A brief review of recent advances in stem cell biology.

Stem cells have the remarkable potential to develop into many different cell types, essentially without limit to replenish other cells as long as the person or animal is still alive, offering immense hope of curing Alzheimer's disease, repairing damaged spinal cords, treating kidney, liver and lung diseases and making damaged hearts whole. Until recently, scientists primarily worked with two kinds of stem cells from animals and humans: embryonic stem cells and non-embryonic "somatic" or "adult" stem cells. Recent breakthrough make it possible to convert or "reprogram" specialized adult cells to assume a stem stem-like cells with different technologies. The review will briefly discuss the recent progresses in this area.

Current progress in embryonic stem cells

Embryonic stem cells (ESCs) are pluripotent stem cells derived from the inner cell mass of the blastocyst (). The scientists first discovered ways to isolate and culture ESCs from early mouse embryos in 1981, nearly 30 years ago (Evans and Kaufman, 1981). They developed a method in 1998 to derive stem cells from frozen human embryos that are no longer needed for in vitro fertilization (Thomson et al., 1998). ESCs are pluripotent. They are able to differentiate into all cell types of an individual. ESC has potentially unlimited capacity for self-renewal, thus if scientists can reliably direct the differentiation of ESCs into specific cell types, they may be able to use the resulting, differentiated cells to treat certain diseases in the future.

A summary of technologies to generate pluripotent stem cells. (a) Illustration on how to generate embryonic stem cells from fertilized egg. (b) Somatic cell nuclear transfer technology to generate embronic stem cell. (c) Reprogrammed somatic mouse cells into pluripotent stem cells by inserting just four unctioning genes into the cells. (d) Stimulus-triggered acquisition of pluripotency.

Somatic cell nuclear transfer (SCNT)

Although ESCs are promising donor sources in cell transplantation therapies, they face an ethical issue regarding the destruction of human embryos. To circumvent this limitation, an existing laboratory technique was revived for creating a blastula with the transfer of a donor nucleus to a denucleated egg (Gurdon, 1962; McGrath and Solter, 1983), laterally called SCNT (). This technique is the basis for cloning animals (such as the famous Dolly the sheep) (Campbell et al., 1996) and in theory could be used to clone humans. One concern is that blastula creation in SCNT-based human stem cell research will lead to the reproductive cloning of humans. A second important concern is the need of appropriate source of eggs that are needed. Thus, the impetus for SCNT-based stem cell research has been decreased by the development and improvement of alternative methods of generating stem cells.

Induced pluripotent stem cell (iPSC)

Dr. Shinya Yamanaka, Nobel prize laureate, announced in June 2006 that he made a breakthrough by identifying conditions that would allow some specialized adult cells to be “reprogrammed” genetically to assume a stem cell-like state, called induced pluripotent stem cell (iPSC) (). They initially reprogrammed mouse skin cells into iPSCs by inserting just four functioning genes (OSKM) into the cells (Takahashi and Yamanaka, 2006). The development of iPSCs from individual skin cells has opened up a new world of research. This embryo-free technique has been proven to be a powerful way to generate cell lines from a patient's own tissues (Takahashi et al., 2007; Yamanaka, 2012). Furthermore, the iPSCs have been directed to make cardiomyocytes, several kinds of neurons, liver cells, hematopoietic stem cells and so on, for possible cell replacement therapy (Robinton and Daley, 2012). Cell reprogramming technology provides a novel approach to derive iPSCs directly from a patient's somatic cells without embryo involvement. Thus, this novel approach overcomes ethical concerns.

Although cell reprogramming is very attractive because of its potential for future cell replacement therapy, several potential challenges need to be overcome before any possible applications can be made. The retroviral system is still one of the most effective approaches by far to mediate the expression of OSKM for producing iPSCs from somatic cells. Unfortunately, most experiments with retrovirus involve integration into the host cell genome with an identified risk for insertional mutagenesis and oncogenic transformation (Sanes et al., 1986). To circumvent such risks, which are deemed incompatible with therapeutic prospects, significant progress has been made with no chromosome integration method or even virus-free reprogramming methods. Life technologies Corporation (USA) has developed a CytoTune® reprogramming vector based on Sendai virus. Unlike other vectors, this viral vector does not integrate into the host genome or alter the genetic information of the host cell (Fusaki et al., 2009; Seki et al., 2010; Ban et al., 2011). Virus-free methods such as direct mRNA, microRNA, or protein delivery have been developed to achieve conversion of adult cells into iPSCs.

Stimulus-triggered acquisition of pluripotency (STAP)

The novel approach developed by Obokata et al. is surprisingly simple. When a dozen cell types, including those from the brain, skin, lung, and liver were exposed to stress, including low pH, about 20% of the cells that survived from stress reprogrammed to multipotent stem cells without introduction of any exogenous genes. Obokata called the phenomena stimulus-triggered acquisition of pluripotency (STAP) (). This is an amazing technique that may allow creating cells with pluripotency from patients without destruction of an embryo or introduction of exogenous genes. If successful, it would open a new era in stem cell biology and research in tumorigenesis. However, scientists need to replicate this exciting result and fully understand the mechanism underlying STAP cells before their full potential is realized and applied in medicine.

“Adult” stem cells

Scientists have found adult stem cells in many tissues or organs playing roles in maintaining and repairing the tissue in which they are found. Typically, the number of “adult” stem cells in each tissue is very small, and once removed from the body, their capacity to divide is limited, making generation of large quantities of stem cells difficult. Currently, blood stem cells are the only type of adult stem cells that are used regularly for treatment; they have been used since the late 1960s in the procedure now commonly known as bone marrow transplant.

Adult neural stem cells

Until very recently, the relative lack of recovery from central nervous system (CNS) injury and neurodegenerative disease resulted in the entire field reaching the conclusion that neurogenesis does not occur in the adult mammalian brain. A series of groundbreaking results from different groups over the last four decades have contradicted this classical view and provided strong evidence that neurogenesis, the birth of new neurons, does extend past embryonic and fetal stages of development and occurs, although with limitations, also in the adult brain.

Joseph Altman and colleagues were the first to use techniques sensitive enough to detect the ongoing cell division that occurs in adult brain and published evidence that neurogenesis constitutively occurs in the hippocampus (Altman and Das, 1965a) and olfactory bulb (Altman, 1969a, b) of the adult mammalian brain. However, the absence of neuron-specific immunocytochemical markers at the time resulted in the identification of putatively newborn neurons being made on purely morphological criteria, which led to a widespread lack of acceptance of these results.

Only two decades ago, technical advances including the use of cell type-specific markers to clearly identify newborn neurons, allowed two independent groups to more definitively show that precursor cells isolated from the forebrain can differentiate into neurons in vitro (Reynolds and Weiss, 1992; Richards et al., 1992). These results led to an explosion of research in the field.

The discovery of adult neurogenesis and of stem cells in the adult brain has changed our view of the mature brain (). Although the bulk of neurogenesis in the mammalian brain occurs during embryonic development (Altman, 1969a; Rakic, 1972; Walsh and Cepko, 1992), a significant number of newborn neurons were generated and integrated into the existing neural network in the adult brain (Altman, 1962; Altman and Das, 1965b). In the late 1990s, Kuhn et al. (1996) and later several other groups revealed hippocampus is one of the regions of the adult brain that can support neurogenesis throughout the life (Kempermann and Gage, 2000; Ming and Song, 2005; Shapiro and Ribak, 2005; Zhao et al., 2006). A large number of newly-generated granular neurons in the adult rat (Cameron and McKay, 2001), primate (Eriksson et al., 1998a; Gould et al., 1999; Kornack and Rakic, 1999) and human (Eriksson et al., 1998b) dentate gyrus (DG) (~9,000 per day) (Cameron and McKay, 2001). Alternatively, the neurons generated from neural precursor cells in the subventricular zone (SVZ) migrate through the rostral migratory stream (RMS) into the olfactory bulb and turn into interneuron (30,000/day) (Lois and Alvarez-Buylla, 1994; Doetsch et al., 1997; Doetsch et al., 1999; Alvarez-Buylla et al., 2001; Alvarez-Buylla and Garcia-Verdugo, 2002; Alvarez-Buylla et al., 2002; Carleton et al., 2003). These postnatal-born granular neurons are electrically active and are capable of firing action potentials and receiving synaptic inputs (Markakis and Gage, 1999; Carlen et al., 2002; van Praag et al., 2002). These adult-born neurons are potential resources for repairing the damaged hippocampus following injury. Adult neurogenesis represents a striking example of structural plasticity in the mature CNS that may be compromised by disease or injury, such as Alzheimer's disease, aging, and traumatic brain injury.

Neural stem cells in the adult hippocampus.

(A) Immunostaining with antibody against enhanced green-fluorescent protein (EGFP) was performed to reveal the neural stem/progenitor cells (NSCs) in the adult hippocampus of Nestin-EGFP transgenic mice. (B) Quiescent neural progenitors (QNPs) and amplifying neural progenitors (ANPs) are distinguishable in the hippocampus of Nestin-EGFP transgenic mice. QNPs are marked by white arrows. While the ANPs are marked by white arrowheads. (C–I) Double immunostaining with antibody against bromodeoxyuridine (BrdU; red) and nestin (green) was performed to visualize the proliferating NSCs in the subgranular zone of the adult hippocampus. (C) BrdU. (D) Nestin. (E) DAPI. (F) Merge of (C) to (F). White arrows indicate proliferating neural stem/progenitor cells. (G–I) Confocal microscopy was performed to verify the colocalization of BrdU with nestin in the cells within the white box in panel (F).

Summary

Advances in stem cell research will provide enormous opportunities for both biological and future clinical applications. Basically, stem cells could replicate any other cells in the body, offering immense hope of curing Alzheimer's disease, repairing damaged spinal cords, treating kidney, liver and lung diseases and making damaged hearts whole. The potential for profit is staggering. However, this field of research still faces myriad biological, ethical, legal, political, and financial challenges. The eventual resolution of these conflicts will determine the success of the research and potentially the face of medicine in the future.