What are Stem Cells?

Stem cells are cells found in most, if not all, multi-cellular organisms. They are characterized by the ability to renew themselves through mitotic cell division and differentiating into a diverse range of specialized cell types. Stem cells are cells that have both the capacity to self-renew (divide into more stem cells) and to differentiate (the process by which a less specialized cell becomes a more specialized cell type) into mature, specialized cells.

Stem cells are the “building blocks” for all cells, tissues and organs in the human body and have the unique ability to develop into a variety of cell types during early life. They are like a blank microchip that can be programmed to perform specific tasks such as serving as an internal repair system, dividing nearly without limit. Under the right conditions, stem cells will begin to develop into specialized tissues and organs and can self-renew, or divide and develop into more stem cells. When it divides, each new cell has the potential to remain a stem cell or become a cell with a more specific function, such as a brain cell, a red blood cell or a muscle cell.

Types of stem cells

There are advantages and disadvantages for the potential use of cell-based regenerative therapies using human embryonic and adult stem cells. The primary difference between adult and embryonic stem cells is in the number of differentiated cell types they can develop into. Embryonic stem cells are pluripotent meaning they can become all cell types of the body. Adult stem cells are limited to only differentiating into cell types of the tissues of origin. Why Are Embryonic Stem Cells So Valuable? Under the right conditions, embryonic stem cell lines can be grown and expanded indefinitely. Most importantly, if they are removed and isolated from conditions that keep them in an undifferentiated, or unspecialized, state, these cells are able to form all the different, specialized cell types of the body. Mouse embryonic stem cells are the most widely studied in research today. From them we have learned a great deal about how pluripotent cells grow and specialize as well as how organisms and diseases develop. Mouse embryonic stem cells have, indeed, proven to be an invaluable tool for medical research and biology. Researchers today have discovered many genes associated with a variety of diseases, have been able to model human diseases and have thus been able to treat certain human disorders in animal models. Human embryonic stem cells are more difficult to work with than mouse stem cells and less is known about them today. However, with human embryonic stem cells, researchers are making great strides in learning about early human developmental processes that enable them to model disease and establish strategies that could lead to therapies which will replace or restore damaged tissues.

Not all stem cells are the same. They vary by type and potency. They include Adult stem cells, or “tissue-specific” stem cells; embryonic stem cells which exist only in the earliest stages of embryonic development; and Induced Pluripotent Stem Cells or IPS cells.

Adult Stem Cells
Adult Stem Cells Adult stem cells, also called tissue-specific or somatic stem cells, are found in a given tissue in the body and generate mature cell types within that tissue or organ. By the adult stage, they are already somewhat specialized and, therefore, only produced a limited number of cell types each. Adult stem cells are thus referred to as “multipotent.” Tissue-specific stem cells have been identified that continuously replenish themselves in organs such as the skin, blood and the gut and in other less regenerative organs such as the brain. The stem cells most readily accessible are tissue-specific cells is the bone marrow found in the center of our bones. Within these cells, there are different types of stem cells to include blood stem cells and mesenchymal stem cells that form tissues such as bone, cartilage and fat.

Embryonic Stem Cells
Embryonic stem cells, are referred to as “pluripotent” stem cells and unlike adult stem cells, they can develop into all cell types of the body. These stem cells can be derived from a variety of animals, including humans. The cells used from humans for research are mostly donated with informed consent of donors from eggs that were fertilized in vitro – in an in vitro fertilization clinic for assisted reproduction. They are not derived from eggs that were fertilized in a woman’s body. Typically, the embryonic stem cells are four or five days old and are in the form of blastocysts, or microscopic balls of 150-200 cells. At this stage, there are no organs or even blood. However, the embryonic stem cells are grown from the inner cell mass of the blastocyst . Stem cells from these blastocysts are those that were no longer needed after in vitro fertilization.

Induced Pluripotent Cells or iPS Cells
Recently, researchers have discovered cells with properties similar to embryonic stem cells, referred to as induced pluripotent stem cells (iPS cells). These cells have been engineered from tissue-specific cells that were ‘induced’ from non-pluripotent cells to become pluripotent. In other words, scientists have discovered methods to ‘reprogram” a cell with a specialized function (for example, a skin cell) to an unspecialized state similar to that of an embryonic stem cell. Producing ‘induced pluripotent cells,’ or iPS cells, is another way to create patient or disease-specific pluripotent stem cells. Though not identical, embryonic stem cells and iPS cells share many similar characteristics such as the ability to give rise to the cells of all organs and tissues. In 2007, several groups reported the generation of human iPS cells by inserting extra copies of three to four genes known to be essential to embryonic stem cells into the genome of the specialized adult cells using viruses. The ‘reprogramming’ process is not yet completely understood making ongoing research necessary. A great deal of work remains before methods to generate iPS cells to create patient and disease-specific cell lines are suitable and safe to develop into effective therapies.   

There are advantages and disadvantages for the potential use of cell-based regenerative therapies using human embryonic and adult stem cells. The primary difference between adult and embryonic stem cells is in the number of differentiated cell types they can develop into. Embryonic stem cells are pluripotent meaning they can become all cell types of the body. Adult stem cells are limited to only differentiating into cell types of the tissues of origin.

Why Are Embryonic Stem Cells So Valuable?

Under the right conditions, embryonic stem cell lines can be grown and expanded indefinitely. Most importantly, if they are removed and isolated from conditions that keep them in an undifferentiated, or unspecialized, state, these cells are able to form all the different, specialized cell types of the body. Mouse embryonic stem cells are the most widely studied in research today. From them we have learned a great deal about how pluripotent cells grow and specialize as well as how organisms and diseases develop. Mouse embryonic stem cells have, indeed, proven to be an invaluable tool for medical research and biology. Researchers today have discovered many genes associated with a variety of diseases, have been able to model human diseases and have thus been able to treat certain human disorders in animal models. Human embryonic stem cells are more difficult to work with than mouse stem cells and less is known about them today. However, with human embryonic stem cells, researchers are making great strides in learning about early human developmental processes that enable them to model disease and establish strategies that could lead to therapies which will replace or restore damaged tissues.