In research, the three most commonly used stem cells are embryonic stem cells (ESCs), induced pluripotent stem cells (iPSCs) and mesenchymal stem cells (MSCs).
Embryonic stem cells
Types of stem cells
ESCs are present in the cores of early-stage embryos – typically in the first week after fertilization before implantation of the embryo has taken place. In more mature embryos, ECSs differentiate, first into three germ layers and then into more specialized cells. ESCs can be cultured in vitro where, under the right conditions, they have the ability to proliferate indefinitely without losing their pluripotency2
. These conditions include specialized culture media, incubators with highly reliable temperature and gas control (for example hypoxic culture), and growth
The unlimited growth potential of ESCs, combined with their potential for differentiating into any cell type in the body, makes them an attractive option for use in research and in the clinic. Functional mature cells and tissues created from ESCs can replace or support damaged tissue – as well as provide a deeper insight into the development of embryos, tissues and organs.
However, the use of human ESCs comes with ethical considerations that go beyond the laboratory. The use of ESCs is ethically and politically controversial, because the derivation of ESCs involves the destruction of embryos. Therefore, several countries have regulated or banned the creation and use of human ESCs.
Nevertheless, in recent years over a thousand scientific papers about human ESCs have been published annually3
, and more than a dozen clinical trials using ESC-derived cells are currently underway with encouraging early results4
. The last decade has also witnessed the rise of iPSCs. These cells are a successful alternative to ESCs, which are created by 'reprogramming' of mature cells and therefore avoid many of the ethical issues surrounding the use of live embryos.Induced pluripotent stem cells
iPSCs are a type of pluripotent cell, created by introducing pluripotency-related genes into the genome of mature, fully differentiated fibroblasts. When cells start expressing these genes, they undergo a transformation into ESC-like cells capable of differentiating into the same range of lineages.
were developed in 2006 by Japanese researchers Shinya Yamanaka and Kazutoshi Takahashi, who used retroviral transduction to introduce four genes (Oct3/4, Sox2, Klf4 and c-Myc) into mouse fibroblasts5
. The same group later showed significanthypoxic improvements in reprogramming efficiency when using conditions6
. Further modifications to the method by other researchers led to reprogramming of neural cells to iPSCs using only a single gene (Oct4)7
Although viral transduction is a relatively efficient method for generating iPSCs, other groups have studied alternative reprogramming strategies that could improve reliability by reducing genetic abnormalities and the risk of tumor formation. These strategies have focused on avoiding the use of viruses, avoiding the need for integrating DNA into the cells’ genome, or both. Examples of viral and non-viral methods include episomal vectors, adenoviral vectors, Sendai viral vectors, plasmids, synthetic mRNA, miRNA, protein transduction and small molecules8,9
Avoiding the ethical issues surrounding ESC isolation has been a key factor in the success of iPSCs. In addition to this, iPSCs also have potential for use in autologous, patient-specific treatments. Because iPSCs originate from adult cells, it is possible to generate iPSCs from a patient’s own cells, opening up possibilities for treatment not possible with non-autologous cells.
However, both ESCs and iPSCs carry important safety concerns in clinical trials. Because both cell types are pluripotent, they have the ability to form tumors when accidentally implanted in an undifferentiated state. This risk means that treatments using cells derived from ESCs or iPSCs need strict quality control to eliminate the risk of tumor formation.
In the years after their discovery, iPSCs have become one of the standard tools available to stem cell researchers, providing a valuable alternative to ESCs for in vitro studies and in therapeutic use10
Mesenchymal stem cells
MSCs are a type of adult stem cell that remain in the body during adulthood and continuously renew and differentiate into specialized cells. MSCs are found in various adult tissues, the main source being bone marrow, but they are also found in adipose tissue, amniotic fluid, umbilical cord blood and the placenta11
A simple and commonly used method to obtain MSCs is to isolate mononuclear cells from bone marrow and discard all cells that do not adhere to a tissue culture plastic. However, there are limitations to this method, especially regarding specificity, and more advanced methods exist whereby the adherent fraction is purified further based on the presence of one or more MSC markers.
Unlike ESCs (which are pluripotent), MSCs are multipotent, meaning they can only differentiate into a small number of cell types – most notably bone, cartilage, muscle and fat cells. Another key difference is that MSCs cultured in vitro do not divide indefinitely, but instead show a decreasing rate of proliferation over time. For these reasons, some scientists prefer to use the term "multipotent (mesenchymal) stromal cell"12
In the human body, MSCs fulfil a variety of roles, for example bone repair after a fracture. When a fracture occurs, MSCs migrate from the bone marrow to the fracture site and differentiate into bone-forming cells that repair the damage13
In research, MSCs are studied for treatment of various conditions, including orthopedic injuries, graft-versus-host disease following bone marrow transplantation, cardiovascular, autoimmune and liver diseases. Because MSCs occur naturally in adults, the risk of tumor formation after implantation is low13