Stem cells have been the object of much excitement and controversy amongst both scientists and the general population. Surprisingly, though, not everybody understands the basic properties of stem cells, let alone the fact that there is more than one type of cell that falls within the “stem cell” category. Here, I’ll lay out the basic concepts of stem cell biology as a background for understanding the stem cell research field, where it is headed, and the enormous promise it offers for regenerative medicine.
Stem cells come in different flavors of potency
Fertilization of an egg cell by a sperm cell results in the generation of a zygote, the single cell that, upon a myriad of divisions, gives rise to our whole body. Because of this amazing developmental potential, the zygote is said to be totipotent. Along the way, the zygote develops into the blastocyst, which implants into the mother’s uterus. The blastocyst is a structure comprising about 300 cells that contains two main regions: the inner cell mass (ICM) and the trophoblast. The ICM is made of embryonic stem cells (ES cells), which are referred to as pluripotent. They are able to give rise to all the cells in an embryo proper, but not to extra-embryonic tissues, such as the placenta. The latter originate from the trophoblast .
Even though it is hard to pinpoint exactly when or by whom what we now call “stem cells” were first discovered, the consensus is that the first scientists to rigorously define the key properties of a stem cell were Ernest McCulloch and James Till. In their pioneering work in mice in the 1960s, they discovered the blood-forming stem cell, the hematopoietic stem cell (HSC) [2, 3]. By definition, a stem cell must be capable of both self-renewal (undergoing cell division to make more stem cells) and differentiation into mature cell types. HSCs are said to be multipotent, as they can still give rise to multiple cell types, but only to other types of blood cells (see Figure 1, left column). They are one of many examples of adult stem cells, which are tissue-specific stem cells that are essential for organ maintenance and repair in the adult body. Muscle, for instance, also possesses a population of adult stem cells. Called satellite cells, these muscle cells are unipotent, as they can give rise to just one cell type, muscle cells.
Therefore, the foundations of stem cell research lie not with the famous (or infamous) human embryonic stem cells, but with HSCs, which have been used in human therapy (such as bone marrow transplants) for decades. Still, what ultimately fueled the enormous impact that the stem cell research field has today is undoubtedly the isolation and generation of pluripotent stem cells, which will be the main focus of the remainder of the text.
Figure 1: Varying degrees of stem cell potency. Left: The fertilized egg (totipotent) develops into a 300-cell structure, the blastocyst, which contains embryonic stem cells (ES cells) at the inner cell mass (ICM). ES cells are pluripotent and can thus give rise to all cell types in our body, including adult stem cells, which range from multipotent to unipotent. Right: An alternative route to obtain pluripotent stem cells is the generation of induced pluripotent stem cells (iPS cells) from patients. Cell types obtained by differentiation of either ES cell (Left) or iPS cells (Right) can then be studied in the dish or used for transplantation into patients. Figure drawn by Hannah Somhegyi.
Reprogramming committed adult cells into stem cells: from frog to man
Martin Evans (Nobel Prize, 2007) and Matt Kauffman were the first to identify, isolate and successfully culture ES cells using mouse blastocysts in 1981 . This discovery opened the doors to the creation of “murine genetic models,” which are mice that have had one or several of their genes deleted or otherwise modified to study their function in disease . This is possible because scientists can modify the genome of a mouse in its ES cells and then inject those modified cells into mouse blastocysts. This means that when the blastocyst develops into an adult mouse, every cell its body will have the modification of interest.
The desire to use stem cells’ unique properties in medicine was greatly intensified when James Thomson and collaborators first isolated ES cells from human blastocysts . For the first time, scientists could, in theory, generate all the building blocks of our body in unlimited amounts. It was possible to have cell types for testing new therapeutics and perhaps even new transplantation methods that were previously not possible. Yet, destroying human embryos to isolate cells presented ethical and technical hurdles. How could one circumvent that procedure? Sir John Gurdon showed in the early 1960s that, contrary to the prevalent belief back then, cells are not locked in their differentiation state and can be reverted to a more primitive state with a higher developmental potential. He demonstrated this principle by injecting the nucleus of a differentiated frog cell into an egg cell from which the nucleus had been removed. (This is commonly known as reproductive cloning, which was used to generate Dolly the Sheep.) When allowed to develop, this egg gave rise to a fertile adult frog, proving that differentiated cells retain the information required to give rise to all cell types in the body. More than forty years later, Shinya Yamanaka and colleagues shocked the world when they were able to convert skin cells called fibroblasts into pluripotent stem cells by altering the expression of just four genes . This represented the birth of induced pluripotent stem cells, or iPS cells (see Figure 1, right column). The enormous importance of these findings is hard to overstate, and is perhaps best illustrated by the fact that, merely six years later, Gurdon and Yamanaka shared the Nobel Prize in Physiology or Medicine 2012 .
The future: stem cell-based personalized regenerative medicine?
Since the generation of iPS cells was first reported, the stem cell ﬁeld has expanded at an unparalleled pace. Today, these cells are the hope of personalized medicine, as they allow one to capture the unique genome of each individual in a cell type that can be used to generate, in principle, all cell types in our body, as illustrated on the right panel of Figure 1. The replacement of diseased tissues or organs without facing the barrier of immune rejection due to donor incompatibility thus becomes approachable in this era of iPS cells and is the object of intense research .
The first proof-of-principle study showing that iPS cells can potentially be used to correct genetic diseases was carried out in the laboratory of Rudolf Jaenisch. In brief, tail tip cells from mice with a mutation causing sickle cell anemia were harvested and reprogrammed into iPS cells. The mutation was then corrected in these iPS cells, which were then differentiated into blood progenitor cells and transplanted back into the original mice, curing them . Even though iPS cells have been found not to completely match ES cells in some instances, detailed studies have failed to find consistent differences between iPS and ES cells . This similarity, together with the constant improvements in the efficiency and robustness of generating iPS cells, provides bright prospects for the future of stem cell research and stem cell-based treatments for degenerative diseases unapproachable with more conventional methods.
Leonardo M. R. Ferreira is a graduate student in Harvard University’s Department of Molecular and Cellular Biology
 “Stem Cell Basics”: http://stemcells.nih.gov/info/basics/Pages/Default.aspx
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