Stem Cell and Regenerative Medicine

Human embryonic stem cell (hESC) research has been a topic of much debate within the ethics community, centering largely on issues relating to the moral status of the embryo. While this discussion has been ongoing, hESC research has progressed at an ever-quickening pace. As this research continues forward, it is imperative that an ethically-optimized framework be established to help guide research. In particular, inadequate attention has been given to issues of social justice and the importance of both protecting vulnerable populations from bearing too great a burden for research while receiving too little of its benefits.

Mammalian stem cells can be obtained mostly at all developmental stages and from numerous anatomical sites. Human adult stem cells are perhaps the most clinically relevant. Due to their broader clinical use, extensive research, and a rather more comprehensive understanding of their physiology, bone marrow-derived cells appear as the first choice for applications in regenerative medicine. Models for addressing fundamental aspects of stem cell biology and behavior have been developed in numerous species including C. elegans, drosophila, rodents, and humans. Extensive research around the world has dramatically increased our understanding of the fundamental aspects of stem cell biology and their applications for the treatment of human and animal diseases. Numerous environmental and trophic factors have been identified to play central roles in regulating the self-renewal, proliferation, migration, differentiation, senescence, and death of stem cells, their derived progeny, and the final differentiated cells that perform all tissue and organ functions. Multipotent mesenchymal stromal cells, derived primarily from the bone marrow, have been examined extensively for their capacity to repair damaged tissues. Besides direct differentiation of the stem cells to the desired mature cell type, other indirect mechanisms have been identified to play important roles in the overall repair of the injured tissue. These included production of paracrine factors, modulation of the host inflammatory response, host cell survival, and recruitment and activation of host tissue stem cells.

Regenerative medicines offer the potential for treatment and possibly cure of debilitating diseases including heart disease, diabetes, Parkinson’s disease and liver failure. Approaches using stem cells from various sources are in pre clinical and clinical testing. The goal of these studies is to deliver cellular products capable of replacing damaged tissue and/or cells. However, the balance between cellular proliferation and differentiation is a carefully controlled process involving a range of growth factors and cytokines produced in large part by tissue stromal cells. These stromal cells make up the tissue microenvironment and appear to be essential for normal homeostasis. We hypothesize that tissue damage in many instances involves damage to the microenvironment resulting in a lack of signals through growth factor networks necessary to maintain survival and proliferation of tissue specific stem cells and progenitor cells. Therefore, optimal repair of disease tissue must account for the damage to the stromal environment. We propose that optimal cellular therapies for regenerative medicine will require combination cellular products consisting of a stromal cell population to reconstitute the microenvironment and to support the survival, proliferation and differentiation of the tissue specific stem cells or progenitor cells.

The application of biomimetic mechanical forces for stem cell differentiation is a technique that has been on the rise in recent years. Bioreactors are being designed and constructed in order to accurately direct these forces onto stem cells in both 2D and 3D configurations. Currently, the most widely investigated mechanical forces are compressive forces and tensile strain, while a small number of researchers are making use of more complex systems of forces such as torsion, shearing and, still more complex, hemodynamic forces. The effects of these forces on mesenchymal stem cells, adipose derived stem cells, and embryonic stem cells are the most commonly explored combinations in functional tissue engineering. Fortunately, recent breakthroughs in the area of adult dental stem cells have brought viable alternatives to the use of these three cell types to the forefront of stem cell research. Yet for certain target cell types, the application of mechanical force alone is not the optimal stimulus for the induction of differentiation programs, which then requires the addition of a chemical stimulus as well. Elucidation of the optimal recipes and how these protocols affect the cells is the ultimate goal of current tissue engineering endeavors. This chapter will summarize the most current findings in functional tissue engineering, explain the importance of engineering in medical research, and describe the ways tissue engineers are attempting to understand what biochemical changes are occurring in the stem cells during the application of mechanical stress.

Adult mesenchymal stem cells (MSCs) hold great promise for engineering replacements for damaged or degraded articular cartilage. This promise has long been contemplated, and its arrival has recently been marked by the implantation of engineered human trachea using autologous adult stem cells [1]. While tracheal cartilage is not the same as the articulating cartilage lining the ends of load bearing joints, the demonstration of in vivo efficacy provides a major step forward clinically. However, not all progress with MSC-based cartilage has been successful and considerable challenges remain in the realization of these constructs for load-bearing applications. Thus the intent of this chapter is to define the functional metrics required for engineering articular cartilage, and to situate the current state of MSC-based constructs within this framework. In doing so, we briefly define the components and function of the native tissue, and review the progress made to date using differentiated cartilage cells (chondrocytes) for cartilage tissue engineering. This discussion includes methods of formation, biochemical formulations for enhancing in vitro development, as well as progress made towards using mechanical forces to further direct maturation. We next overview the origins and applications of adult multi-potential stem cells, and discuss how routes towards cartilage tissue engineering with stem cells match (or fail to match) those approaches that were successful using differentiated cells. In particular, we describe new requirements for cartilage formation with MSCs, and outline several research areas that may inform this new direction in cartilage tissue engineering.

Therapeutic angiogenesis represents a molecular and cellular approach to the treatment of CAD that may be an alternative or additive to traditional pharmacology and interventional cardiology. The goal of angiogenic therapy is to activate endogenous angiogenic and arteriogenic pathways and stimulate revascularization of ischemic myocardial tissue. The feasibility of such a strategy has now been established through the results of studies over the past two decades, and clinical trials involving more than 1000 patients have been implemented. In this review we will discuss the results from these trials, tracing the progression of the technology from the delivery of recombinant proteins through gene and stem cell therapies. Critical evaluations reveal that neither proteins nor genes delivered by transient expression vectors provide an optimal therapy. Similarly, stem cell therapy is not achieving the level of improvement that was expected or predicted from preclinical results. The future of therapeutic angiogenesis lies in the use of permanent gene delivery vehicles expressing regulated genes and/or stem cells appropriately engineered with regulated genes.

The use of progenitor/stem cells to modulate the sensory systems in chronic pain is a new field in translational research. This follows 30 years of non-human cell therapy approaches to elucidate which tissue source, cell phenotype, neurotransmitter, or peptide might be antinociceptive in models of pain. Stem or progenitor approaches have been tested in cardiac myopathies, liver dysfunction, stroke, and genetic abnormalities, but almost none have applied progenitor cells to the relief of neuropathic, pain. Perhaps the best studied neural progenitor cell line NT2, has recently resulted in two NT2-derived cell lines: hNT2.17, secreting the inhibitory neurotransmitters GABA and glycine; and hNT2.19, secreting the neurotransmitter serotonin. Each of these NT2-lines has demonstrated antinociceptive potential in models of SCI-related neuropathic pain, in peripheral neuropathy, and diabetic neuropathic pain. These human progenitors may prove to be useful in the relief of chronic pain and open the way to other regenerative approaches to pain management.

The devastation caused by disruption of the central nervous system and the increased prevalence of chronic neural diseases in our aging population led to enthusiastic and rapid acceptance of neural stem cells as an effective tool to reverse central nervous system pathology over a decade ago. Shortly after human embryonic stem cells were identified, results which held that functional neurons generated from stem cells reversed severe damage to the central nervous system were widely disseminated and avidly endorsed. Subsequent reports claimed neural localization and proliferation of adult stem cells, trans-differentiation of mesenchymal stem cells into functional neurons, and stunning therapeutic effects of human neural stem cells in animal models.

Despite these claims, effective therapy with neural stem cells has not been realized. Early results have been attributed to factors released by infused cells and investigator bias but not to tissue regeneration. The identity of stem cell progeny identified as neurons by antigen expression and morphology has been questioned, leading to the re-interpretation of these results by many of the investigators who first reported them. Therapeutic expectations have subsided while reports of success in animal models continue to appear. The thought that neural stem cells, as currently defined, will reverse brain injury or pathology has been largely dismissed.

Despite these diminished expectations, herein we note that regenerative cells, or structures that mimic the function regenerative cells possess, are present in germinal areas of the adult human brain, albeit in limited numbers. Evidence does suggest that damaged brain tissue does, in some patients, regenerate with recovery of lost function. These cellular entities have not been widely studied, characterized or cultured, but they may be similar to structures generated in neural tissue of primitive vertebrates which have a remarkable capability to regenerate intact, functional brain. These structures can potentially be expanded using methods that differ vastly from stem cell culture methods employed to date. Successfully expanded and stored, these structures may provide an effective means to regenerate brain tissue after stroke and traumatic brain injury in humans.

The existence of clinically successful cell therapies (islet transplantation) for type 1 diabetes has stoked a keen interest in developing alternative, inexhaustible sources of insulin-producing cells. In this chapter we will broadly cover the state of the art regenerative therapies for the endocrine component of the pancreas, from stem cells to transdifferentiation. In particular, we will review the basics of pancreatic development, whose recapitulation remains the subject of a plethora of in vitro differentiation strategies using both embryonic and adult stem cells. Then we will examine the leading theories about the cellular and molecular mechanisms behind the in vivo regeneration of the organ that is observed under specific circumstances, as well as the purported ability of some tissues to turn into pancreatic endocrine cells when subjected to specific interventions (transdifferentiation). Finally, we will conclude with a general overview of the remaining challenges and clinical perspectives of all the above strategies, with a special emphasis on the immunological hurdles to be overcome for these approaches to find their way to standard clinical practice.

Stem cells have been isolated and characterized from embryonic, fetal, and adult tissues. The therapeutic and clinical application of embryonic stem cells and fetal stem cells is challenging to the many ethical and political controversies concerning their use. Adult stem cells have been isolated and characterized from a wide variety of tissues including bone marrow, brain, skin, hair follicles, skeletal muscle, adipose tissue, cord blood, dental tissue, and their differentiation potential may reflect their local environment. To date, several sources of dental stem cells have been isolated and being characterized as dental epithelial stem cells, dental pulp stem cells, dental follicle precursor cells, stem cells from human exfoliated deciduous teeth, stem cells from apical papilla, and periodontal ligament stem cellsDental stem cells have been shown to have multipotential by their ability to differentiate into neuronal, adipogenic, myogenic, chondrogenic, osteogenic and dentinogenic cells when cultured under specific conditions. These facilitated studies to address an important property of stem cells, that is, the capacity of a given stem cell population to regenerate an organized, functional tissue following transplantation in vivo. Furthermore, the ready availability of tooth tissues from redundant teeth such as third molars can provide a good supply of dental stem cells that may be utilized for regenerating other body parts or organs.

The human cornea is a site of tissue-specific adult progenitor cells, residing between cornea and conjunctiva in the Palisade of Vogt of the limbus region. Advances in molecular and cell culture techniques presently provide new platforms to investigate the intrinsic biological roles and properties of cornea epithelial progenitor cells (CEPCs), which is known to maintain corneal homeostasis throughout human life. Although specific molecular markers of CEPCs are still to be discovered, results of recent research provide new information to apply them for cell replacement in damaged tissues. Cultured CEPCs, with the aid of external support, have been used for ex vivo cornea therapy with satisfactory clinical outcome While the niche environment, i.e., the extracellular matrix, growth factors and cytokines, provide regulatory measures in the proliferation of CEPCs. The recent discovery of CEPC specific microRNAs opens a new direction of research on the biological properties of CEPC and stem cells of other resources. This should facilitate to address important questions regarding CEPC functions and therapeutic strategies in health and diseases.