Stem cells, by definition, can undergo infinite number of cell division while remain in an undifferentiated state. However, stem cells can also give rise to differentiated daughter cells that are committed to specialised cell fate in all three primary germ layers: oderm, mesoderm and endoderm.
During embryogenesis, the degree of this differentiation becomes gradually restricted as the foetus develops and the potential of various stem cells is limited by the class they belong to. While both totipotent and pluripotent stem cells can generate every type of cells found in the body, only totipotent ones are able to form an entire organism. In contrast, multipotent stem cells are more mature in developmental age compared to totipotent and pluripotent stem cells, and therefore, can only give rise to a limited population of cells within a specific lineage.
The particularly extrinsic and intrinsic molecular signalling network that confers their self-renewal and differentiation remain fairly uncharacterised. However, a consistent requirement for the Oct4, Nanog and Sox2 transcription factors in maintaining pluripotency seems to be evolutionarily conserved between mouse and human.
The Different Types of Stem Cells:
Embryonic Stem Cells of both mouse and human are pluripotent stem cells harvested from the inner cell mass of a pre-implanted blastocyst at day 3.5 and 5, respectively. Human embryonic stem cell lines are derived from donated embryos leftover from in vitro fertilisation. While the technical methods of isolating embryonic stem cells between mouse and human are similar, these two cell types vary in the specific gene expression and in various cellular behaviours. For instance, the key renewal factor for mouse embryonic stem cells is leukaemia inhibitory factor (LIF) while basic fibroblast growth factor (bFGF) is a necessary component in maintaining long-term self-maintenance in human embryonic stem cells.
Adult Stem Cells are small populations of undifferentiated multipotent cells found within differentiated tissues or organs. Similar to embryonic stem cells, they can self-renew and differentiate upon receiving specific signals. However, stem cells only have limited abilities to differentiate into specific cell types and are thought to repopulate dying cells and regenerate damaged tissues. Although there are no ethical and political concerns over the usage of adult stem cells for research purposes, they are technically challenging to use since these cells exist in tissues at low frequencies. Furthermore, there are limited numbers of known defined markers to identify these cells, making it difficult to isolate them for research purposes.
Mesenchymal Stem Cells are multipotent stromal cells found in many adult tissues, such as endometrial polyps, bone marrow and adipose tissues. Beyond having shown to differentiate into in vitro osteoblasts, adipocytes and chondrocytes, they also can give rise to bone and cartilage after ectopic implantation in vivo.
Induced Pluripotent Stem Cells are somatic cells, such as skin fibroblasts, reprogrammed into an embryonic cell-like state through somatic cell nuclear transfer or through the ectopic expression of pluripotency-specific transcription factors and culturing under embryonic stem cell conditions. Human induced pluripotent stem cells hold great medical hope for treating degenerative diseases in a patient-specific manner, since they provide a source for autologous cells.
References:
RH Xu et al. Nat Methods. 2005 Mar; 2(3):185-90.
The Promise of Stem Cell Research
The first isolation of human embryonic stem cells was published in 1994 (Bongso et al, 1994), where the inner cell mass from human blastocysts was derived and propagated for two passages. However, seminal research carried out by Thomson and colleagues four years later demonstrated the first continuous culture of five human embryonic stem cell lines using mouse embryonic fibroblasts as feeder cells (Thomson et al, 1998). Since then, dramatic advances in the field of stem cell biology, especially in the discovery of induced pluripotent stem cells, provide new excitement in regenerative medicine and drug discovery.
Research Tool
Human embryonic stem cells offer a unique insight into basic human development in vitro and provide a model for the study of embryo implantation and development, which will have an impact in infertility medicine. Additionally, using pluripotent stem cells, one can learn about the plasticity of the human epigenetics. Several published studies have already begun to demonstrate that although induced pluripotent stem cells exhibit similar epigenetic markers as embryonic stem cells, they are not identical. Therefore, further research is clearly needed in order to accelerate the current slow rate of somatic cell reprogramming.
Disease Modelling
Until the discovery of induced pluripotent stem cells, modelling human diseases in pluripotent cell culture system often involved genetic modification of existing human embryonic stem cells or generating new cell lines from embryos harbouring those mutations via preimplantation genetic diagnosis. However, the advent of induced pluripotent stem cells now allows researchers to generate and differentiate induced pluripotent stem cells from patients with intractable diseases. This provides an invaluable tool to study the biological properties of cells carrying the same genetic materials as a patient suffering from a specific illness. In principle, these cells will reflect a particular disease phenotype more accurately than other cellular or animal model systems, allowing for faster diagnosis and custom therapeutic discoveries.
Regenerative Medicine
Stem cells have the ability to proliferate indefinitely, giving rise to an unlimited source of cells for differentiation into a particular cell lineage. Therefore, this provides a great advantage for scientists and clinicians in harvesting stem cells and inducing them into a particular cell type, such as cardiomyocytes and dopaminergic neurons to repopulate infarcted hearts from ischemia and to treat Parkinson’s disease, respectively.
Pharmacological and Toxicology Screening
Current methods in using established cell lines, primary explanted tissues and animal-modelled systems all have limitations in their ability to accurately predict the efficacy and safety of new drugs. Therefore, stem cells provide a benefited in vitro disease model for small molecule drug identifications in high throughput screens.
References:
JA Thomson et al. Science. 1998 Nov 6; ;282(5391):1145-7.
A Bongso et al. Hum Reprod. 1994 Nov;9(11):2110-7.
Limitations and Challenges of Stem Cells
Since stem cell biology is a relatively new field of research, several challenges must be overcome before pluripotent stem cells can be used in clinical settings.
First, although stem cells provide an abundant resource of pluripotent cells, present knowledge is limited in how to efficiently push all stem cells into a particular lineage during differentiation. Present methods in directed differentiation are laborious and low yield, even when supplemented with known growth factors. To overcome this issue, more studies are needed in order to elucidate the molecular and cellular mechanisms of cell differentiation and basic human developmental biology.
Second, although induced pluripotent stem cells provide an attractive in vitro model for the study of diseases in a patient-specific manner, it is unclear whether results from these studies accurately predict diseases in vivo. Standard pre-clinical evaluation of potential drug candidates will still require in vivo testing.
Third, since a hallmark characteristic of pluripotent stem cells is the ability to form teratomas, stem cells must first be terminally differentiated before using them as cell-based therapies. It is unclear whether introducing induced pluripotent stem cell-derived cells into matched-patients will cause abnormal teratoma growth since the frequency of teratoma formation seems to directly correspond to the degree of immunosuppression, as shown in animal studies. Furthermore, it is unknown how the host environment will react to the introduced cells, regardless of the cell source, after cell transplantation. It might not be possible to rely on cell-based therapy as a mode to treat autoimmune diseases since the host immune system may also target and destroy these transplanted cells, as they already eradicate endogenous host cells.