The role of stem cells
Stem cells are unspecialized cells that can develop into other types of cell. They have the ability to divide by mitosis to produce new cells.
Types and Potency of Stem Cells
The capacity of a stem cell to produce different cell types is called its potency, which varies. There are several types:
Totipotent Stem Cells: These cells can mature into any type of body cell. In mammals, totipotent cells are only present for a limited time in the first few cell divisions of an embryo, including cells that make up the placenta.
Pluripotent Stem Cells: Found in mammalian embryos, these cells can differentiate into any type of body cell except placental cells. Embryonic stem cells, obtained from embryos (e.g., created by IVF) at an early stage (4-5 days old), are pluripotent and can divide an unlimited number of times.
Multipotent Stem Cells: These are found in mature mammals and can differentiate into a few different types of cells. For example, stem cells in bone marrow are multipotent and can produce various blood cells like red blood cells, monocytes, neutrophils, and lymphocytes.
Unipotent Stem Cells: These are also found in mature mammals and can only differentiate into one type of cell. For instance, a type of unipotent stem cell produces only epidermal skin cells. Some scientists believe old or damaged cardiomyocytes (heart muscle cells) can be replaced by new ones derived from a small supply of unipotent stem cells in the heart.
Role in Cell Division and Specialization
Stem cells are crucial for generating new cells and tissues because they divide by mitosis to produce daughter cells. The process by which a stem cell becomes specialized is called differentiation. This specialization occurs because, during development, stem cells only transcribe and translate part of their DNA. Under specific conditions, certain genes are expressed (activated) while others are switched off, leading to the production of proteins that modify the cell's structure and control its processes, resulting in specialization.
Functions in the Body
Stem cells are essential for several biological processes:
Growth of Multicellular Organisms: Repeated cell divisions from a single-celled zygote allow an organism to grow into a multicellular adult.
Replacement and Repair: Cells are constantly dying or becoming damaged, and stem cells divide to produce new, identical cells to replace them and repair tissues. This occurs rapidly in tissues like skin and the lining of the gut.
Asexual Reproduction: In some organisms, mitosis, driven by stem cells, is the basis of asexual reproduction, producing genetically identical offspring.
Immune Response: The cloning of B- and T-lymphocytes during the immune response is dependent on mitosis, originating from stem cells in the bone marrow.
Medical Applications (Stem Cell Therapies)
Scientists believe stem cells can replace damaged tissues in a range of diseases due to their ability to differentiate into specialized cell types.
Existing Therapies: Bone marrow transplants are an established therapy that uses stem cells from bone marrow to treat blood and immune system diseases like leukaemia (cancer of blood or bone marrow), lymphoma (cancer of the lymphatic system), and genetic disorders such as sickle-cell anaemia and severe combined immunodeficiency (SCID). The stem cells in transplanted bone marrow divide and specialize to produce healthy blood cells.
Future Therapies and Research: Research is ongoing for treating conditions such as:
Spinal cord injuries (replacing damaged nerve tissue).
Heart disease and damage from heart attacks (replacing damaged heart tissue).
Bladder and respiratory conditions (growing whole bladders or covering windpipes with stem cell-generated tissue).
Organ transplants (growing new organs from stem cells to reduce donor waiting lists and potential rejection).
Benefits: Stem cell therapies could save many lives (e.g., for organ transplants) and improve the quality of life for many people (e.g., replacing damaged eye cells for the blind). If made genetically identical to a patient's own cells, rejection of transplants could be avoided.
Sources of Stem Cells
Human stem cells can be obtained from three main sources:
Adult Stem Cells: Obtained from body tissues of an adult (e.g., bone marrow). The procedure is relatively simple with low risk, but they are less flexible (multipotent) than embryonic stem cells.
Embryonic Stem Cells: Obtained from early-stage embryos (4-5 days old) created by in vitro fertilization (IVF). These are pluripotent and can divide indefinitely.
Induced Pluripotent Stem Cells (iPS cells): Created in the lab by "reprogramming" specialized adult body cells to become pluripotent. This involves expressing transcription factors associated with pluripotency. iPS cells have the potential to be as flexible as embryonic stem cells but without the same ethical issues, especially if made from a patient's own cells.
Ethical Considerations
The use of embryonic stem cells raises ethical issues because it involves the destruction of an embryo that could become a fetus. Some people believe that life begins at fertilization and that destroying embryos is wrong. Others have fewer objections to using artificially activated unfertilized egg cells as they cannot survive beyond a few days or develop into a fetus. The development and use of iPS cells are seen as a way to potentially bypass these ethical concerns. Society, with the help of regulatory authorities, must consider both the potential benefits and the ethical issues when making decisions about stem cell research.
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