Stem Cells

Unlike a regular cell, which can only replicate to create more of its own kind of cell, stem cells can develop into any one of several cell types. Stem cells also have the capability to self-renew – they can reproduce themselves many times over. Stem cells in the embryo develop into all the different types of cells of the body. Adult stem cells can only form some cell types.

Human Stem Cell. Source:Wikimedia Commons

Types

There are two types of stem cells: embryonic stem cells and adult stem cells. Embryonic stem cells come from an embryo. An embryo is the mass of cells in the earliest stage of human development that, if implanted in a woman's womb, will eventually grow into a fetus. When the embryo is between three and five days old, it contains stem cells, which are busily working to create the various organs and tissues that will make up the fetus.

Adults have stem cells in the heart, brain, bone marrow, lungs and other organs. These adult stem cells are the built-in repair kits, regenerating cells damaged by disease, injury and everyday wear and tear.

Description

Embryonic stem cells are present in the blastocyst, which is an embryo that has developed for three to five days. A blastocyst is a mass of approximately 100 cells. The stem cells are the inner cells of the blastocyst.

Role of Stem Cells in the Body

Embryonic stem cells are considered to be blank slates, meaning that their fate is undecided. However, they can develop into any cell, any tissue and any organ in the human body. Thus, embryonic stem cells are pluripotent. Pluripotent cells can develop into any 1 of the 220 different human cell types.

Stem cells make up the germ layers of the developing embryo. Human embryos have three germ layers: the ectoderm, endoderm, and mesoderm. The ectoderm is the outermost germ layer, and stem cells in this layer develop into skin and the nervous system. Cells in the innermost layer, the endoderm, develop into cells that line the gastrointestinal system and glands. The mesoderm lies between the two other germ layers. Stem cells in the mesoderm develop into the heart, many large blood vessels, bone, muscle, the urinary system, the reproductive system, as well as many other tissues and organs.

Adult stem cells were once believed to be more limited than embryonic stem cells, only giving rise to the same type of tissue from which they originated. In this way, they help regenerate and replenish cells that have die or that have become injured. Many cells in the human body need to be regenerated. Bone marrow stem cells supply the body with red blood cells, which have a lifespan of approximately 120 days. Some stem cells develop into cells that line the stomach and intestines, areas where cells are continually dying. However, research has revealed that some adult stem cells are pluripotent and others are multipotent progenitor cells. The latter cells have the potential to generate other types of cells, although they cannot develop into as many different cell types as pluripotent stem cells. For example, multipotent progenitor liver cells may be coaxed, in a lab, to produce insulin just like cells in the pancreas. These liver cells, however, cannot develop into bone, nerve cells, or many other tissues in the body. The ability of cells to change function is called plasticity.

Research

Both embryonic and adult stem cells are being research intensively for their medicinal potential. Embryonic stem cells are studied by removing the cells from the bastocyst, or even earlier stages of embryo development, and culturing them (grow them in a nutrient-rich solution) in petri dishes. After the cells have replicated several times and are becoming too numerous for the culture dish, they are removed and placed into several other dishes. In just a few months, several stem cells can become millions of stem cells. Embryonic stem cells that have been cultured for several months without differentiating are referred to as a stem cell line. Cell lines can be frozen and shared between laboratories.

Adult stem cells are much harder for scientists to work with because they are more difficult to extract and culture than their embryonic counterparts. Stem cells not only are hard to find in adult tissue, but scientists also have difficulty getting them to replicate in the laboratory. Stem cells derived from bone marrow or umbilical cord blood have the greatest potential as medicinal agents.

Applications

Stem cells are already being used in the treatment of some diseases. Bone marrow stem cells are used to replenish blood cells during treatment of bone or blood cancers. This approach works well because bone marrow cells naturally develop into blood cells. However, coaxing stem cells to develop into cells that they would normally not develop into is a challenge. Yet, having a stem cell line that could develop into many different cells and tissues would be useful because many tissues, such as the brain or heart, do not continuously regenerate. If scientists can overcome this challenge, stem cells will have much medicinal potential.

First, pluripotent stem cells can be used to test new medications for safety and effectiveness. A medication could be tried out on a specific type of cell to gauge its response far more quickly than it could be tested in clinical trials. For example, scientists could use a cancer stem cell line to investigate whether a new anti-tumor drug stopped the cancer from growing.

Stem cells could also be used to repair cells or tissues that have been damaged by disease or injury. This type of treatment is known as cell-based therapy. One potential application is to inject embryonic stem cells into the heart to repair cells that have been damaged by a heart attack. In one Mayo Clinic study, researchers induced a heart attack in rats, and then injected rodent embryonic stem cells into the damaged hearts. Eventually, the stem cells regenerated the damaged muscle tissue, improving the rats' heart function.

Stem cells may also one day be used to repair brain cells in patients with Parkinson disease. The brains of people with Parkinson disease lack nerve cells that release a chemical messenger called dopamine. Without dopamine, their movements become jerky and uncoordinated, and they suffer from uncontrollable tremors. In studies, researchers have injected rodent embryonic stem cells into the brain of rats with Parkinson disease. The stem cells generated dopamine-producing nerve cells, improving the rats' condition.

Eventually, scientists might even be able to grow entire organs in a laboratory to replace ones that have been damaged by disease. They would create a scaffold out of a biodegradable polymer to shape the organ, and then seed it with either embryonic or adult stem cells. Growth factors specific to the organ would be added to guide the organ's development. The tissue-covered scaffold would then be implanted into the patient. As the tissue grew from the stem cells, the scaffold would degrade, leaving a complete ear, liver or other organ. In 2008, researchers built a functioning rat heart in a petri dish.^[1] In the procedure, the cells inside rat hearts were removed, leaving behind the outermost layer of the heart. Stem cells from other rats were infused into the empty cavities of the hearts. Under the appropriate electrical stimulation and the influence of growth factors, the cells eventually integrated with the heart shells and formed fully-functioning hearts.

Some other diseases that may one day be treated with cell-based therapy include the following:

• diabetes
• heart disease
• spinal cord injury
• burns
• Alzheimer disease
• vision loss

Controversy

Stem cell research has become one of the biggest issues dividing the scientific and religious communities around the world. At the core of the issue is the question of "When does life begin?"

To get stem cells, scientists either have to use an embryo that has already been conceived, or clone an embryo using a cell from a patient's body and a donated egg. Either way, to harvest an embryo's stem cells, scientists must destroy it. Although that embryo may only contain four or five cells, some religious leaders say that destroying it is the equivalent of taking a human life.

Even as scientists move forward in their understanding of stem cells and their ability to manipulate them, the ethical and political debates rage on. Many governments have placed tight restrictions on stem cell research or have tightly limited funding for it.

To bridge the debate, scientists are exploring less controversial avenues of research, using adult stem cells that are trained to act like embryonic stem cells, instead of creating a new embryo. Although they are not as pluripotent as embryonic stem cells, new research suggests that adult stem cells might be more flexible than scientists once imagined. Even if the outcome of the debate favors the use of stem cells, it will likely be at least a few more decades until stem cell therapies come into widespread use.

Related Videos

Dr. Story Landis is Director of the National Institute of Neurological Disorders and Stoke, and the chair of the NIH Stem Cell Task Force. She reminds that some cell-based therapies, like bone marrow transplants, are already being used.


Co-Director of Harvard Stem Cell Institute David Scadden reveals how 1960’s research into surviving radiation stumbled upon stem cells, in this video from BigThink:

Dr. Scadden also explores the hypothesis that cancer grows from stem cells and how future cancer treatment might focus on stem cells, in this additional BigThink video:

David Scadden talks about Shinya Yamanaka’s stem cell research could pave the way for peace between scientists and ethicists, in this BigThink video "Ending the Stem Cell Debate Forever"

Dr. Scadden envisions a future where medicines activate stem cells already in our bodies, and explains this concept in the video below:

Like stem cells, immature progenitor cells have the unique ability to build brain tissue from a single cell. NIH researchers have discovered how to get these cells in a line, light them up and, using a computer-driven system, split them out.

References

1. ↑ Ott HC, Matthiesen TS, Goh SK et al. Perfusion-decellularized matrix: using nature's platform to engineer a bioartificial heart. Nat Med. 2008 Feb;14(2):213-21. Abstract | Press Release