Stem cells are special types of cells which have the characteristic ability to renew themselves through mitotic cell division and differentiating into a diverse range of specialized cell types.
[Mitosis is a kind of cell division in which two similar cells are produced from one cell.]
Stem cells are present in most multi-cellular organisms.
The two broad types of mammalian stem cells are:
Embryonic stem cells
These cells are isolated from the inner cell mass of blastocysts in the embryo. In a developing embryo, stem cells can differentiate into all of the specialized embryonic tissues.
Adult stem cells
These are found in adult tissues and participate in the repair system for the body, replenish specialized cells and maintain the normal turnover of regenerative organs, such as blood, skin or intestinal tissues.
Stem cells can now be grown and transformed into specialized cells with characteristics consistent with cells of various tissues such as muscles or nerves through cell culture.
Highly plastic adult stem cells from a variety of sources, including umbilical cord blood and bone marrow, are routinely used in medical therapies. Embryonic cell lines and autologous embryonic stem cells generated through therapeutic cloning are promising candidates for future therapies.
Stem Cell Properties
The classical definition of a stem cell requires that it possess two properties:
- Self-renewal: The ability to go through numerous cycles of cell division while maintaining the undifferentiated state.
- Potency: The capacity to differentiate into specialized cell types. These can be
Totipotent or omnipotent stem cells
These cells can differentiate into embryonic and extraembryonic cell types. Such cells can construct a complete, viable, organism. These cells are produced from the fusion of an egg and sperm cell.
Pluripotent stem cells
These are the descendants of totipotent cells and can differentiate into nearly all cells i.e. cells derived from any of the three germ layers.
Multipotent stem cells
These can differentiate into a number of cells, but only those of a closely related family of cells.
Oligopotent stem cells
These cells can differentiate into only a few cells, such as lymphoid or myeloid stem cells.
These cells can produce only one cell type, their own, but have the property of self-renewal.
Embryonic Stem Cells
Embryonic stem cells, as their name suggests, are derived from embryos.
Specifically, embryonic stem cells are derived from embryos that develop from eggs that have been fertilized in vitro—in an in vitro fertilization clinic—and then donated for research purposes with informed consent of the donors.
They are not derived from eggs fertilized in a woman’s body. The embryos from which human embryonic stem cells are derived are typically four or five days old and are a hollow microscopic ball of cells called the blastocyst.
The blastocyst includes three structures
- Trophoblast, which is the layer of cells that surrounds the blastocyst
- Blastocoel, which is the hollow cavity inside the blastocyst
- Inner cell mass, which is a group of approximately 30 cells at one end of the blastocoel.
Embryonic stem cells can develop into each of the more than 200 cell types of the adult body when given sufficient and necessary stimulation for a specific cell type.
Most of the research on embryonic stem cell has been done on mouse embryonic stem cells or human embryonic stem cells.
A human embryonic stem cell is also defined by the presence of several transcription factors and cell surface proteins.
The cell surface antigens most commonly used to identify human embryonic stem cells are
- Glycolipids SSEA3 and SSEA4
- Keratan sulfate antigens Tra-1-60 and Tra-1-81.
Embryonic stem cells, being pluripotent cells, require specific signals for correct differentiation. Because of their combined abilities of unlimited expansion and pluripotency, embryonic stem cells remain a theoretically potential source for regenerative medicine and tissue replacement after injury or disease.
This video is about eight minutes long and carries a good explanation about stem cells.
Adult Stem Cells
An adult stem cell is an undifferentiated cell found among differentiated cells in a tissue or organ. These cells can renew themselves and can differentiate to yield the major specialized cell types of the tissue or organ.
The primary role of the adult stem cell in a living organism is to maintain and repair the tissue in which it is found.
They are also termed as somatic stem cells. The origin of adult stem cells in mature tissues is unknown.
Adult stem cells have been found in many more tissues than they once thought possible. In the right environment, certain kinds of adult stem cells seem to have the ability to differentiate into a number of different cell types, given the right conditions.
There are a very small number of stem cells in each tissue. They may remain non-dividing for many years until they are activated by disease or tissue injury.
The adult tissues reported to contain stem cells include brain, bone marrow, peripheral blood, blood vessels, skeletal muscle, skin and liver.
Struggle, however, is to grow adult stem cells in cell culture and manipulate them to generate specific cell types so they can be used to treat injury or disease.
How To Identify Adult Stem Cells?
One or more of the following three methods are used to identify adult stem cells
- Labeling the cells in a living tissue with molecular markers and then determining the specialized cell types they generate
- Removing the cells from a living animal, labeling them in cell culture, and transplanting them back into another animal to determine whether the cells repopulate their tissue of origin
- Isolating the cells, growing them in cell culture, and manipulating them, often by adding growth factors or introducing new genes, to determine what differentiated cells types they can become.
Growing cells in the laboratory is known as cell culture.
- Embryonic stem cells are isolated by transferring the inner cell mass into a plastic laboratory culture dish that contains a nutrient broth known as culture medium.
- The cells divide and spread over the surface of the dish. The inner surface of the culture dish is typically coated with mouse embryonic skin cells that have been treated so they will not divide. This coating layer of cells is called a feeder layer. The reason for having the mouse cells in the bottom of the culture dish is to give the inner cell mass cells a sticky surface to which they can attach. Also, the feeder cells release nutrients into the culture medium.
- Over the course of several days, the cells of the inner cell mass proliferate. When this occurs, they are removed gently and plated into several fresh culture dishes. The process of replating the cells is repeated many times and for many months, and is called subculturing.
- After six months or more, the original 30 cells of the inner cell mass yield millions of embryonic stem cells. Embryonic stem cells that have proliferated in cell culture for six or more months without differentiating, are pluripotent, appear genetically normal and are referred to as an embryonic stem cell line.
- Once cell lines are established, or even before that stage, batches of them can be frozen and shipped to other laboratories for further culture and experimentation.
Adult Stem cells versus Embryonic Stem Cells
Apart from the differences in sources, human embryonic and adult stem cells each offer advantages and disadvantages when channeled for potential use for cell-based regenerative therapies.
They differ in number and type of differentiated cell types they can become.
While embryonic stem cells can become all cell types of the body because they are pluripotent, adult stem cells are limited to differentiating into their tissue of origin.
Embryonic stem cells can be grown relatively easily in culture. Adult stem cells are rare and isolating these cells from an adult tissue is difficult. Moreover, ways to increase their population in cell culture is yet to be found.
This is an important difference, as large numbers of cells are needed for stem cell replacement therapies.
Embryonic and adult stem cells may differ in the likelihood of being rejected after transplantation. Though not supported by any scientific evidence, 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 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 serious side effects.
Fetal Stem Cells
Research with fetal tissue so far has been limited to only a few cell types: neural stem cells, including neural crest cells; hematopoietic stem cells; and pancreatic islet progenitors.
Neural stem cells, which are numerous in the fetal brain, can be isolated and grown in an undifferentiated form in culture, and they have been shown to differentiate into the three main types of brain cells.
These cells have been used in rodent models of Parkinson’s disease.
The fetal liver and blood are rich sources of hematopoietic stem cells, which are responsible for generating multiple cell types in blood, but their properties have not been extensively investigated. The umbilical cord and placenta are also rich sources of hematopoietic stem cells.