Stem Cells: Understanding Types and Therapeutic Potential
Understanding Stem Cells: Types and Applications
A stem cell is an undifferentiated cell that can self-renew and differentiate into various specialized cell types. Stem cells serve as a continuous source for the diverse cells that constitute the tissues and organs of animals and plants. There is significant interest in stem cells due to their powerful potential for developing therapeutic approaches to replace damaged or diseased cells resulting from various disorders and injuries.
Stem cells are totipotent progenitor cells capable of self-renewal and differentiation into multiple cell lineages. While the human body comprises many cells and tissues, these stem cells are the origin of all tissues and organs. Stem cells are identified by their unique ability to regenerate and divide into identical daughter cells, while also giving rise to differentiated cells.
Stem cells play a crucial role in repairing and replacing damaged tissues in the body, possessing significant healing properties. This regenerative capacity is a primary function of stem cells.
Types of Stem Cells
Stem cells are broadly categorized based on their origin and differentiation potential. The main types include:
Embryonic Stem Cells (ESCs)
Embryonic stem cells are derived from the inner cell mass of the blastocyst, a hollow sphere of dividing cells formed at the very beginning of embryonic development. Embryonic stem cells from human embryos and embryos of other mammals can be cultured in vitro.
Human Embryonic Stem Cells (hESCs)
Human embryonic stem cells can form different types of in vitro tissue and develop teratomas when transplanted into immunodeficient mice. While it is not definitively known whether these cells can form all tissue types of the human embryo, it is widely believed that, based on some of their characteristics, they are truly pluripotent cells. Consequently, they are considered a potential source for various cellular therapies, aiming to replace diseased or damaged patient cells with healthy ones. Many cell types, such as dopamine-secreting neurons for Parkinson’s disease treatment and pancreatic beta cells that produce insulin for diabetes treatment, can potentially be produced from embryonic stem cells. Cells for these purposes were previously available only from highly limited sources, such as pancreatic beta cells from cadaveric organ donors.
Adult Stem Cells
These stem cells are obtained from developed organs and tissues. They can repair and replace damaged tissues within the region where they are located. For example, hematopoietic stem cells are found in the bone marrow and are used in bone marrow transplants to treat specific types of cancers.
Other tissues of the adult body, such as the skin epidermis, the small intestine lining, and the bone marrow, constantly renew their cells. These tissues contain resident stem cells, along with a large number of “progenitor cells” that originate from stem cells and undergo a limited number of divisions before differentiating. Stem cells reside in “niches” composed of other cells that produce substances essential for stem cell survival and function. Other tissue types, such as liver tissue, show little cell division or divide cells only when injured. In these tissues, there is almost no dedicated stem cell population, and any cell can contribute to tissue regeneration when needed.
Epithelial Stem Cells
Keratinocytes are cells present in the epidermis of the skin. Only the basal layer, near the dermis, contains dividing cells. While some of these are stem cells, most are progenitor cells. Keratinocytes slowly migrate upwards through the epidermis as they mature, eventually dying and being shed from the skin surface. The epithelium of the small intestine forms finger-like projections called villi, which are connected by invaginations called crypts. Dividing cells are found in crypts, with stem cells located near the base of each crypt. Cells are continuously produced in crypts, migrate to the villi, and are eventually shed into the intestinal lumen.
Hematopoietic Stem Cells (HSCs)
The bone marrow contains hematopoietic stem cells. These produce all types of blood cells, including immune cells. Large numbers of hematopoietic stem cells are found in umbilical cord blood, and smaller numbers are found in peripheral blood. In the bone marrow, hematopoietic stem cells are associated with osteoblasts of trabecular bone and blood vessels. They produce lymphocytes, granulocytes, red blood cells (RBCs), and other cell types, with their differentiation influenced by growth factors in the surrounding microenvironment.
Induced Pluripotent Stem Cells (iPSCs)
These cells have been generated by reprogramming adult somatic cells into an embryonic-like pluripotent state in the laboratory. These cells are accepted as an important tool to learn about normal development, disease onset, and progression, and are also helpful in testing various drugs. These stem cells share similar characteristics with embryonic cells and have the potential to give rise to all the different types of cells in the human body.
Mesenchymal Stem Cells (MSCs)
These cells are primarily found in the connective tissues surrounding other tissues and organs, known as the stroma. These mesenchymal stem cells are also accurately called stromal cells. The first mesenchymal stem cells were found in the bone marrow and are capable of differentiating into bone, fat, and cartilage cells.
Applications of Stem Cells
Stem cells hold immense promise across various medical fields due to their regenerative capabilities:
Tissue Regeneration
A primary application of stem cells is their use to grow specific types of tissue or even organs. This can be potentially helpful in kidney and liver regeneration, reducing the need for transplants. Doctors have successfully used stem cells from beneath the epidermis to develop skin tissue for grafting, repairing severe burns and other injuries.
Treatment of Cardiovascular Disease
A team of researchers has developed blood vessels in mice using human stem cells. Within two weeks of implantation, the blood vessels formed their own network and were as efficient as natural vessels.
Treatment of Brain Diseases
Stem cells can also treat diseases such as Parkinson’s disease and Alzheimer’s. These can help to replenish damaged brain cells. Researchers have successfully differentiated embryonic stem cells into these specific cell types, making it possible to treat such diseases.
Blood Disease Treatment
Adult hematopoietic stem cells are used to treat cancers, sickle cell anemia, and other immunodeficiency diseases. These stem cells can be used to produce healthy red and white blood cells in the body.
Conclusion
Stem cell technology continues to advance through multidisciplinary efforts, with clinical applications of modified stem cells including transplantation and gene therapy. The practice of cloning and nuclear transfer for stem cell research raises several ethical and technical considerations. Improved understanding of the interactions between cytokines and the extracellular matrix will be crucial for successful ex vivo manipulation of stem cells. Cytokines may reduce the binding capacity of stem cells to components of the stromal microenvironment, potentially facilitating their transfer into the peripheral bloodstream.