STEM CELL IN CELLULAR THERAPEUTICS AND REGENERATION

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1 : , December, 2010 ISSN (Print) X ISSN (Online) X REVIEW STEM CELL IN CELLULAR THERAPEUTICS AND REGENERATION VIDHATRI TIWARI*, PANKAJ GOEL, AJAY BHAMBHAL and SUDHANSHU SAXENA Peoples College of Dental Sciences & Research Centre, Bhopal, Madhya Pradesh, India ABSTRACT In biology, an organism is said to regenerate a lost or damaged part if it regrows and restores the original function. Regenerative capacity is inversely related to complexity: the more complex an animal is the less regeneration it is capable of. Newts, for example, can regenerate severed limbs, mammals cannot. Simpler animals have an enhanced capacity to regenerate due to the retainment of clusters of stem cells within their bodies that in need can migrate to the parts of the body in need for healing. There they divide and differentiate to provide the required missing tissue. Certain parts of human skeleton that are capable of regeneration, such as the finger tips in children age 10 yrs or less (Putta, 2004; Andrew, 2008), the ribs, liver and kidney. According to recent studies it has been accounted that cementum and periodontal ligament fibres on the root can also regenerate, so that a new attachment of injured tooth with their adjacent bone and connective tissues are established. Stem cells are found in most, if not all, multi-cellular organisms are characterized by the ability to renew themselves through mitotic cell division and differentiate into a diverse range of specialized cell types. Research in the stem cell field grew out of the findings by the Canadian scientists (Beeker et al., 1996; Siminovitch et al., 1963). The two broad types of mammalian stem cells are: embryonic stem cells that are isolated from the Inner Cell Mass (ICM) of blastocysts, and adult stem cells that are found in adult tissues. In a developing embryo, stem cells can differentiate into all of the specialized embryonic tissues. In adult organisms, stem cells and progenitor cells act as a repair system for the body, replenish specialized cells and maintain the normal turnover of regenerative organs, such as blood, skin or intestinal tissues.this review attempts to highlight the potential of stem cell in various regeneration processes. KEY WORDS: Stem Cell, Regeneration, Dentistry, Craniofacial INTRODUCTION Stem cells can now be grown and transformed into specialized cells with characteristics consistent with cells of various tissues such as muscles or nerves through cell culture. Highly plastic adult stem cells from a variety of sources, including umbilical cord blood and bone marrow, are routinely used in medical therapies. Embryonic cell lines and autologous embryonic stem cells generated through therapeutic cloning have also been proposed as promising candidates for future therapies (Tuch, 2006). The classical definition of a stem cell requires that it possess two properties: 1. Self-renewal - the ability to go through numerous cycles of cell division while maintaining the undifferentiated state. 2. Potency - the capacity to differentiate into specialized *Corresponding author ; dr_vidhatri15@rediffmail.com cell types. In the strictest sense, this requires stem cells to be either totipotent or pluripotent - to be able to give rise to any mature cell type, although multipotent or unipotent progenitor cells are sometimes referred to as stem cells Types of Stem Cells 1. Embryonic stem cells (ES cells) are pluripotent stem cells derived from the inner cell mass of an early stage embryo known as a blastocyst (Company claims, 2010). 2. Fetal stem cells are primitive cell types found in the organs of fetuses (Ariff & Eng Hin Lee, 2005). The classification of fetal stem cells remains unclear and this type of stem cell is currently often grouped into an adult stem cell. However, a more clear distinction between the two cell types appears necessary. 118

2 3. Adult stem cells are undifferentiated cells, found throughout the body after embryonic development, that multiply by cell division to replenish dying cells and regenerate damaged tissues. Also known as somatic stem cells, they can be found in juvenile as well as adult animals and human (Jiang et al., 2002). 4. Amniotic stem cells are found in amniotic fluid, are very active, expand extensively without feeders and are not tumorogenic. These are multipotent and can differentiate in cells of adipogenic, osteogenic, myogenic, endothelial, hepatic and also neuronal line (Coppi et al., 2007). 5. Induced pluripotent stem cells, commonly abbreviated as ips cells or ipscs, are a type artificially derived from a non-pluripotent cell, typically an adult somatic cell, by inducing a forced expression of certain gene (Baker & Monya, 2007). Stem cells are the master cells of the body defined by their ability to make many different types of cells and to divide many more times than regular cells. Scientists are, therefore extremely enthusiastic about using stem cells for regenerative medicine. However, a major question is where to get these powerful stem cells? Recently, scientists have discovered that there are stem cells in teeth that can be collected from teeth that are falling out or removed during surgery that would otherwise be thrown away. Stem Cell in Therapeutics Stem cell treatments are a type of cell therapy that introduces new cells into damaged tissue in order to treat a disease or injury. Many medical researchers believe that stem cell treatments have the potential to change the face of human disease and alleviate suffering. The ability of stem cells to self-renew and give rise to subsequent generations that can differentiate offer a large potential to culture tissues that can replace diseased and damaged tissues in the body, without the risk of rejection (Weissman, 2002). A number of stem cell treatments exist, although most are still experimental and/or costly, with the notable exception of Bone Marrow Transplantation. Medical researchers anticipate that one day they would be able to use technologies derived from adult and embryonic stem cell research to treat cancer, Type 1 diabetes mellitus, Parkinson s disease, Huntington s disease, cardiac failure, muscle damage and neurological disorders, along with many others (Singec et al., 2007). For over 30 years, bone marrow, and more recently, umbilical cord blood stem cells have been used to treat cancer patients with conditions such as leukemia and lymphoma. During chemotherapy, the cytotoxic agents kill most growing cells including the leukemia or neoplastic 119 cells as well as the haematopoietic stem cells within the bone marrow. The stem cell transplant attempts to reverse this side effect of chemotherapy; the donor s healthy bone marrow reintroduces functional stem cells to replace those lost during the treatment. Stroke and traumatic brain injury lead to cell death, characterized by a loss of neurons and oligodendrocytes within the brain. Healthy adult brains contain neural stem cells which divide and maintain general stem cell numbers or become progenitor cells. In healthy adult animals, progenitor cells migrate within the brain and function primarily to maintain neuron populations for olfaction (the sense of smell). Stem cells may also be used to treat brain degeneration in Parkinson s and Alzheimer s disease (Cell Basics, 2009). Adult neural stem cell, injected into the brains of dogs has been very successful in treating cancerous tumors. With traditional techniques brain cancer is almost impossible to treat due to its rapid spread. Researchers at the Harvard Medical School induced intracranial tumours in rodents and injected human neural stem cells. Within few days these cells migrated into the cancerous area, produced cytosine deaminase (enzyme that converts a non-toxic pro-drug into a chemotherapeutic agent) and resulted in reduction of tumor mass by 81 percent. The stem cells neither differentiated nor turned tumorigenic (Douglas, 2000). The key to finding a cure for cancer is to inhibit cancer stem cells at the site of cancerous tumor. Currently, cancer treatments are designed to kill all cancer cells, but through this method, researchers would be able to develop drugs to specifically target these stem cells (Cancer stem cell Hint at curve, 2000). Umbilical cord blood derived multipotent adult stem cell, injected a patient suffering from spinal cord injury and unable to walk for 19 long years had the potential for cure. Human embryonic stem cells regenerate not only the neurons, but also the cells of the myelin sheath, a layer of cells which insulates neural impulses and increases speeds, facilitating communication with the brain (damage to which is often the cause of neurological injury in humans). Researchers have transformed human blastocyst stem cells into neural stem cells, then into the beginnings of motor neurons, and finally into spinal motor neuron cells. Which transmit messages from the brain to the spinal cord in the human body. The newly generated motor neurons exhibited electrical activity, indicating their action (Kang et al., 2005). Several clinical trials targeting heart disease have shown that adult stem cell therapy is safe and effective, and is equally efficient in old as well as recent infarcts (Strauer et al., 2009). Adult stem cell therapy for heart disease is commercially available in at least five continents and the possible mechanisms involve generation of heart

3 muscle cells, stimulation of growth of new blood vessels that repopulate the heart tissue, secretion of growth factors, rather than actual incorporation into the heart and assistance via some other mechanism. It may be possible to have adult bone marrow cells differentiate into heart muscle cells (Cell Basics, 2009). Fully mature human red blood cells may be generated ex vivo by hematopoietic stem cells (HSCs), which are precursors of red blood cells. In this process, HSCs are grown together with stromal cells creating an environment that mimics the conditions of bone marrow, the natural site of red blood cell growth. Erythropoietin, a growth factor, is added, coaxing the stem cells to complete terminal differentiation into red blood cells (Giarratana et al., 2005). Further research into this technique should have potential benefits for gene therapy, blood transfusion, and topical medicine. Hair follicles also contain stem cells, and some researchers predict that these follicle stem cells may lead to successes in treating baldness through hair multiplication, also known as hair cloning. This treatment is expected to work through taking stem cells from existing follicles, multiplying them in cultures, and implanting the new follicles into the scalp. Later treatments may be able to simply signal follicle stem cells to give off chemical signals to nearby follicle cells which have shrunk during the aging process, which in turn respond to these signals by regenerating and once again making healthy hair (Hair Cloning, 2004). There has been success in regrowing cochlea hair cells with the use of stem cells (Gene Therapy, 2005). Using embryonic stem cells, scientists are able to grow a thin sheet of totipotent stem cells in the laboratory. When these sheets are transplanted over the damaged retina, the stem cells stimulate renewed repair, eventually restoring vision (Fetal Tissue, 2004). Stem cells have cured rats with an Amyotrophic lateral sclerosis-like disease. The rats were injected with a virus to kill the spinal cord motor nerves related to leg movement, succeeded by injections of stem cells into their spinal cords. These migrated (passed through many layers of tissues) to the sites of injury where they were able regenerated the dead nerve cells restored the walking ability of rats (Gearhart & Kerr, 2001). Scientists were able to reverse the learning deficits in the offspring of pregnant mice, neural stem cell exposed to heroin and the pesticide organophosphate. This was done by direct neural stem cell transplantating into the brains of the offspring. The recovery was almost 100 percent, in which the treated animals showed normal behavior and learning scores. On the molecular level, brain chemistry was also restored to normal. Scientists are now developing procedures to administer the neural stem cells in the least invasive way probably via blood vessels, making therapy practical and clinically feasible. Researchers also plan to work on developing methods to take cells from the patient s own body, turn them into stem cells, and transplant them back into the patient s blood via the blood stream. Besides decreasing the chances of immunological rejection, the approach will also eliminate the controversial ethical issues involved in the use of stem cells from human embryos (Yanai, 2008). Human embryonic stem cells may be grown in cell culture and stimulated to form insulin-producing cells that can be transplanted into the patient (Cell Basics, 2009). However, success will depends on developing procedures where the cells proliferate and will generate sufficient tissue differentiate into the right cell type, survive in the recipient without rejection, integrate with the surrounding tissue in the body and function appropriate in long-term. Clinical cases, in the treatment of orthopedic conditions, have been reported. MRI evidence of increased cartilage and meniscus volume in individual human subjects are available to date (Centeno et al., 2008). In an experimental method in regenerative medicine, stem cells are used to stimulate the growth of human tissues. In an adult, wounded tissue is most often replaced by scar tissue, which is characterized in the skin by disorganized collagen structure, loss of hair follicles and irregular vascular structure. In the case of wounded fetal tissue, however, wounded tissue is replaced with normal tissue through the activity of stem cells. A possible method for tissue regeneration in adults is to place adult stem cell seeds inside a tissue bed soil and allow the stem cells to stimulate differentiation in the tissue bed cells. This method elicits a regenerative response more similar to fetal wound healing than adult scar tissue formation. Researchers are still investigating different aspects of the soil tissue that are conducive to regeneration (Gurtner et al., 2007). There is scientific evidence supporting that stem cells can improve healing by providing an anti-inflammatory effect, homing to damaged tissues and recruiting other cells, such as endothelial progenitor cells, that are necessary for tissue growth, supporting tissue remodeling over scar formation, inhibiting apoptosis and differentiating into bone, cartilage, tendon, and ligament tissue (Richardson, 2007; Csaki, 2007). Currently, research is being conducted to develop stem cell treatments for horses, dogs and cats. The former suffering from COPD (Chronic obstructive pulmonary disease), neurologic disease, and laminitis; and the latter 120

4 two suffering from heart, liver, kidney and neurologic diseases to immune-mediated disorders. Stem cells taken from the patient are coaxed in the lab turn into a tooth bud which, when implanted in the gums, gives rise to a new tooth, that takes nearly two months to grow (Teeth from scratch). It fuses with the jawbone and releases chemicals that encourage nerves and blood vessels to get connected with the new tooth. The process is similar to what happens when original adult teeth grow. This discovery has revolutionized the whole scenario of dentistry and periodontics and has opened a new passageway in the field of cranio-facial surgery (Yen & Sharpe, 2008). Stem Cell Research: Craniofacial Surgery and Dentistry Tissue Engineering and Dental and Craniofacial Regenerative Medicine Program (Lumelsky, 2008) supports basic and translational research on the reconstruction, remodeling and repair of the oral and craniofacial tissues damaged by disease or injury. Research in this particular field takes advantage of advances in biology, chemistry, material science, Nano technology, computer science, and engineering to develop tissue constructs that mimic structure and function of native oral and craniofacial tissues including bone, cartilage, muscle, vascular and neural components of cranium and temporomandibular joint, teeth, periodontal ligament, and oral and facial epithelium. It is concerned with craniofacial tissue damage resulting from injury, trauma, infection, inflammation, lesion, radiation, and surgery, and the regenerative capacity of these tissues. Thus, the cellular and molecular determinants of responses to damaging, degenerative and apoptotic pathways, mechanisms of regeneration, and augmentation of regenerative processes are emphasized. The particular areas include: 1. Destruction and regeneration of the periodontium, inflammatory bone erosion associated with periodontal diseases, 2. Distinct molecular and cellular mechanisms of intramembranous and endochondral bone regeneration, 3. Osteogenesis, angiogenesis and matrix remodeling during bone regeneration, 4. Augmentation of craniofacial bone regeneration, 5. Characterization of in situ stem cell populations and stem cell niches that contribute to tissue regeneration of the craniofacial complex, 6. Responses of fibrocartilage to injury and trauma, mechanisms of degeneration and regeneration, 7. Dentin-pulp complex homeostasis, injury and therapy, 8. Wound healing, connective tissue remodeling and scar less regeneration and 9. Impact of biomechanical forces on tissue damage and regeneration. Research on embryonic and adult stem cell biology helps elucidate the complex events that occur during oral, dental, and craniofacial development and disease as well as the repair and restoration of affected tissues focusing on the use of human embryonic stem cell lines for examining mechanisms of progenitor cell differentiation on the characteristics that make them potentially useful for stem cell-based therapy and on adult stem cell in the orofacial tissues, bone marrow, fat and blood that have the ability to give rise to oral, dental, and craniofacial tissues and organs (Gronthos & Stan, 2005). In order to take advantage of the new opportunities in human stem cell research the NIDCR encourages investigation on the identification, purification and characterization of stem cell populations that result in the formation of teeth, salivary glands, oral epithelium, cartilage, bone and smooth muscles. Some examples of potential research areas are: 1. Identification and characterization of the stem cell population(s), in terms of molecular markers and cell lineage in the oral, dental and craniofacial tissues and organs; 2. Design of conditions for maintenance, ex vivo, of cell populations that retain their pluripotency/ multipotency; 3. Use of stem cells to understand the genetic mechanisms that regulate development of orofacial structures, how tissues are maintained in health and how tissues are repaired or regenerated following trauma and disease; 4. Development of markers that distinguish stem and progenitor populations and gene profiles that characterize all stages of differentiation; 5. Understanding the mechanisms that regulate selfrenewal of stem cells in the oral epithelium, salivary gland, tooth structures and other craniofacial structures (e.g., bone, cartilage, muscles and nerves); 6. Use of stem cell lines to identify the environmental cues and conditions that are required for a human embryonic stem cell to give rise to cells that make up the different tissues of the orofacial complex; 7. Identification of signals, signaling pathways, components, and transcriptional factors that regulate the fate(s) of transplanted human stem cells and their derivatives; 8. Identification, characterization and reproduction of 121

5 the microenvironment ( niches ) of stem and progenitor cells; 9. Identification of the optimal type of stem cell or stem cell derivative for specific assays and cell therapy for orofacial diseases and disorders; 10. Use of animal models of oral, dental and craniofacial diseases and disorders for screening and comparing the functional capabilities of implanted human stem cells and their progeny (Bianco & Gehron, 2001; Donavon & Gearhart, 2001; Fuchs & Segre, 2000; Ghazizadeh & Taichman, 2001; Gronthos et al., 2000; Reya et al., 2001; Slack, 2000; Temple, 2001). Craniofacial tissue engineering promises the regeneration or, de novo formation of dental, oral, and craniofacial structures lost due to congenital anomalies, trauma and diseases. All craniofacial structures are derivatives of mesenchymal cells. The mesenchymal stem cell is derived from mesenchymal cells following asymmetrical cell division and resides in various craniofacial structures in the adult. Mesenchymal stem cell-mediated tissue regeneration is a promising approach for a wide range of applications (stem cells from apical papilla). Using minipig model, scientists transplanted human SCAP and periodontal ligament stem cells (PDLSCs) to generate a root/ periodontal complex capable of supporting a porcelain crown, resulting in normal tooth function. The particular study entails a stem cell-mediated tissue regeneration strategy, engineered materials for structure, and current dental crown technologies. The hybridized tissue engineering approach led to recovery of tooth strength and appearance (Sonoyama et al., 2006; Chai & Slavkin, 2003). CONCLUSION Advances in the adult stem cell biology have provided a great deal of impetus for the biomedical community to translate these findings into clinical applications. The stem cells reproducibly reform bone marrow, cementum, dentin, and perhaps even periodontal ligaments. Also complete restoration of the hard tissue in the oral cavity using patient s own cells, thereby avoiding issues of histocompatibility. Bone marrow derived cells can be reprogrammed to give rise to ameloblast-like cells and offers novel possibilities for tooth-tissue engineering and the study of the simultaneous differentiation of one bone marrow cell sub-population into cells of two different embryonic lineages. Human postnatal stem cells have been identified in periodontal ligament, with the potential to regenerate the periodontium in vivo. Cells with characteristics of putative mesenchymal stem cells are present in regenerating periodontal tissues, implying their involvement in periodontal regeneration (Lin, 2008). Mesenchymal stem cells and hematopoietic stem cells in the bone marrow are not involved in the regeneration of the periodontium (Ohta, 2008).Cells migrate from the residual periodontal ligament regenerate new alveolar bones at early stages and dentin in the cavity. Human dental pulp cells (DPSCs) are capable of forming ectopic dentin and associated pulp tissue in vivo. DPSCs are also able to differentiate into adipocytes and neural like cells. DPSCs possess stem-cell-like qualities, including self-renewal capability and multilineage differentiation (Batouli, 2003). Human periodontal ligament cells (PDL) possess stem cell properties of selfrenewal and multipotency. They also express the mesenchymal stem cell markers on their cell surfaces. Thus PDL cells can also be used for periodontal regenerative procedures (Nagatomo, 2006). Primary human oral keratinocytes cultured artificially, have the ability to regenerate oral mucosal graft. This implies that small-sized cultured oral keratinocytes contain an enriched population of progenitor/stem cells (Izumi et al., 2007). 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