The appeal of stem cells is their potential. Early embryonic stem cells are pluripotent with the ability to develop into any of the tissues found in the human body. But the early embryo is not the only source of stem cells. Pools of adult stem cells are found in many organs of the body including the bone marrow, skin and gut. These types of stem cells are multipotent and they can become one of several specialized cell types.
Stem cells offer numerous possibilities for both learning about the biology of development and for clinical applications. Since researchers discovered how to isolate and culture mouse embryonic stem cells in 1981, the stem cell and regenerative medicine fields have made significant strides in understanding some of the factors and conditions that allow stem cells to become specialized organ and tissue cells, as well as those factors that facilitate self-renewal and maintenance of an undifferentiated, non-specialized state.
Defining stem cell types
While only the very earliest embryonic cells (those few cells in the newly fertilized embryo) can morph into an entire new organism, embryonic stem cells from both the pre and post-implantation embryo represent a spectrum of pluripotent states that can be captured and maintained in culture.
From mouse embryos, two types of pluripotent stem cells have been successfully derived and defined. The most undifferentiated are embryonic stem cells (ESCs), derived from the mouse embryo before it has been implanted on about day 5 of development. The window to derive these ESCs is small. At 5 days, these ESCs begin to form the inner cell mass which then begins to differentiate into the three primary embryonic germ layers (i.e. the ectoderm, mesoderm and endoderm) which give rise to the body’s tissues and organs. This differentiation begins soon after the embryo implants. While ESCs are referred to as 'naïve', the second type of pluripotent stem cells from the post-implantation embryo -called epiblast stem cells (EpiSCs)- are known as 'primed' pluripotent cells. While EpiSCs can also develop into each of the three germ layers, they behave differently from ESCs in culture and have different features including active signaling pathways, molecular signatures and morphology. Human ESCs share some of the features of mouse ESCs. Both human and mouse ESCs express some of the same transcription factors (TF) involved in maintaining the undifferentiated state and self-renewal including Nanog, SOX2, and Oct4. Relatively high levels of all three TFs are necessary to maintain the pluripotent stem cell state. The levels of these proteins can be readily assessed using immunocytochemistry and immunofluorescence.
More recently, another type of stem cell has been created by genetically reprogramming differentiated adult cells to a more multipotent or pluripotent state through expression of the factors and genes, including the TFs mentioned above, that are necessary to maintain ESCs and epiSCs. These induced pluripotent stem cells (iPSCs) were named the Breakthrough of the Year by the journal Science back in 2008. Both mouse and human iPSCs are defined by their phenotype. Mouse iPSCs can form tumors that contain all three embryonic germ layers and can contribute to the formation of different tissues when injected into an early mouse embryo. Human iPSCs are able to form cells with the characteristics of each of the three embryonic germ layers.
The first successful demonstration of mouse iPSC creation was in 2006 from mouse tail-derived skin cells. The transformation required either four genes (Oct4, SOX2, c-Myc, and Klf4) or just Oct4, SOX2, and Klf4 alone. Starting in 2007, researchers used the same genes to create human iPSCs from adult skin and connective tissues as well as from stomach and liver. The human iPSC transformation has been since shown to require either the same genes as in the mouse or Oct4, SOX2, Nanog, and Lin28.
Clinical and Research Applications
The study of the genes and factors involved in both de-differentiation of adult cells and maintenance of pluripotency is helping to define various pluripotent stem cell states in order to advance ways to develop and maintain human and mouse stem cells in culture. Understanding the key events and factors necessary will allow researchers to better control the reprogramming process, advancing knowledge of how stem cells behave and differentiate and uncovering the gene regulatory networks involved.
The ability of pluripotent stem cells to generate almost any cell type offers the potential to use these cells to treat diseases and disorders associated with damage to tissue or specific cell loss such as age-related macular degeneration, spinal cord injury, stroke, diabetes, Alzheimer’s disease, Parkinson’s disease, and blood or kidney diseases. Human iPSCs and ESCs also have the potential to serve as model systems to study development and understand the course of disease as well as for efficacy and tissue-specific toxicity drug effects. For these and other research and clinical applications, clearly defining cell populations via stem cell markers can be important. Immunophenotyping using antibodies and sorting of cells by flow cytometry allows defining such cell populations. The technique is particularly amenable to multiparametric analysis, required for immunophenotyping of PSCs. With rigorously tested antibodies to known stem cell factors, Abcam offers a comprehensive line of stem cell and lineage markers to identify and define stem cell populations for laboratory use.
|Stem cell marker||PSC type||Cellular localization||Description|
|Nanog||mESC, hESC, hiPSC||Nuclear||TF. Together with Oct-4 and Sox2, is necessary to maintain pluripotency. Expression is controlled by factors including Oct-4 and Sox2.|
|Oct4||ESC, iPSC||Nuclear||TF. Together with Nanog and Sox2, is necessary to maintain pluripotency.|
|SOX2||miPSC, hiPSC||Nuclear||TF. Together with Oct-4 and Sox2, is necessary to maintain pluripotency.|
|c-Myc||miPSC, hiPSC||Nuclear||TF and oncogene. Used to create human and mouse PSCs.|
|Klf4||miPSC, hiPSC||Nuclear||Zinc-finger TF. Used to create human and mouse PSCs.|
|Lin28||hiPSC||Cytoplasmic, nuclear||MiRNA binding protein. Used to create human PSCs.|
|TRA-1-60||hiPSC, hESCs||Cell membrane||Protein present on cell membrane of human stem cells and embryonic germ cells, but not mouse embryonic stem cells.|
|SSEA-4||hESC, hiPSC||Cell membrane||Glycolipid carbohydrate on surface of human ESCs and germ cells, but not mouse ESCs.|
|SSEA-1||mESCs||Cell membrane, Golgi apparatus||Oligosaccharide on surface of mouse embryonic and germ cells but only expressed on human germ cells, not human ESCs.|
Takahashi K, Yamanaka S. (2006). Induction of Pluripotent Stem Cells from Mouse Embryonic and Adult Fibroblast Cultures by Defined Factors. Cell, 126(4): 663-76.