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Stem cells have the remarkable potential to develop into many different cell types in the body during early life and growth. In addition, in many tissues they serve as a sort of internal repair system, dividing essentially without limit to replenish other cells as long as the person or animal is still alive. When a stem cell divides, each new cell has the potential either to remain a stem cell or become another type of cell with a more specialized function, such as a muscle cell, a red blood cell, or a brain cell.
What are stem cells, and why are they important?
Adult stem cells
Throughout the life of the organism, populations of adult stem cells serve as an internal repair system that generates replacements for cells that are lost through normal wear and tear, injury, or disease. Adult stem cells have been identified in many organs and tissues and are generally associated with specific anatomical locations. These stem cells may remain quiescent (non-dividing) for long periods of time until they are activated by a normal need for more cells to maintain and repair tissues.
Pluripotent stem cells
Early mammalian embryos at the blastocyst stage contain two types of cells – cells of the inner cell mass, and cells of the trophectoderm. The trophectodermal cells contribute to the placenta. The inner cell mass will ultimately develop into the specialized cell types, tissues, and organs of the entire body of the organism. Previous work with mouse embryos led to the development of a method in 1998 to derive stem cells from the inner cell mass of preimplantation human embryos and to grow human embryonic stem cells (hESCs) in the laboratory. In 2006, researchers identified conditions that would allow some mature human adult cells to be reprogrammed into an embryonic stem cell-like state. Those reprogramed stem cells are called induced pluripotent stem cells (iPSCs).
What are the unique properties of all stem cells?
Stem cells have unique abilities to self-renew and to recreate functional tissues.
Stem cells have the ability to self-renew.
Unlike muscle cells, blood cells, or nerve cells—which do not normally replicate— stem cells may replicate many times. When a stem cell divides, the resulting two daughter cells may be: 1) both stem cells, 2) a stem cell and a one more differentiated cell, or 3) both more differentiated cells. What controls the balance between these types of divisions to maintain stem cells at an appropriate level within a given tissue is not yet well known.
Discovering the mechanism behind self-renewal may make it possible to understand how cell fate (stem vs. non-stem) is regulated during normal embryonic development and post-natally, or misregulated as during aging, or even in the development of cancer. Such information may also enable scientists to grow stem cells more efficiently in the laboratory. The specific factors and conditions that allow pluripotent stem cells to remain undifferentiated are of great interest to scientists. It has taken many years of trial and error to learn to derive and maintain pluripotent stem cells in the laboratory without the cells spontaneously differentiating into specific cell types.
Stem cells have the ability to recreate functional tissues.
Pluripotent stem cells are undifferentiated; they do not have any tissue-specific characteristics (such as morphology or gene expression pattern) that allow them to perform specialized functions. Yet they can give rise to all of the differentiated cells in the body, such as heart muscle cells, blood cells, and nerve cells. On the other hand, adult stem cells differentiate to yield the specialized cell types of the tissue or organ in which they reside, and may have defining morphological features and patterns of gene expression reflective of that tissue.
Different types of stems cells have varying degrees of potency; that is, the number of different cell types that they can form. While differentiating, the cell usually goes through several stages, becoming more specialized at each step. Scientists are beginning to understand the signals that trigger each step of the differentiation process. Signals for cell differentiation include factors secreted by other cells, physical contact with neighboring cells, and certain molecules in the microenvironment.
How do you culture stem cells in the laboratory?
How are stem cells grown in the laboratory?
Growing cells in the laboratory is known as “cell culture.” Stem cells can proliferate in laboratory environments in a culture dish that contains a nutrient broth known as culture medium (which is optimized for growing different types of stem cells). Most stem cells attach, divide, and spread over the surface of the dish.
The culture dish becomes crowded as the cells divide, so they need to be re-plated in the process of subculturing, which is repeated periodically many times over many months. Each cycle of subculturing is referred to as a “passage.” The original cells can yield millions of stem cells. At any stage in the process, batches of cells can be frozen and shipped to other laboratories for further culture and experimentation.
How do you “reprogram” regular cells to make iPSCs?
Differentiated cells, such as skin cells, can be reprogrammed back into a pluripotent state. Reprogramming is achieved over several weeks by forced expression of genes that are known to be master regulators of pluripotency. At the end of this process, these master regulators will remodel the expression of an entire network of genes. Features of differentiated cells will be replaced by those associated with the pluripotent state, essentially reversing the developmental process.
How are stem cells stimulated to differentiate?
As long as the pluripotent stem cells are grown in culture under appropriate conditions, they can remain undifferentiated.
To generate cultures of specific types of differentiated cells, scientists may change the chemical composition of the culture medium, alter the surface of the culture dish, or modify the cells by forcing the expression of specific genes. Through years of experimentation, scientists
have established some basic protocols, or “recipes,” for the differentiation of pluripotent stem cells into some specific cell types (see Figure 1 below).
Figure 1. Directed differentiation of mouse embryonic stem cells.
What laboratory tests are used to identify stem cells?
At various points during the process of generating stem cell lines, scientists test the cells to see whether they exhibit the fundamental properties that make them stem cells. These tests may include:
- Verifying expression of multiple genes that have been shown to be important for the function of stem cells.
- Checking the rate of proliferation.
- Checking the integrity of the genome by examining the chromosomes of selected cells.
- Demonstrating the differentiation potential of the cells by removing signals that maintain the cells in their undifferentiated state, which will cause pluripotent stem cells to spontaneously differentiate, or by adding signals that induce adult stem cells to differentiate into appropriate cell phenotypes.
Could stem cell therapy help cure osteoarthritis?
What are stem cells and how can they help?
What if it were possible to stop the degradation of the cartilage and replace the chondrocytes? This is what stem cell therapy hopes to achieve. Adult stem cells are a special type of cell that is scattered throughout the body. These cells are responsible for helping regenerate our tissues.
A good example of this regeneration is red blood cells. We make 200 billion new red blood cells every day and the adult stem cells that are responsible for giving rise to red blood cells are found in the bone marrow. If stem cells naturally replenish our tissues can they be used to replenish the chondrocytes in patients with osteoarthritis
Scientists have been investigating the use of mesenchymal stromal cells to treat osteoarthritis. These adult stem cells give rise to bone, cartilage, connective and fat tissues and are also found in the bone. The mesenchymal stromal cells are thought to help treat osteoarthritis in a number of different ways. The first is that these cells may mature into functional chondrocytes that are capable of replenishing cartilage and replacing the damaged tissues. Mesenchymal stem cells are capable of stopping inflammation and tamping down the immune response. Researchers think this is important in helping to combat osteoarthritis because it prevents the chondrocytes that are already in the joint from dying.
Scientist use bone marrow transplants to help treat arthritis
In a recent Canadian study published in Stem Cells Translational Medicine, scientists wanted to know if it would be safe to use mesenchymal stromal cells to treat patients with osteoarthritis. This was the first clinical trial of its kind to take place in Canada.
This clinical trial was a phase I/II trial which means the main goal of the study was to determine whether or not stem cell treatment was safe. The group enrolled 12 patients (seven men and five women) into the trial. They were aged 40-65 and had with osteoarthritis in the knee. The scientists then extracted bone marrow from the hip of each patient. They then isolated the mesenchymal stromal cells and grew these cells on a petri dish until there were enough cells to transplant back into the patient. Each patient received their own cells via an injection into the knee. The patients were split into four groups with each receiving a different number of cells. The patients were monitored for up to two years after injection.
Stem cell therapy is a safe treatment for knee osteoarthritis
The scientists found that the stem cell injections were safe, there were no serious side effects and only four of the patients suffered pain or swelling at the injection site which cleared without intervention. The scientist asked the patients to report on their perceived pain and they were able to show a significant improvement after treatment. This study does not account for the placebo effect however, as the scientist did not include a cell-free control group. This makes it difficult to determine whether the improvement in pain management was genuine.
The researchers also used magnetic resonance images (MRI) of the knee and found no improvements to the cartilage after 12 months. The researchers then screened blood and urine to detect molecules associated with chondrocyte degradation. The level of these molecules remained constant over a 12-month period indicating that the stem cells may have a protective effect on the chondrocytes. The scientists also found a decrease in pro-inflammatory markers present in the knee. This suggests that the stem cells may suppress the immune responses in this tissue. Although the data presented in this study are intriguing, only twelve patients were observed. This small sample size makes it difficult to draw relevant conclusions about the broader population.