Stem cells are the foundation for every organ and tissue in your body. There are many different types of stem cells that come from different places in the body or are formed at different times in our lives.
These include embryonic stem cells that exist only at the earliest stages of development and various types of tissue-specific (or adult) stem cells that appear during fetal development and remain in our bodies throughout life.
All stem cells can self-renew (make copies of themselves) and differentiate (develop into more specialized cells).
Beyond these two critical abilities, though, stem cells vary widely in what they can and cannot do and in the circumstances under which they can and cannot do certain things. This is one of the reasons researchers use all types of stem cells in their investigations.
In this section:
- Embryonic stem cells
- Tissue-specific stem cells
- Mesenchymal stem cells
- Induced pluripotent stem cells
Embryonic stem cells
Embryonic stem cells are obtained from the inner cell mass of the blastocyst, a mainly hollow ball of cells that, in the human, forms three to five days after an egg cell is fertilized by a sperm.
A human blastocyst is about the size of the dot above this “i.”
In normal development, the cells inside the blastocyst divide for a short time, then begin developing into more specialized cells that give rise to the entire body—all of our tissues and organs.
Scientists can extract the inner cell mass and grow these in the lab. These are embryonic stem cells, and under the right conditions, they can grow indefinitely in the lab.
Embryonic stem cells are pluripotent, meaning they can give rise to every cell type in the fully formed body, but not the placenta and umbilical cord.
These cells are incredibly valuable because they provide a renewable resource for studying normal development and disease, and for testing drugs and other therapies.
Human embryonic stem cells have been derived primarily from blastocysts created by in vitro fertilization (IVF) for assisted reproduction that were no longer needed.
Tissue-specific stem cells
Tissue-specific stem cells (also referred to as somatic or adult stem cells) are more specialized than embryonic stem cells.
Typically, these stem cells can generate different cell types for the specific tissue or organ in which they live.
For example, blood-forming (or hematopoietic) stem cells in the bone marrow can give rise to red blood cells, white blood cells and platelets.
However, blood-forming stem cells don’t generate liver or lung or brain cells, and stem cells in other tissues and organs don’t generate red or white blood cells or platelets.
Some tissues and organs within your body contain small caches of tissue-specific stem cells whose job it is to replace cells from that tissue that are lost in normal day-to-day living or in injury, such as those in your skin, blood, and the lining of your gut.
Tissue-specific stem cells can be difficult to find in the human body, and they don’t seem to self-renew in culture as easily as embryonic stem cells do.
However, study of these cells has increased our general knowledge about normal development, what changes in aging, and what happens with injury and disease.
Mesenchymal Stem Cells
You may hear the term “mesenchymal stem cell” or MSC to refer to cells isolated from stroma, the connective tissue that surrounds other tissues and organs.
Cells by this name are more accurately called “stromal cells” by many scientists. The first MSCs were discovered in the bone marrow and were shown to be capable of making bone, cartilage and fat cells. Since then, they have been grown from other tissues, such as fat and cord blood.
Various MSCs are thought to have stem cell, and even immunomodulatory, properties and are being tested as treatments for a great many disorders, but there is little evidence to date that they are beneficial.
Scientists do not fully understand whether these cells are actually stem cells or what types of cells they are capable of generating.
They do agree that not all MSCs are the same, and that their characteristics depend on where in the body they come from and how they are isolated and grown.
Induced pluripotent stem cells
Induced pluripotent stem (iPS) cells are cells that have been engineered in the lab to behave like embryonic stem cells.
IPS cells are critical tools to help scientists learn more about normal development and disease onset and progression, and they are also useful for developing and testing new drugs and therapies.
While iPS cells share many of the same characteristics of embryonic stem cells, including the ability to give rise to all the cell types in the body, they aren’t exactly the same. Scientists are exploring what these differences are and what they mean.
For one thing, the first iPS cells were produced by using viruses to insert extra copies of genes into tissue-specific cells.
Researchers are experimenting with many alternative ways to create iPS cells so that they can ultimately be used as a source of cells or tissues for medical treatments.
What are the similarities and differences between embryonic and adult stem cells?
Human embryonic and adult stem cells each have advantages and disadvantages regarding potential use for cell-based regenerative therapies.
One major difference between adult and embryonic stem cells is their different abilities in the number and type of differentiated cell types they can become. Embryonic stem cells can become all cell types of the body because they are pluripotent.
Adult stem cells are thought to be limited to differentiating into different cell types of their tissue of origin.
Embryonic stem cells can be grown relatively easily in culture. Adult stem cells are rare in mature tissues, so isolating these cells from an adult tissue is challenging, and methods to expand their numbers in cell culture have not yet been worked out.
This is an important distinction, as large numbers of cells are needed for stem cell replacement therapies.
Scientists believe that tissues derived from embryonic and adult stem cells may differ in the likelihood of being rejected after transplantation.
We don’t yet know for certain whether tissues derived from embryonic stem cells would cause transplant rejection, since relatively few clinical trials have tested the safety of transplanted cells derived from hESCS.
Adult stem cells, and tissues derived from them, are currently believed less likely to initiate rejection after transplantation.
This is because a patient’s own cells could be expanded in culture, coaxed into assuming a specific cell type (differentiation), and then reintroduced into the patient.
The use of adult stem cells and tissues derived from the patient’s own adult stem cells would mean that the cells are less likely to be rejected by the immune system.
This represents a significant advantage, as immune rejection can be circumvented only by continuous administration of immunosuppressive drugs, and the drugs themselves may cause deleterious side effects.
What are the potential uses of human stem cells and the obstacles that must be overcome before these potential uses will be realized?
There are many ways in which human stem cells can be used in research and the clinic. Studies of human embryonic stem cells will yield information about the complex events that occur during human development.
A primary goal of this work is to identify how undifferentiated stem cells become the differentiated cells that form the tissues and organs.
Scientists know that turning genes on and off is central to this process. Some of the most serious medical conditions, such as cancer and birth defects, are due to abnormal cell division and differentiation.
A more complete understanding of the genetic and molecular controls of these processes may yield information about how such diseases arise and suggest new strategies for therapy.
Predictably controlling cell proliferation and differentiation requires additional basic research on the molecular and genetic signals that regulate cell division and specialization.
While recent developments with iPS cells suggest some of the specific factors that may be involved, techniques must be devised to introduce these factors safely into the cells and control the processes that are induced by these factors.
Human stem cells are currently being used to test new drugs. New medications are tested for safety on differentiated cells generated from human pluripotent cell lines.
Other kinds of cell lines have a long history of being used in this way. Cancer cell lines, for example, are used to screen potential anti-tumor drugs.
The availability of pluripotent stem cells would allow drug testing in a wider range of cell types. However, to screen drugs effectively, the conditions must be identical when comparing different drugs.
Therefore, scientists must be able to precisely control the differentiation of stem cells into the specific cell type on which drugs will be tested.
For some cell types and tissues, current knowledge of the signals controlling differentiation falls short of being able to mimic these conditions precisely to generate pure populations of differentiated cells for each drug being tested.
Perhaps the most important potential application of human stem cells is the generation of cells and tissues that could be used for cell-based therapies.
Today, donated organs and tissues are often used to replace ailing or destroyed tissue, but the need for transplantable tissues and organs far outweighs the available supply.
Stem cells, directed to differentiate into specific cell types, offer the possibility of a renewable source of replacement cells and tissues to treat diseases including macular degeneration, spinal cord injury, stroke, burns, heart disease, diabetes, osteoarthritis, and rheumatoid arthritis.