Stem cells explained: What are they, and how do they work?

What are stem cells and how do they work?

There are three types of stem cells. Each has potential for medical research and clinical applications based on its unique properties.

Peter Hoey, Special To The Chronicle

Stem cells are the building blocks of the human body. At the start of life, they divide over and over again to create a full person from an embryo. As we age, they replenish cells in our blood, bone, skin and organs. Stem cells could be powerful tools in treating injury and illness.

Embryonic Stem Cells

The first cells to form after a sperm fertilizes an egg.

Blank slate cells: can become every other kind of cell in the body.

Can divide and multiply endlessly.

Controversial in medicine because embryos must be destroyed to obtain stem cells.

Adult Stem Cells

Mature stem cells that replenish blood, skin, gut and some other cells.

In some cases, can replace cells damaged by illness or injury.

Limited ability to become other types of cells.

Limited ability to divide and multiply.

Induced-pluripotent Stem Cells

Adult cells that are reprogrammed to look and act like embryonic stem cells.

Can be made from skin, blood and other adult cells.

From their embryonic-like state, can be further altered to become any other type of cell.

Good potential use in medicine, but still a new area of research.

Embryonic

What are they?

Embryonic stem cells are the starter cells of the human body. They are undifferentiated, which means they have not matured and specialized, and they are able to become any other kind of cell in the body.

In embryos, these cells multiply and differentiate to become organs, bones and muscles. In the laboratory, they can be multiplied to create stem cell lines for study or for therapy.

Scientists harvest embryonic stem cells from three- to five-day-old embryos donated by people who have gone through in-vitro fertilization. Scientists isolated the first human embryonic stem cells in 1998.

What makes them different from other stem cells?

These are the only stem cells that naturally are able to become any other cell type and to multiply endlessly. Under the right circumstances in a lab, they can be nudged to become specific cell types.

Why do these characteristics give these cells medical potential?

Because of their ability to differentiate and multiply, embryonic stem cells long were thought to be the most powerful, and thus have the most potential for treating injury and disease. If scientists are able to control how they differentiate and how often, embryonic stem cells could be used to replace any damaged part of the body from missing insulin-making cells in people with Type 1 diabetes to brain cells lost in Parkinsons disease or skin cells scarred by burns.

What are the limitations of these therapies?

Many people have ethical problems using human embryos for scientific study. Also, embryonic stem cells ability to replicate endlessly means they may develop mutations that can interfere with their growth or allow them to keep dividing to the point of causing harm. Finding the right medical applications for embryonic stem cells is challenging.

Adult

What are they?

Adult stem cells are so-named because they are more mature than embryonic stem cells, though they dont necessarily have to come from adults. Their maturity means that they are limited in their ability to differentiate. Pockets of adult stem cells are found in many of our organs and they replenish cells in the organs in which they reside. Types of adult stem cells include:

Hematopoietic

Found in bone marrow and umbilical cord blood, they become blood and immune cells. They are the only stem cells approved by the FDA for therapy, for treatment of certain blood cancers.

Mesenchymal

These cells are found throughout the body, including in bone marrow, fat tissue and organs. They turn into the connective tissue found throughout the body, though the specific cell they become is related to the organ in which theyre located. These stem cells may decrease inflammation.

Fetal

Stem cells from fetuses are more mature, and therefore less able to differentiate, than embryonic stem cells, but they may be more multipurpose than other adult stem cells. For example, neural stem cells from fetal brain tissue can become several kinds of neurons, but neural stem cells from the adult brain are rare and have very limited ability to differentiate.

What makes them different from other stem cells?

Adult stem cells are limited in their abilities. They can only become certain types of cells they are called multi-potent, instead of pluripotent, for that reason and there is a limit to how often they can divide.

Why do these characteristics give these cells medical potential?

Adult stem cells are less powerful than embryonic, but they are easier to use, since all humans have their own supply of these cells. They may be useful for reducing inflammation.

What are the limitations of these therapies?

Its unclear how useful these stem cells could be given their limited abilities. Though the idea of tapping into a persons own source of adult stem cells and using them for treatment is appealing, these cells cannot repair serious injuries or replace cells lost to disease, like neurons or insulin-producing cells.

Induced-pluripotent

What are they?

Induced-pluripotent stem (IPS) cells are adult cells often skin or blood cells that have been taken from an individual and reprogrammed in a lab to become like embryonic stem cells. Then, like embryonic stem cells, they can be developed into any other type of cell. So a skin cell could be turned into an embryonic-like cell and then further turned into a heart cell.

What makes them different from other stem cells?

Like embryonic stem cells, except they are manufactured in a lab. And since they come from an individual, they are an exact match to that individual. Scientists are still studying whether IPS cells could be used interchangeably with embryonic stem cells.

Why do these characteristics give these cells medical potential?

They could be used to replace cell types lost to disease or injury. In addition, IPS cells can be used to study human diseases in Petri dishes or in animals. Scientists can take skin cells from a person with a genetic mutation, convert those cells to IPS cells, then study those cells as a living model of how the mutation functions.

What are the limitations of these therapies?

Making IPS cells can be a time- and resource-consuming process. But mostly, IPS cells have the same limits as embryonic stem cells.

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Stem cells explained: What are they, and how do they work?

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