Stem cells are unique cells within the body that possess two defining characteristics: the ability to self-renew, meaning they can create more copies of themselves, and the capacity to differentiate, which allows them to develop into various specialized cell types. These properties make stem cells a subject of considerable interest in medicine, as they hold potential for repairing damaged tissues, regenerating organs, and advancing our understanding of diseases.
Obtaining Stem Cells from Embryos
Embryonic stem cells (ESCs) are derived from early-stage human embryos, specifically from the inner cell mass of a blastocyst. A blastocyst is a structure that forms approximately five to seven days after an egg is fertilized, typically consisting of about 50 to 150 cells. To obtain ESCs, cells are isolated from this inner cell mass. These isolated cells are then cultured in a laboratory setting.
Embryonic stem cells are characterized by their pluripotent nature, meaning they have the ability to differentiate into any cell type found in the adult body. However, the process of obtaining these cells typically involves the destruction of the blastocyst. This aspect has led to significant ethical debates and concerns, as it raises questions about the moral status of the embryo and the permissibility of its use for scientific purposes.
Obtaining Stem Cells from Adult Tissues
Adult stem cells, also known as somatic stem cells, are found in small numbers within mature tissues throughout the body. Unlike embryonic stem cells, adult stem cells are generally multipotent, meaning they can differentiate into a limited range of cell types, typically those within their tissue of origin. These cells play a role in maintaining and repairing the tissues where they reside, replacing cells lost due to normal turnover or injury.
One common method for obtaining adult stem cells is through bone marrow aspiration, typically from the hip bone (iliac crest). This procedure involves inserting a needle into the bone to extract a small amount of marrow, a traditional source for hematopoietic stem cells used in treating blood disorders. Adipose (fat) tissue is another source, from which stem cells can be isolated after a liposuction procedure. Adipose tissue is an abundant source of mesenchymal stem cells, often yielding more cells than bone marrow for a given volume.
Umbilical cord blood is collected from the umbilical cord and placenta immediately after birth. This non-invasive method provides a rich supply of stem cells. Peripheral blood can also be a source of stem cells, though typically fewer are present. To obtain a sufficient number, a process called apheresis is used, where stem cells are mobilized from the bone marrow into the bloodstream through injections and then collected from the blood.
These methods are generally less ethically controversial than those involving embryos, as they do not involve embryo destruction. However, the cell yield and differentiation potential of adult stem cells can vary depending on the source and the donor’s age.
Creating Induced Pluripotent Stem Cells
Induced pluripotent stem cells (iPSCs) are not directly obtained from natural sources like embryonic or adult stem cells. Instead, they are created in a laboratory by genetically reprogramming specialized adult somatic cells. This innovative process involves introducing specific genes, known as Yamanaka factors (Oct4, Sox2, Klf4, and c-Myc), into cells like skin or blood cells. These genes effectively “reset” the adult cells, causing them to revert to an embryonic-like pluripotent state.
The discovery of iPSCs in 2006 by Shinya Yamanaka and his team revolutionized stem cell research. This technology offers a way to generate pluripotent stem cells without the ethical concerns associated with the use of human embryos. A significant advantage of iPSCs is their potential to create patient-specific stem cell lines. Since these cells are derived from an individual’s own tissues, they can be used for therapies without the risk of immune rejection, which is a common challenge in transplantation. They also serve as valuable tools for disease modeling and drug screening.