On Arthritis and Musculoskeletal and Skin Diseases Generation of replacement cells and tissue to treat diseases: Because stem cells constitute a self-renewing population of cells, they can be cultured to generate greater numbers of bone or cartilage cells than could be obtained from a tissue sample. Equally important, if a self-renewing population of new stem cells can be established in a transplant recipient, it could effect long-term correction of many diseases and degenerative conditions in which bone or cartilage cells are deficient in numbers or defective in function. This could be done either by transplanting the stem cells from a healthy donor to a recipient, or by genetically modifying a person's own stem cells and returning them to the marrow. Such an approach holds great promise for genetic disorders of bone and cartilage, such as osteogenesis imperfecta and the various chondrodysplasias. In a somewhat different application, stem cells could be stimulated in culture to develop into either bone or cartilage-producing cells. These cells could then be introduced into the damaged areas of joint cartilage in cases of osteoarthritis, or into large gaps in bone that can arise from fractures or surgery. This sort of repair would have a number of advantages over the current practice of tissue grafting. Improve understanding of normal and abnormal development: The ability to isolate and manipulate stem cells in culture will provide experimental access to the processes that regulate the differentiation of bone and cartilage cells. This will enable investigators to identify the molecules that control the proliferation of stem cells, or induce or inhibit stem cells' progression to mature functional cell types. In turn, testing for the presence and activity of these regulatory molecules in healthy and diseased tissues will indicate conditions in which defects of stem cell regulation or differentiation underlie pathology. Improve development and testing of drugs: A detailed understanding of stem cell regulation and the molecules that affect it would provide new targets for pharmacological interventions. Stem cell culture systems would also make possible rapid and economical testing of candidate agents for both effectiveness and safety.
Mental Health Research There is good evidence that many of the mental and behavioral disorders such as schizophrenia, autism, manic-depressive illness and memory disorders, result from permanent disruption of brain circuitry or brain chemistry. The National Institute of Mental Health (NIMH) is currently supporting many research grants to determine exactly where and how such brain disruptions may occur, and is encouraging a vigorous program of research, including stem cell research with animals, to develop disease models and understand basic neuronal processes. For example, scientists are using non-human primate models to explore the hypothesis that schizophrenia is the result of damage to brain cells that mature at a particular stage of human development. If this hypothesis is supported, pluripotent stem cell research might provide a window into what goes wrong in the brain in schizophrenia, as well as potential interventions to remediate this devastating illness. Similar hope may apply to the severe developmental disorders, such as autism. In another arena, investigators are examining stress- and toxin-induced loss of cells in a brain structure important for memory, the hippocampus. Animal studies have shown that natural replacement of cells in the hippocampus, via stem cells, results in improved memory performance as long as another important structure for memory, the cerebral cortex, is largely intact. Human pluripotent stem cells might ultimately be important to the development of replacement cells in the hippocampus of humans suffering from memory loss caused by selective damage to the hippocampus.
On Eyes Treatment of Retinal Degenerations: Some promising results have been obtained transplanting retinal cells and tissues in an effort to "treat" animal models of retinal degeneration. However, the results have been mixed and many questions remain. The immunologic issues governing transplant survival are complex and only partially understood. Possible strategies for overcoming these problems are suggested from ongoing investigations of the development and maturation of the normal retina. Cell lineage analysis has shown that retinal cells are generated from progenitor cells throughout development. The cell types generated in vitro can be influenced by the environment, and certain growth factors added to retinal cell cultures can lead to shifts in the types of cells produced. Growth factors can also influence the survival of retinal cells in vitro or in vivo. In addition to the effect of extrinsic cues, intrinsic properties of progenitor cells contribute to the genesis of retinal cell types as well. These types of experiments may lead to more effective strategies whereby manipulation of these progenitor cells could be exploited for retinal transplantation therapies. However, these experiments may also reveal insurmountable difficulties associated with this limited approach. In which case, use of pluripotent stem cells would become essential to overcome the immunological or other potential problems that may be encountered. Treatment of Ocular Surface Disorders: There is a significant clinical need for improved techniques to promote conjunctival and corneal healing during disease or after injury. Conventional surgery is not consistently successful in treating persistent corneal ulcers, chemical or thermal injury, bullous keratopathy, and various cicatrical diseases. Transplantation with pluripotent stem cells could provide a means of facilitating epithelialization of the ocular surface, reducing inflammation, vascularization, and scarring.
Dental and Craniofacial Research Pluripotent human stem cells offer an important new tool and resource in biomedical research. Research using animal pluripotent stem cells is already helping to improve our understanding of the complex events of tissue development and regeneration. In addition, it is providing new approaches for the development and testing of new drugs and therapies and is contributing to the development of new technologies for the repair and replacement of organs, which have been damaged by disease or injury. This is but a glimpse of what promises to become a rapidly expanding research portfolio at NIDCR. Stem cell research should better allow us to understand the biology of inherited craniofacial anomalies such as cleft lip and cleft palate and also the way normal cells can become malignant in orofacial and pharyngeal cancer. This should provide new information to prevent and treat these diseases. In addition, stem cell research could lead to the engineering of specialized cells such as bone, cartilage and salivary cells, which can be used as replacement for organs damaged by disease or injury. Examples include the treatment of temporomandibular joint disorders (TMDs), the replacement of skeletal elements lacking or damaged in diseases such as fibrous dysplasia of bone using cells grown in special natural or synthetic scaffolding materials, and the replacement of salivary cells damaged by disease (Sj鰃ren's Syndrome) or radiation for head and neck cancer.
Human Embryonic Stem Cell Embryonic stem (ES) cells are continuously growing stem cell lines of embryonic origin first isolated from the inner cell mass of developing mouse blastocysts. More recently, it has been shown that embryonic germ (EG) cell lines, established from primitive reproductive cells of the fetus, are functionally equivalent to ES cells. The distinguishing features of ES cells are their capacity to be maintained in an undifferentiated state indefinitely in culture and their potential to develop into every cell of the body. The most rigorous test of the developmental potential of mouse ES cells is their ability to contribute to all cell lineages—including the germ–line—of chimeric animals. In addition, under appropriate culture conditions, ES cells differentiate into a broad spectrum of cell types and when injected into immunocompromised animals, they form teratomas composed of multiple lineages. It is this ability to develop into a wide range of cell types that has drawn so much attention to ES cells as a basic research tool and as a novel source of cell populations for new clinical therapies.
Given the outstanding potential demonstrated by mouse ES cells, it was anticipated by many that the development of human ES cell lines was not far off. The wait ended in November last year when John Gearhart's group at the Johns Hopkins Hospital reported on the development of what seemed to be a human EG line and James Thomson's group at the University of Wisconsin described the isolation of human cells with the properties of ES cells. When discussing these studies, the first critical question that comes to mind is: do these cell lines possess the characteristics of true EG and ES cells? The fact that these lines were generated from appropriate cell populations, that they both have extensive proliferative potential and display a capacity for multilineage differentiation suggests that they do indeed represent EG and ES cells. In these studies, the EG cell lines demonstrated some potential to undergo spontaneous differentiation in culture although the efficiency and extent of this differentiation remains to be fully defined. The pluripotential nature of the human ES cells was demonstrated in teratomas generated following their injection into immunocompromised mice. But further characterization of the developmental potential of both types of cells in culture is necessary. To appreciate the potential importance of human ES/EG cell lines, it is worth examining recent advances made with mouse ES cells. Although mouse ES cells are most noted for their ability to produce new strains of animals with specific genetic alterations, it is their capacity to differentiate and generate multiple lineages in culture that is most relevant to human cell lines. When removed from conditions that maintain them in an undifferentiated state, ES cells will spontaneously differentiate to form multicellular structures known as embryoid bodies (EBs) that contain elements of all three embryonic germ layers: ectoderm, mesoderm and endoderm. As differentiation continues, a wide range of cell types, including hematopoietic, endothelial, muscle, and neuronal develop within the EBs in a defined and reproducible temporal order. When analyzed in detail, the developmental programs associated with lineage commitment in EBs show remarkable similarities to those found in the normal embryo. Thus, EBs provide a rich source of normal developing cell populations with both embryonic and adult phenotypes. Given this potential, what opportunities do human ES and EG cell lines offer? The most obvious application of human ES/EG cells and the one that receives the most attention is in cell–replacement therapies: to replace diseased or degenerating tissues, or cell populations (such as those of the hematopoietic system) that have been destroyed by chemotherapy. In theory, EBs could provide an unlimited supply of specific cell types for transplantation. To date, ES cell–derived cardiomyocytes, neural precursors and hematopoietic precursors have been transplanted into recipient animals. Although the analyses of the long–term outcome of such experiments are limited, the findings suggest that the transplanted cells are able to function in the host animal. These preliminary studies are encouraging and suggest that ES cell–based therapies for certain neuronal, hematopoietic and cardiovascular diseases will be possible. However, before this goal can be reached a number of obstacles need to be overcome, the most significant of which is donor/recipient compatibility and graft rejection. Possible solutions to this problem include the banking of large numbers of ES/EG cell lines encompassing a significant fraction of the histocompatibility types in the population, and/or the genetic modification of the stem cells to make the graft more acceptable to the recipient. In addition, recent cloning studies suggest a third solution in which nuclear transfer could be used to produce starting material that would allow the isolation of individualized ES cell lines. In this situation, the cells used for transplantation would be genetically identical to those of the patient. Another application of ES/EG cells is for the study of development in both human and animal model systems. Embryoid bodies provide an unprecedented opportunity to study the events regulating lineage commitment, and tissue growth and maturation during embryonic development. This approach includes the identification and isolation of novel precursor cells and of medically important genes. Such genes might encode proteins that have direct therapeutic applications, such as growth factors, or that are important targets for drug development. The ability to experimentally manipulate this system opens many new avenues of research as access to normal embryo–derived precursors at comparable stages of development is often very difficult in animal models, and is almost impossible in humans because of ethical considerations. The ES cell differentiation system can also be used to develop a rapid assay to study the function of genes. In the mouse, most gene function studies require the generation of transgenic and/or gene knockout animals, which can be considered long–term assays. The ES cell differentiation system offers a rapid complementary in vitro approach that is not subject to the limitations of in vivo studies. The system is well suited to understanding the role of genes through 'loss–of–function' studies by developing ES cell lines that lack a specific gene. Understanding how the absence of a gene influences the normal growth and development of specific tissues is an important first step towards elucidating its function. The ES cell differentiation system is also ideal for 'gain–of–function' studies, as it is possible to analyze the consequences of overexpression of specific genes on the development of different cell lineages. With the development of human ES/EG cells, these same approaches can now be applied to the study of human genes using a human in vitro developmental system. Human ES/EG cells will be valuable as a test system for evaluating the toxicity and efficacy of new medicines or chemicals. The wide range of cell types and tissues that develop in EBs represent a biological system that mimics many of the complex interactions of the cells and tissues of the body, and as such provides an attractive screening tool. This type of assay could have wide applications in the pharmaceutical, cosmeceutical and agrochemical industries. It has the potential to reduce the need for animal testing and to increase the efficiency and reduce the costs of developing safe and effective drugs and chemicals. Given the progress with mouse ES cells, it is important to ask how close we are to similar experiments with human cells? Before one can fully answer this question, a number of basic issues need to be addressed. First, we need a more thorough understanding of the efficiency and regulation of human ES/EG cell differentiation in culture. There are also basic technical issues relating to the growth and propagation of these cells in culture that need to be overcome. Specifically, the reported difficulty in dissociating the human ES/EG cell clusters into viable single cells is problematic, particularly for gene–targeting experiments. In addition, we need more data on the reproducibility and efficiency of human ES/EG cell isolation. Will it be possible for other investigators to establish comparable cell lines, and if so, at what frequency?
