Stem cells are a type of cells that can divide and differentiate into various types of cells in the body. They are the building blocks of the body, and they play a crucial role in the growth, repair, and regeneration of tissues. Their fantastic properties mean that they can be used for all kinds of regenerative therapy.
Our bodies are made up of trillions of individual cells of hundreds of different types of cells. Stem cells can become any of them. Observing and studying how stem cells (let’s say blank cells) become specific cells with specific functions is a massive field of research for medical professionals. It is fascinating and promising for humanity as a whole and even today there are real-world applications for the limited knowledge that we currently possess.
Differentiation potential classification of stem cells
Stem cells can be classified into different potency levels based on their ability to differentiate into different cell types. The four main potency classifications of stem cells are:
- Totipotent stem cells: These are stem cells that can differentiate into any type of cell in the body, as well as the extraembryonic membranes that support fetal development. Totipotent stem cells are only found in the early stages of embryonic development, typically up to the 4-cell stage.
- Pluripotent stem cells: These are stem cells that can differentiate into any type of cell in the body, but not the extraembryonic membranes. Pluripotent stem cells are typically found in the inner cell mass of the blastocyst, a structure that forms about 5-7 days after fertilization.
- Multipotent stem cells: These are stem cells that can differentiate into multiple types of cells, but not all cell types in the body. For example, hematopoietic stem cells in the bone marrow can differentiate into various types of blood cells, but not into neurons or muscle cells.
- Unipotent stem cells: These are stem cells that can only differentiate into one type of cell. For example, muscle stem cells can only differentiate into muscle cells.
Differentiation leads to subclassification and here are some examples of stem cells that go through different stages.
Examples of stem cells
Induced pluripotent stem cells (iPSCs): These are adult cells that have been reprogrammed to have the properties of ESCs. This is done by introducing specific genes into the cells, making them pluripotent.
Mesenchymal stem cells (MSCs): These are found in bone marrow, fat, and other tissues. They have the ability to differentiate into several types of cells, including bone, cartilage, and fat cells.
Neural stem cells (NSCs): These are found in the brain and spinal cord. They have the ability to differentiate into various types of neural cells, including neurons and glial cells.
Hematopoietic stem cells (HSCs): These are found in the bone marrow and have the ability to differentiate into all types of blood cells, including red blood cells, white blood cells, and platelets.
Endothelial stem cells (ESCs): These are found in blood vessels and have the ability to differentiate into endothelial cells, which line the inside of blood vessels.
Just the variation of these stem cells is mind-boggling. Stem cells have the potential to revolutionize medicine by offering new therapies for a wide range of diseases and conditions.
From embryo to fetus to adult stem cells
There are several types of stem cells, including:
Embryonic stem cells (ESCs): These are the most widely known to the general public, think of school biology classes. These types of stem cells are derived from embryos that are three to five days old. They have the ability to differentiate into all the cells in the body, making them pluripotent. Everyone was at some stage of their development almost entirely made of embryonic stem cells.
Fetal stem cells: These are derived from the fetus and are found in the liver, bone marrow, and blood. They are also multipotent.
Adult stem cells: These are found in various tissues throughout the body, including bone marrow, blood, brain, and muscle. They are multipotent, meaning they can differentiate into several types of cells within their tissue of origin. However, fetal stem cells display many properties that make them superior to adult cells for use in regenerative medicine applications, including greater plasticity in differentiation potential, faster growth in culture, and increased survival at low oxygen tension.
Obtaining stem cells for research and treatments
The main difference between embryonic, fetal, and adult stem cells is their source and the stage of human development at which they are created and therefore obtained.
Embryonic stem cells (ESCs) are obtained from the blastocyst stage of development, which occurs approximately 4-5 days after fertilization. At this stage, the developing embryo consists of a small cluster of cells that will eventually differentiate into all the cells and tissues of the body. ESCs are pluripotent, meaning they can differentiate into any cell type in the body.
Fetal stem cells, on the other hand, are obtained from the developing fetus during gestation. They can be found in various tissues, such as the liver, bone marrow, and blood. These stem cells are multipotent, meaning they can differentiate into a limited number of cell types within their tissue of origin.
Adult stem cells are found in various tissues throughout the body, such as the bone marrow, brain, and muscle. They are multipotent and can differentiate into a limited number of cell types within their tissue of origin. Adult stem cells play an important role in tissue repair and regeneration.
In terms of the ratios of these stem cells, embryonic stem cells are present only in the very early stages of development, while fetal stem cells can be obtained throughout gestation, although their availability and numbers may vary depending on the tissue. Adult stem cells, on the other hand, are present in most tissues throughout the entire lifespan of an individual, but their numbers may decline with age.
It is important to note that the use of embryonic stem cells is controversial due to ethical concerns related to their source, as they are obtained from the destruction of embryos. In contrast, fetal and adult stem cells are obtained without harming the donor, and therefore, their use is generally less controversial.
Bone marrow stem cells
Bone marrow is one of the most important parts of the human body for the regulation of multiple systems. Bone marrow contains a type of stem cell called hematopoietic stem cells (HSCs) that can differentiate into various types of blood cells. These blood cells include red blood cells, which carry oxygen throughout the body; white blood cells, which fight off infections; and platelets, which help with blood clotting.
In addition to blood cells, bone marrow stem cells may also have the potential to differentiate into other cell types, such as bone, cartilage, and fat cells. However, the differentiation potential of bone marrow stem cells is still an area of active research so we have limited knowledge.
Some people may require a bone marrow transplant, also known as a stem cell transplant if their bone marrow is not functioning properly. You may have seen television shows where bone marrow is mentioned because it is tremendously important. Bone marrow may not function properly and this may occur due to certain types of cancers, such as leukemia or lymphoma, or due to other diseases that affect the bone marrow, such as aplastic anemia.
During a bone marrow transplant, the patient receives a transfusion of stem cells from a donor, which can help to replace the diseased or damaged bone marrow. In the best-case scenario, the donated stem cells then migrate to the patient’s bone marrow and begin to produce healthy blood cells.
Bone marrow transplants can be an effective treatment option for some people, but in the worst-case scenario, they also carry risks and potential complications, such as graft-versus-host disease (GVHD), where the transplanted cells attack the patient’s own tissues. Therefore, careful consideration is necessary when deciding whether a bone marrow transplant is appropriate for a particular individual. Finding a suitable donor takes a lot of effort and extensive testing.
Stem cell exhaustion (depletion) and aging
Stem cells have the ability to divide and produce copies of themselves, as well as differentiate into different cell types. However, there are several factors that can cause stem cells to stop reproducing and differentiate:
- Environmental factors: Exposure to toxins, radiation, or other environmental factors can damage stem cells and cause them to stop dividing.
- Telomere shortening: Telomeres are protective caps on the ends of chromosomes that shorten each time a cell divides. Eventually, telomeres become too short, and the cell stops dividing. This is known as replicative senescence.
- Epigenetic changes: Changes in the expression of genes that regulate stem cell division can also cause stem cells to stop dividing and differentiate instead.
All of these occur over time and typically correlate with aging, but when stem cells stop performing their function is when aging accelerates. Hence, the exhaustion of stem cells is a viable measurement of aging.
Stem cell exhaustion is a term used to describe the gradual decline in the number and function of stem cells that occurs with aging, however, damage from the environment such as smoking, and exposure to radiation will also deplete and destroy the potency of stem cells.
As we age, our bodies produce fewer and less functional stem cells, which can contribute to a range of age-related diseases and conditions. For example, the decline in hematopoietic stem cells in the bone marrow can lead to a weakened immune system and anemia. Older people have typically a weaker immune system and cells that fight infections are created in the bone marrow from stem cells.
Similarly, the decline in neural stem cells in the brain can contribute to cognitive decline and neurodegenerative diseases such as Alzheimer’s and Parkinson’s. Thus, stem cell exhaustion is considered a hallmark of aging, and understanding the underlying mechanisms of this process is an active area of research in the field of aging biology.
Future of stem cell research for aging
Stem cell research can be an expensive field due to the need for specialized equipment, reagents, and highly trained personnel. However, with advances in technology and an increased understanding of stem cell biology, the costs of stem cell research have been decreasing in recent years.
The development of AI and machine learning in particular will contribute immensely to the study of stem cells.
Stem cell research has the potential to revolutionize medicine and our understanding of aging in several ways. Here are a few examples:
- Regenerative medicine: Stem cells can be used to replace damaged or diseased tissues and organs, potentially providing a cure for a wide range of diseases and injuries. For example, stem cell therapies are already being used to treat conditions such as spinal cord injury, heart disease, and diabetes.
- Drug discovery: Stem cells can be used to create disease models in a lab, allowing researchers to study the underlying mechanisms of diseases and test potential drugs. This can accelerate the drug discovery process and lead to more effective treatments for a range of diseases.
- Aging research: Stem cells play a key role in the aging process, and studying stem cell biology may provide insights into how and why we age. This could lead to new approaches for extending the human lifespan and improving health in old age.
The future of stem cell research is bright but it will come quicker if we pull together the resources towards this highly-promising field.