Stem Cell Generation: How Scientists Create Stem Cells
Explore the scientific methods for making stem cells available, from their isolation in tissues to their creation and cultivation in a laboratory setting.
Stem cells possess the unique ability of self-renewal and differentiation into various specialized cells. These properties make them a focal point of research for understanding development and creating potential new therapies. The methods for obtaining these cells vary depending on the stem cell type, each involving specific scientific procedures.
Derivation of Embryonic Stem Cells
Embryonic stem cells (ESCs) are pluripotent, meaning they can develop into any cell type in the body. They are sourced from the inner cell mass of a blastocyst, an early-stage embryo about five to six days old. The blastocysts used for this purpose are donated from in vitro fertilization (IVF) procedures with informed consent from the donors.
Deriving an ESC line begins with isolating the inner cell mass. The outer layer of the blastocyst, the trophectoderm, is carefully removed using antibodies or precision laser technology. This process destroys the blastocyst.
The isolated inner cell mass is placed onto a culture plate with a nutrient medium that supports cell survival and proliferation. The ESCs multiply and divide, establishing a population of undifferentiated cells called a stem cell line. This line can be maintained and grown for research.
Isolation and Collection of Adult Stem Cells
Adult stem cells, or somatic stem cells, are found in various tissues and act as an internal repair system. Unlike ESCs, adult stem cells are multipotent, meaning their ability to differentiate is limited to the cell types of their tissue of origin. Their generation involves isolation and purification from mature tissues.
A primary source is bone marrow, which contains hematopoietic stem cells that form all blood cells. One collection method is bone marrow aspiration, where a needle withdraws marrow from the hip bone. Another technique, apheresis, draws blood, separates the stem cells with a machine, and returns the remaining blood to the donor. Apheresis is often used after medication has increased the number of stem cells in the blood.
Adipose (fat) tissue is another source for mesenchymal stem cells (MSCs), collected through liposuction. In the lab, the tissue is washed and treated with enzymes like collagenase to break down the extracellular matrix and free the stem cells. The mixture is then filtered and centrifuged to separate the stem cell population.
Generating Induced Pluripotent Stem Cells (iPSCs)
Induced pluripotent stem cells (iPSCs) are created by reprogramming specialized adult cells, like skin or blood cells, to revert to an embryonic-like state. This process allows for the creation of pluripotent cells genetically matched to an individual, which is useful for studying diseases and developing personalized therapies.
The technique was pioneered by Shinya Yamanaka, who found that introducing four specific genes—Oct4, Sox2, Klf4, and c-Myc—could transform adult cells. These “Yamanaka factors” are delivered into adult cells using a vector, such as a retrovirus, that integrates the genes into the cells’ DNA. The cells are then cultured under conditions that support pluripotency.
The reprogramming process is slow, taking several weeks, with only a small fraction of cells successfully converting. The resulting iPSC colonies behave much like embryonic stem cells, able to self-renew and differentiate into all three primary germ layers.
For safety in clinical applications, scientists have developed non-integrating methods for delivering the factors. These methods include using Sendai viruses or directly delivering synthetic mRNA or proteins into the cells. These techniques avoid the risk of genetic mutations from viral integration, making iPSCs safer for therapeutic use.
Culturing and Expansion of Stem Cell Lines
After stem cells are derived or isolated, they must be grown in the laboratory to produce the large quantities needed for research or therapeutic applications. This process, known as in vitro culture, requires highly specialized conditions to maintain the cells’ defining properties. Without the precise environment, the cells may die, differentiate spontaneously, or undergo genetic changes.
Stem cells are grown in culture dishes on a substrate that provides a surface for them to attach and grow. They are bathed in a complex culture medium—a liquid mixture of nutrients, salts, and proteins. This medium is supplemented with specific growth factors that instruct the cells to remain in their undifferentiated state.
As the cells multiply, they must be periodically subcultured, or “passaged.” This involves detaching the cells and transferring a small number to a new dish with fresh medium, allowing for the generation of billions of cells. Researchers must constantly monitor the cultures for contamination and test the cells to ensure they have maintained their genetic stability.
Ethical Considerations in Stem Cell Sourcing
The sourcing of embryonic stem cells is contentious because it involves the destruction of a human blastocyst. This raises fundamental questions about the moral status of the embryo. The debate centers on when human life begins and whether an embryo should be considered a person with rights.
To navigate these concerns, many countries have established regulations. A common framework permits using embryos from IVF that are no longer needed for reproduction and are donated with informed consent. The use of existing, established ESC lines is also encouraged to minimize the destruction of additional embryos.
The development of iPSCs avoids using embryos, bypassing some of these ethical issues. However, iPSC technology has its own considerations, such as the ability to create human gametes from iPSCs, which opens up complex questions about reproduction. The sourcing of adult stem cells, while less controversial, still requires strict informed consent from the donor.