medicine has made in recent years, from Dr. Robert Gallo, the co-discoverer of HIV, and his team at the Institute of Human Virology, to Dr. Claire Fraser-Liggett and her team at the University of Maryland Institute for Genome Sciences,” Dr. Civin says. “I was encouraged by how happy these people were, and by the collaborative culture the school maintains for all its faculty members. I’m looking forward to working with my new colleagues.”The Department of Pediatrics faculty, which Dr. Civin joins as a professor, is eagerly anticipating his arrival, according to Steven Czinn, MD, chair of the Department of Pediatrics of the University of Maryland School of Medicine and director of the University of Maryland Hospital for Children at the University of Maryland Medical Center.“Dr. Civin will be a tremendous asset to our department,” says Dr. Czinn. “We’re certain he will energize our research program and help us find new treatments for children who desperately need them.”Dr. Civin also will become part of the University of Maryland Marlene and Stewart Greenebaum Cancer Center. The National Cancer Institute recently honored the Greenebaum Cancer Center with its distinguished designation — as one of only 64 such centers nationally— recognizing the Center’s high quality research and research-related patient care. “Dr. Civin has helped lead the field of stem cell biology, and his discoveries have had a critical impact on approaches to cancer with his contributions to the practice of bone marrow transplantation,” says Kevin J. Cullen, MD, professor of medicine and director of the Greenebaum Cancer Center.“More recently, his work is providing essential insights into the role of stem cells not just in leukemia and hematologic malignancies, but throughout the emerging field of regenerative medicine,” Dr. Cullen says.In his administrative position as associate dean for research, Dr. Civin will assist Dr. Jarrell, executive vice dean, in managing the Office of Research and Graduate Studies. Dr. Civin’s duties will include supporting the School of Medicine investigators and working to enhance the school’s research activities by promoting and managing a widely respected research enterprise.“Dr. Civin will be a tremendous catalyst for creativity across the clinical and basic science departments, and core infrastructure,” says Dr. Jarrell. “He is a proven leader and welcome collaborator who will enhance the University of Maryland’s already outstanding productivity.”

EMBO Reports (2019) e48172 Stem cell‐based regenerative medicine has experienced turbulent times in recent years that have involved several scandals implicating prominent institutions and numerous retractions of papers from leading journals. But it has also seen real progress being made on the way to the clinic. It reflects the tension between human ambition and hope on one side and the need for hard evidence and solid data on the other side in a field where early expectations ran far ahead of reality. This tension may help explain the recent scandal involving stem cell surgeon Paolo Macchiarini, who was found guilty of scientific misconduct regarding an article in The Lancet , which has since been retracted. It is an ongoing saga as Macchiarini is still publishing in leading journals, even though his reputation as a surgeon and stem cell researcher has been compromised. > … the whole field of regenerative medicine had been tainted by a number of cases where […] researchers had taken short cuts and neglected basic science. The story dates back to 2004, when Macchiarini's team extracted some pig intestine and removed the cells to leave a collagen matrix that was then repopulated with muscle and fibroblasts to generate connective epidermal tissue. The patch was implanted in the airway of a 58‐year‐old patient suffering from respiratory problems after surgery for lung cancer [1]. Macchiarini then expanded this technique to whole tracheas using plastic scaffolds in addition to natural tissue from human donors. In another clinical case, the bare plastic scaffold was populated with stem cells taken from the patient's own bone marrow and implanted in a young female patient in 2008. This propelled Macchiarini almost to rock star status and an appointment by the Karolinska Institute, where he refined his technique by replacing donor windpipes with plastic …

An interview with Norio Nakatsuji.

What's in a Name?

5 ISSN 1746-0751 10.2217/RME.09.84 © 2010 Future Medicine Ltd Regen. Med. (2010) 5(1), 5–6 “To further the advancement of responsible stem cell research, the Canadian Stem Cell Foundation ... released the Stem Cell Charter in September 2009 ... The promulgation and promotion of this Charter aims to prospectively frame stem cell research and to serve as a benchmark for the guidance of international and national initiatives in both scientific research and policymaking.” Bartha Maria Knoppers Author for correspondence: Director, Centre of Genomics & Policy, Faculty of Medicine, Department of Human Genetics, McGill University, 740 Dr. Penfield Avenue, Suite 5214, Montreal, Quebec, H3A 1A4 Canada Tel.: +1 514 398 8866; Fax: +1 514 398 8954; bartha.knoppers@mcgill.ca

recent Nobel Prize in medicine was awarded to two stemcell researchers, John Gurdon and Shinya Yamanaka, for theirachievements in stem cell research and reprogramming ofsomatic cells. Flow cytometry is by nature the ideal tool toidentify, characterize, and isolate stem and progenitor cells forresearch and potential clinical use (1). The major strength offlow cytometry is its ability to rapidly perform highlymultiplexed quantitative measurements on single cells withina heterogeneous cell population. However, when the cell typeof interest is extremely rare, as most stem and progenitor cellsare, several sources of artifact must be addressed. The impor-tance of flow cytometry as a driving force for stem cellresearch was demonstrated in a focused issue of the journalexactly three years ago (1). This current focus issue of Cytome-try A is devoted to the topic of stem cells due to numerouscurrent innovations and discoveries.Applications of stem cells include several disciplines,from embryogenesis, adult tissue maintenance, and repair,and more recently, cancer as well as for toxicity screening anddisease modeling. All of these topics are represented in thisissue, with special emphasis on the role of analytic and pre-parative flow cytometry in the elucidation of stem cell pheno-type and function, and best laboratory practices as they applyto flow cytometry. Image and flow cytometry together withcell sorting have revolutionized the study of stem cell biologyand the implications of these cells and their progeny indevelopmental biology, tissue engineering, and cellulartherapy. The number of parameters and the speed of theirsimultaneous measurements in single cells has continued toincrease with advances in hardware, reagents, and analyticalsoftware (2).

Too Much Carrot and Not Enough Stick in New Stem Cell Oversight Trends.

See related article, pages 648–656 The ability to generate induced pluripotent stem (iPS) cells from somatic cells by the overexpression of a limited number of stem cell-related genes has generated great excitement and interest in the biomedical research community including cardiovascular researchers. The pioneering study by Yamanaka and colleagues showing that overexpression of Oct3/4 , Sox2 , Klf4 , and c-Myc could reprogram mouse fibroblasts to a pluripotent state similar to that of embryonic stem (ES) cells opened major new avenues of research.1 This epigenetic reprogramming was rapidly extrapolated to the human system using either the same combination of reprogramming factors or a slightly different combination of transgenes ( OCT4 , NANOG , SOX2 , LIN28 ).2–4 Like embryonic stem (ES) cells, iPS cells can be used for basic developmental biology research and also as a cell source to generate theoretically unlimited quantities of desired cell types such as cardiomyocytes. Such differentiated cells types can be used in a wide range of basic research studies and potentially in clinical applications, which not only include cellular therapies but also drug discovery and safety testing. One appealing aspect of human iPS cells compared to human ES cells is that they can be more readily generated without specialized expertise and access to human embryos, which also avoids the ethical challenges associated with human embryo research. Potentially the most powerful advantage of iPS cells over ES cells is that they can be generated from any patient to produce genetically identical pluripotent cells that can create human disease models or generate patient-specific cells for therapy. Already a number of iPS cell human disease models have been generated,5,6 and proof-of-principle iPS cellular therapies have been pioneered in mouse models.7–9 Despite the speed at which the iPS cell field is racing forward, we …

The discovery that somatic mammalian cells can be epigenetically reprogrammed to induced pluripotent stem cells (iPSCs) through the exogenous expression of the Oct4, Sox2, Klf4 and c-Myc (OSKM) has demonstrated a new way for cell-replacement therapy in regenerative medicine (Li et al., 2013; Nishimura and Takahashi, 2013; Takahashi and Yamanaka, 2013). This novel technology has opened new therapeutic opportunities to generate stem cells in any tissue for cell replacement therapy in a number of disorders (Yamanaka, 2012; Li et al., 2013; Nishimura and Takahashi, 2013; Takahashi and Yamanaka, 2013). Just last week, two papers published in Nature, describing a surprisingly simple method to turn mature cells into embryonic-like stem cells by culturing cells in a low pH medium (Obokata et al., 2014a, 2014b). This method by Obokata and colleagues is truly the simplest, cheapest, and fastest method ever achieved for reprogramming somatic cells into multipotent stem cells. Stem cells have the remarkable potential to develop into many different cell types, essentially without limit to replenish other cells as long as the person or animal is still alive. Thus, stem cell research holds the possible cure for many of the maladies that cripple, blind, and disable a significant portion of our population. Until recently, scientists primarily worked with two kinds of stem cells from animals and humans: non-embryonic “somatic” or “adult” stem cells and embryonic stem cells (ESCs). Scientists have found adult stem cells in many tissues or organs playing roles in maintaining and repairing the tissue in which they are found. Typically, the number of “adult” stem cells in each tissue is very small, and once removed from the body, their capacity to divide is limited, making generation of large quantities of stem cells difficult. Currently, blood stem cells are the only type of adult stem cells that are used regularly for treatment; they have been used since the late 1960s in the procedure now commonly known as bone marrow transplant. ESCs are pluripotent stem cells derived from the inner cell mass of the blastocyst (Figure 1a). The scientists first discovered ways to isolate and culture ESCs from early mouse embryos in 1981, nearly 30 years ago (Evans and Kaufman, 1981). They developed a method in 1998 to derive stem cells from frozen human embryos that are no longer needed for in vitro fertilization (Thomson et al., 1998). ESCs are pluripotent. They are able to differentiate into all cell types of an individual. ESC has potentially unlimited capacity for self-renewal, thus if scientists can reliably direct the differentiation of ESCs into specific cell types, they may be able to use the resulting, differentiated cells to treat certain diseases in the future. Although ESCs are promising donor sources in cell transplantation therapies, they face an ethical issue regarding the destruction of human embryos.Figure 1: Technologies to generate pluripotent stem cells.To circumvent this limitation, an existing laboratory technique was revived for creating ablastula with the transfer of a donor nucleus to a denucleated egg (Gurdon, 1962; McGrath and Solter, 1983), laterally called somatic cell nuclear transfer (SCNT; Figure 1b). This technique is the basis for cloning animals (such as the famous Dolly the sheep) (Campbell et al., 1996) and in theory could be used to clone humans. One concern is that blastula creation in SCNT-based human stem cell research will lead to the reproductive cloning of humans. A second important concern is the need of appropriate source of eggs that are needed. Thus, the impetus for SCNT-based stem cell research has been decreased by the development and improvement of alternative methods of generating stem cells. Dr. Shinya Yamanaka, Nobel prize laureate, announced in June 2006 that he made a breakthrough by identifying conditions that would allow some specialized adult cells to be “reprogrammed” genetically to assume a stem cell-like state, called (iPSC; Figure 1c). They initially reprogrammed mouse skin cells into iPSCs by inserting just four functioning genes (OSKM) into the cells (Takahashi and Yamanaka, 2006). The development of iPSCs from individual skin cells has opened up a new world of research. This embryo-free technique has been proven to be a powerful way to generate cell lines from a patient's own tissues (Takahashi et al., 2007; Yamanaka, 2012). Furthermore, the iPSCs have been directed to make cardiomyocytes, several kinds of neurons, liver cells, hematopoietic stem cells, and so on, for possible cell replacement therapy (Robinton and Daley, 2012). Cell reprogramming technology provides a novel approach to derive iPSCs directly from a patient's somatic cells without embryo involvement. Thus, this novel approach overcomes ethical concerns. Although cell reprogramming is very attractive because of its potential for future cell replacement therapy, several potential challenges need to be overcome before any possible applications can be made. The retroviral system is still one of the most effective approaches by far to mediate the expression of OSKM for producing iPSCs from somatic cells. Unfortunately, most experiments with retrovirus involve integration into the host cell genome with an identified risk for insertional mutagenesis and oncogenic transformation (Sanes et al., 1986). To circumvent such risks, which are deemed incompatible with therapeutic prospects, significant progress has been made with no chromosome integration method or even virus-free reprogramming methods. Life technologies Corporation (USA) has developed a CytoTune® reprogramming vector based on Sendai virus. Unlike other vectors, this viral vector does not integrate into the host genome or alter the genetic information of the host cell (Fusaki et al., 2009; Seki et al., 2010; Ban et al., 2011). Virus-free methods such as direct mRNA, microRNA, or protein delivery have been developed to achieve conversion of adult cells into iPSCs. The novel approach developed by Obokata et al is surprisingly simple. When a dozen of cell types, including those from brain, skin, lung, and liver were exposed to stress, including low pH, about 20% of the cells that survived from stress reprogrammed to multipotent stem cells without introduction of any exogenous genes. Obokata called the phenomena stimulus-triggered acquisition of pluripotency (STAP; Figure 1d). This is an amazing technique that may allow creating cells with pluripotency from patients without destruction of an embryo or introduction of exogenous genes. If successful, it would open a new era in stem cell biology and research in tumorigenesis. However, scientists need to replicate this exciting result and fully understand the mechanism underlying STAP cells before their full potential is realized and applied in medicine.

A Great Match

Human pluripotent stem cells comprise induced pluripotent and embryonic stem cells, which have tremendous potential for biological and therapeutic applications. The development of efficient technologies for the targeted genome alteration of stem cells in disease models is a prerequisite for utilizing stem cells to their full potential. Genome editing of stem cells is possible with the help of synthetic nucleases that facilitate site-specific modification of a gene of interest. Recent advances in genome editing techniques have improved the efficiency and speed of the development of stem cells for human disease models. Zinc finger nucleases, transcription activator-like effector nucleases, and clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated system are powerful tools for editing DNA at specific loci. Here, we discuss recent technological advances in genome editing with site-specific nucleases in human stem cells.

To the Editor: The molecular mechanisms and the control of smooth muscle cell (SMC) differentiation have been extensively investigated because of its therapeutic potential.1 To date, different cell types have been used to study SMC differentiation, including a variety of mouse embryonic stem cells,2 adult stem cells,3,4 and others.5 Because several fundamental differences exist between mouse and human embryonic development,6 lack of a good model system to study human SMC differentiation has hampered the progress of translating SMC knowledge to novel clinical therapies. Human embryonic stem (hES) cells provide a valuable source of cells for studying human cell differentiation and developing therapeutic potentials in regenerative medicine. Since the initial report describing the derivation of hES cells,7 a variety of studies have established in vitro differentiation strategies to several lineages. Recently, it has been demonstrated that vascular progenitors derived from hES cells could be differentiated into endothelial cells and SMCs by endothelial …

After the biotech medicine era, symbolized by the progress of genetic engineering and the developments of physiologically active substances such as proteins and cytokines as medicine, the regenerative medicine era, when cells and tissues become medicine, is going to start and getting popular in recent years in the world. In particular, regenerative medicine is highly expected as curative-therapeutic treatments that can cure patients with obstinate diseases and physically impaired function, while the conventional symptomatic treatments are unable to do so. For supporting and enhancing the progress of regenerative medicine, remarkable advancements in regenerative medical sciences, and cell and tissue engineering are desired worldwide. Furthermore, to achieve new treatments based on regenerative medicine, multidisciplinary approaches from various fields such as molecular biology, cell biology, science and engineering, and pharmaceutical and medical sciences are essential. Regenerative medicine should be recognized as a completely new interdisciplinary academic discipline, which is unable to be obtained from the extrapolations of vertically connected and conventional academic disciplines. Therefore, for realizing regenerative medicine, not only the reconstruction of conventional medical science but also the establishment of new academic field, which integrates and assimilates scientific and engineering technologies, and biotechnology, become important. For example, technology controlling the expansion and differentiations of embryonic stem (ES) cells and induced pluripotent stem (iPS) cells is essential for obtaining the sufficient numbers of the cells allowing desired medical treatments to be realized. For separating remaining small amounts of undifferentiated ES and iPS cells from the differentiated cells with a high accuracy, an interdisciplinary research project having the horizontally integration of science, engineering, medicine should be considered. Further, the developments of technologies, which can efficiently transplant somatic cells in the body, are more important than those of cell sources, and new cooperative and integrative systems having various academic disciplines including medicine, science, engineering, and pharmacy are extremely important. For example, harvested cells, which are obtained from culture dishes by treating them with enzymes including trypsin or dispase after being cultured and expanded, are damaged in their structures and functions by enzymatically cleaving their cell-membrane proteins. Upon the direct injection of suspension containing damaged cells following the enzyme treatments to the tissue or organ, approximately 95% of the cells fail to stay in the target, and only less than several percent of the cells transplanted in the organ is speculated to be effective (Hofmann et al., 2005). The unwelcome fact that possible therapeutic effects are expected with a small number of transplanted cells should be noticed seriously. Tissue engineering is extremely important in terms of the more efficient engraftment of cell transplantation and is expected to proceed further with the integration of the technology of biomaterials contacting cell directly and bio- and medical-technologies. One of the tissue engineering approaches is biodegradable polymer scaffold-based methods (Langer and Vacanti, 1993). It is important to promote research that investigates how the shape and function of cells cultured in the scaffolds are maintained after transplantation. In this special issue, the recent progress of tissue engineering approaches using biodegradable polymer scaffolds are reviewed by Rui Reis and his colleagues on periodontal tissue, Charles Vacanti and Koji Kojima on trachea, Toshiharu Shinoka and his colleagues on vasculature, Dietmar Hutmacher and his colleagues on osteochondral tissue, and Stephan Badylak and his colleagues on skeletal muscle. For maintaining cell-dense thicker tissue viability, sufficient amounts of oxygen and nutrients should be supplied into the tissue. However, scaffold-based tissue has a heavy burden to have supplies of oxygen and nutrients from the scaffold surface by diffusion, which is hampered with increasing cell density. The developments of new technologies to break through this problem would be desired. Another approach for tissue engineering is cell sheet-based methods (Yang et al., 2007). Dense monolayered cell sheets that are harvested from temperature responsive culture dishes are fully viable and almost all of the cells are engrafted and keep their function after transplantation. In this special issue, the cutting edge research using cell sheet-based tissue engineering methods are reviewed by Teruo Okano and Kazuo Ohashi on liver and islets, Tatsuya Shimizu and Katsuhisa Matsuura on cardiac tissue, Masayuki Yamato and his colleagues on periodontal tissue, and Masato Sato and his colleagues on articular cartilage. In the cornea, myocardium, periodontal ligament, esophagus, and articular cartilage, the clinical studies with this cell-sheet technology have been already initiated, and the clinical efficacies of these cell-sheet transplantations have been confirmed. Remarkable research projects investigating the preparation of three-dimensional (3-D) cell-sheet tissues by layering cell sheets have been started. Especially challenging is to make capillaries in 3-D layered cell-sheet tissue and allow them to connect with living blood vessels (Sekine et al., 2013) or the micro-channels of artificial vascular beds (Sakaguchi et al., 2013). From these successful studies, the preparations of various organs such as liver, kidney, pancreas, and so forth are expected to be accelerated. Various stem cell types are currently under investigation. Among them, the application of tissue stem cells, also called somatic stem cells, is being developed with remarkable progress (Korbling and Estrov, 2003). Some tissue stem cells have proceeded to clinical trials and even further to general medical treatment. Hematopoietic stem cells (HSCs) are representative of such cells (Weissman and Shizuru, 2008). HSCs have been successfully administered to patients with leukemia for over 50 years (Thomas, 1983). In this special issue, Atsushi Iwama and his colleagues, Yaeko Nakajima-Takagi and Mitsujiro Osawa, review HSCs, including the methods used for identifying and expanding HSCs ex vivo, and the niche cells that control the self-renewal, differentiation, and homing of HSCs. One special type of tissue stem cell has attracted great attention. Mesenchymal stem cells have been intensively studied for more than a decade, with new knowledge accumulating from both basic and applied studies (Kuroda et al., 2011). Importantly, more than 450 clinical studies are currently being performed throughout the world, targeting various diseases, and there are several venture capital companies focusing on mesenchymal stem cells. Mesenchymal stem cells are indeed a hot topic and a major field in basic science, clinical medicine, and medical industry. In this special issue, Yasumasa Kuroda and Mari Dezawa provide an overview of mesenchymal stem cell studies, focusing on three representative sources, bone marrow, adipose tissue, and umbilical cord, and describe the similarities and differences among them. They also describe recent advances in clinical studies and discuss perspectives of mesenchymal stem cell use. Finally, they introduce multilineage-differentiating stress enduring (Muse) cells, which are recently discovered pluripotent stem cells that make up a large percentage of mesenchymal stem cells. Muse cells have attracted attention not only because they explain the triploblastic differentiation ability of mesenchymal stem cells, but also because they are pluripotent but non-tumorigenic. Their review discusses the great potential of Muse cells for regenerative medicine. Neural regeneration is another hot topic in regenerative medicine (Gage, 2000). While there are some candidate stem cells that relate to neural regeneration, in this special issue James St. Johns and Jenny Ekberg review olfactory ensheathing cells. Established central nervous system tissue does not possess regenerative capacity; therefore, the only way to restore damaged brain regions and spinal cord is to supply new neural cells and reconstruct neuronal circuits. Neural stem cells have attracted great attention for quite some time, but the number of available cells from fetal or adult brain is limited. Due to their regenerative capacity and feasibility of application, olfactory ensheathing cells are considered one of the strongest candidates for the treatment of spinal cord injury, and in fact, they have been already applied to patients in some countries. For diseases of complex tissues, such as the kidney and eye, the search for tissue stem cells has not been as simple and straightforward. Motoko Yanagida and her colleague, Koji Takaori, are pioneers in the area of kidney regeneration and stem cells. Here in this special issue, they review kidney stem cells. The retina is an organ with highly sophisticated function, such as information processing ability comparable to that of the brain. In mammals, retinal stem/progenitor cells (RPCs) are suggested to be possible retinal stem cells that are quiescent and exhibit very little activity. A number of different cellular sources of RPCs have been identified in the vertebrate retina. These include RPCs at the retinal margin; pigmented cells in the ciliary body, iris, and retinal pigment epithelium; and Müller cells within the retina. Henry Yip describes the isolation and expansion of RPCs from immature and mature eyes, and their potential application to transplantation in degenerated retinal tissue. James Trosko, the pioneer of tissue stem cells and cancer stem cells, poses questions about the reprogramming hypothesis of iPS cells. His review proposes novel insights into the relation between iPS cells and cancer stem cells. A survey of the stem cell world reveals that tissue stem cells are diverse and complex, just as our bodies are composed of complex sophisticated systems. Our bodies are able to maintain tissue homeostasis perhaps because of the evolutionary development of tissue stem cells to acquire a variety of functions. We hope that this special issue will stimulate development of additional tissue engineering and tissue stem cell studies.

The ISSCR: Who Are We and Where Are We Going?