Sources and types of stem cells

Cells in the body have specific purposes, but stem cells are cells that do not yet have a specific role and can become almost any cell that is required.

Stem cells are undifferentiated cells that can turn into specific cells, as the body needs them.

Scientists and doctors are interested in stem cells as they help to explain how some functions of the body work, and how they sometimes go wrong.

Stem cells also show promise for treating some diseases that currently have no cure.

Sources of stem cells
Stem cells originate from two main sources: adult body tissues and embryos. Scientists are also working on ways to develop stem cells from other cells, using genetic "reprogramming" techniques.

Adult stem cells
Stem cells can turn into any type of cell before they become differentiated.

A person's body contains stem cells throughout their life. The body can use these stem cells whenever it needs them.

Also called tissue-specific or somatic stem cells, adult stem cells exist throughout the body from the time an embryo develops.

The cells are in a non-specific state, but they are more specialized than embryonic stem cells. They remain in this state until the body needs them for a specific purpose, say, as skin or muscle cells.

Day-to-day living means the body is constantly renewing its tissues. In some parts of the body, such as the gut and bone marrow, stem cells regularly divide to produce new body tissues for maintenance and repair.

Stem cells are present inside different types of tissue. Scientists have found stem cells in tissues, including:

• the brain
• bone marrow
• blood and blood vessels
• skeletal muscles
• skin
• the liver

However, stem cells can be difficult to find. They can stay non-dividing and non-specific for years until the body summons them to repair or grow new tissue.

Adult stem cells can divide or self-renew indefinitely. This means they can generate various cell types from the originating organ or even regenerate the original organ, entirely.

This division and regeneration are how a skin wound heals, or how an organ such as the liver, for example, can repair itself after damage.

In the past, scientists believed adult stem cells could only differentiate based on their tissue of origin. However, some evidence now suggests that they can differentiate to become other cell types, as well.

Embryonic stem cells
From the very earliest stage of pregnancy, after the sperm fertilizes the egg, an embryo forms.

Around 3–5 days after a sperm fertilizes an egg, the embryo takes the form of a blastocyst or ball of cells.

The blastocyst contains stem cells and will later implant in the womb. Embryonic stem cells come from a blastocyst that is 4–5 days old.

When scientists take stem cells from embryos, these are usually extra embryos that result from in vitro fertilization (IVF).

In IVF clinics, the doctors fertilize several eggs in a test tube, to ensure that at least one survives. They will then implant a limited number of eggs to start a pregnancy.

When a sperm fertilizes an egg, these cells combine to form a single cell called a zygote.

This single-celled zygote then starts to divide, forming 2, 4, 8, 16 cells, and so on. Now it is an embryo.

Soon, and before the embryo implants in the uterus, this mass of around 150–200 cells is the blastocyst. The blastocyst consists of two parts:

an outer cell mass that becomes part of the placenta
an inner cell mass that will develop into the human body
The inner cell mass is where embryonic stem cells are found. Scientists call these totipotent cells. The term totipotent refer to the fact that they have total potential to develop into any cell in the body.

With the right stimulation, the cells can become blood cells, skin cells, and all the other cell types that a body needs.

In early pregnancy, the blastocyst stage continues for about 5 days before the embryo implants in the uterus, or womb. At this stage, stem cells begin to differentiate.

Embryonic stem cells can differentiate into more cell types than adult stem cells.

Mesenchymal stem cells (MSCs)
MSCs come from the connective tissue or stroma that surrounds the body's organs and other tissues.

Scientists have used MSCs to create new body tissues, such as bone, cartilage, and fat cells. They may one day play a role in solving a wide range of health problems.

Induced pluripotent stem cells (iPS)
Scientists create these in a lab, using skin cells and other tissue-specific cells. These cells behave in a similar way to embryonic stem cells, so they could be useful for developing a range of therapies.

However, more research and development is necessary.

To grow stem cells, scientists first extract samples from adult tissue or an embryo. They then place these cells in a controlled culture where they will divide and reproduce but not specialize further.

Stem cells that are dividing and reproducing in a controlled culture are called a stem-cell line.

Researchers manage and share stem-cell lines for different purposes. They can stimulate the stem cells to specialize in a particular way. This process is known as directed differentiation.

Until now, it has been easier to grow large numbers of embryonic stem cells than adult stem cells. However, scientists are making progress with both cell types.

Types of stem cells
Researchers categorize stem cells, according to their potential to differentiate into other types of cells.

Embryonic stem cells are the most potent, as their job is to become every type of cell in the body.

The full classification includes:

Totipotent: These stem cells can differentiate into all possible cell types. The first few cells that appear as the zygote starts to divide are totipotent.

Pluripotent: These cells can turn into almost any cell. Cells from the early embryo are pluripotent.

Multipotent: These cells can differentiate into a closely related family of cells. Adult hematopoietic stem cells, for example, can become red and white blood cells or platelets.

Oligopotent: These can differentiate into a few different cell types. Adult lymphoid or myeloid stem cells can do this.

Unipotent: These can only produce cells of one kind, which is their own type. However, they are still stem cells because they can renew themselves. Examples include adult muscle stem cells.

Embryonic stem cells are considered pluripotent instead of totipotent because they cannot become part of the extra-embryonic membranes or the placenta.