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Types of stem cells and their uses

Stem cells are the cells responsible for producing new cells. This is important for growth, replacement, and repair. Here, we give you a short overview of different stem cell types before comparing the progress made towards therapies for patients, and the challenges or limitations that still need to be addressed. 

Embryonic stem cells (ESCs)

Pluripotent stem cells can make any type of specialised cell in the body (eg., embryonic stem cells). Multipotent stem cells can make several different specialised cells (eg., haematopietic stem cells).  Unipotent stem cells are responsible for producing a single type of specialised cells (eg., germline stem cells). 

 

Embryonic stem cells (ESCs) have unlimited potential to produce specialised cells of the body. This suggests enormous possibilities for disease research and for providing new therapies. ESCs are pluripotent: they can differentiate into any cell type of the body. 

Human ESCs were first grown in the lab in 1998. These cells are derived from a very early stage of development. The embryo consists of about 100 cells, forming a structure called a blastocyst, and has not yet implanted into the uterus. 

Hand-drawn diagram showing the inncer cell mass of a blastocysts

Not every experiment using ESCs requires a new blastocyst. ESCs derived in the lab can be maintained and grown in large numbers for long periods of time. In fact, it seems possible to continue growing them indefinitely. When scientists know that a particular group of ESCs can be maintained and grown indefinitely, they can save samples to deposit in stem cell banks, so that other scientists can use them too. These saved samples are referred to as cell lines. Cell lines are often used in the early stages of experiments, as their properties are well-known, and it makes it easier to compare results between other trials using the same line. 

ESCs can be used to study how specific tissues develop. They can also be used for drug-testing, and for modelling diseases in different tissues without collecting tissue samples from patients. 

There are still some practical challenges around the use of ESCs for research or in the clinic. Researchers need to ensure that ESCs fully differentiate into the required tissue type, and that tissues grown from ESCs will behave the same way as those which grow in the body. There are also ethical considerations around the use of cells obtained from a human blastocyst. 

Induced Pluripotent Stem Cells (iPSCs)

In 2012, Shinya Yamanaka and John B. Gurdon received the Nobel Prize for their discovery of iPSCs.

The discovery that specialised, mature adult cells can be ‘reprogrammed’ into cells that behave like embryonic stem cells was a breakthrough in the field of stem cell research. This process is sometimes called de-differentiation. 

iPSCs offer a source of ESC-like cells which can be accessed without the creation of a blastocyst. They can used to study developmental process, the factors involved in the growth of specific tissue types, and as a way of modelling diseases in the lab. 

From the clinical perspective, the discovery of these induced pluripotent stem cells (iPSCs) raised hopes that cells could theoretically be made from a patient’s own skin (or other tissues) in order to treat their disease. This would avoid the risk of immune rejection. It may also the generation of iPSC cell banks, which would function like blood banks: matching patients with suitable donors. 

Similar to ESCs, iPSCs must be shown to completely and consistently differentiate into the required types of specialised cells to make them effective tools for research, and eventually to meet standards suitable for use in patients. 

Tissue-specific (adults) stem cells

Many tissues in the human body are maintained and repaired throughout life by stem cells. These tissue-specific stem cells are very different from embryonic stem cells. 

Tissue-specific stem cells are not pluripotent like ESCS. They have partially differentiated or matured, and so they are no longer capable of producing everyu cell type. They are still capable of self-renewal. Most adults stem cells are multipotent, capable of producing only a limited number of specialised cell types. Some are even unipotent: they produce only one type of cell. 

Adult stem cells can only make a limited number of specialised cell types. For example, neural stem cells can only differentiate into brain cells; haematopoietic stem cells (blood stem cells) can only differentiate into specialised blood and immune cells; and germline cells can only produce sperm or ova.

What is the importance of stem cells in research?

Stem cells can be used to study development

Stem cells may help us understand how a complex organism is maintained. In the laboratory, scientists can ‘track’ individual stem cells as they divide and become specialised, to maintain skin, bone, muscle, and other tissues. Identifying the signals and mechanisms that determine whether a stem cell chooses to carry on replicating itself or differentiate into a specialised cell type, and into which cell type, will help us understand how a healthy body maintains itself. 

Stem cells can be used to study developmental conditions and cancers

Some of the most serious medical conditions, such as cancer and birth defects, are due to abnormal cell division and differentiation. Understanding the genetic and molecular controls of these processes may provide information about how such diseases arise, and suggest new strategies for therapy. 

Stem cells could be used to study genetic disease

In many cases it is difficult to obtain the cells that are damaged in a disease, and to study them in detail. Stem cells, either carrying the disease gene or engineered to contain disease genes, offer a viable alternative. Scientists could use stem cells to model disease processes in the laboratory, and better understand what goes wrong. 

Stem cells could provide a resource for testing new medical treatments

New medications could be tested for safety on specialized cells generated in large numbers from stem cell lines – reducing the need for animal testing. Other kinds of cell lines are already used in this way. Cancer cell lines, for example, are used to screen potential anti-tumour drugs. 

Stem cells have the potential to replace damaged cells and treat disease

This property is already used in the treatment of extensive burns, and to restore the blood system in patients with leukaemia and other blood disorders. 

Stem cells may also hold the key to replacing cells lost in many other devastating diseases for which there are currently no sustainable cures. Today, donated tissues and organs are often used to replace damaged tissue, but the need for transplantable tissues and organs far outweighs the available supply. 

If we can isolate stem cells from the body and grow them in large numbers, this could provide a renewable source of replacement cells and tissues. This would be valuable in treating diseases including Parkinson's disease, stroke, heart disease and diabetes. This prospect is an exciting one, but there are significant technical challenges to address in order to ensure this approach is safe and effective. 

 

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