Stem cells are cells in the body of multicellular organisms, which form the basis of all tissues and organs , can transform into different cell types, renew themselves, and have different structures and stages of development . To put it simply , stem cells are cells in the body that have the potential to turn into more than one cell type during development and growth . While these cells take on the most important role during embryonic development , they play an important role in the development of tissues and the body during growth. When it is an adult , it helps the tissues to renew themselves.
Stem cells differ from other cell types with some features. For example, these cells are not specialized and can renew themselves by undergoing cell division. In addition, non-specialized cells can be transformed into "specialized tissue or organ cells" in a natural or laboratory environment with special signals. Another difference from other cell types is that stem cells divide continuously in some tissues and organs and ensure the continuity of the environment, and in some tissues and organs, they never divide except in special cases.
The development and understanding of stem cell biology provides enormous advantages for us humans. Humanity; It has made many attempts to prolong life, treat diseases, delay or stop aging. Today, with the developing technology, the use of stem cells in treatment has become widespread in areas such as molecular biology and genetic engineering.
With stem cell technology, radical changes can be made in the treatment procedures of very serious diseases such as organ transplantation, immune system diseases, irreversible cell and tissue damage. As a result of the researches, it has been found that stem cells can provide possible treatment for incurable diseases thanks to their ability to replace defective cells and regenerate organs.
In addition, the unique regeneration and specialization abilities of stem cells show that they can be used for treatment purposes in tissue and organ degenerations , diseases such as diabetes and heart diseases. However, this field is still an emerging field, and scientists continue to discover new properties and capabilities of stem cells through laboratory studies. There are already many experiments carried out on the therapeutics of stem cells. In addition to these experiments, scientists are trying to understand how a living thing originates from a single cell with the help of stem cells.
Stem Cell Resources
Stem cells can be found in many different places in the human body, but they are basically divided into two main groups according to their location. These:
- Embryonic stem cells (Eng: "embryonic stem cells" or "ESC for short"),
- Non-embryonic, adult (or somatic ) stem cells.
Totipotent, Pluripotent, Multipotent, Oligopotent and Unipotent Cells
These cells are divided into five according to their development and differentiation potential:
The zygote formed as a result of fertilization undergoes successive mitotic divisions. The cell formed by each division is called a "blastomere". The first cell to form is embryonic cells with " totipotent " characteristics: In other words, they have the potential to form the whole organism . Between the middle of the 5th day and the end of the 2nd week after fertilization, the cells in the "blastocyst" stage cannot form a complete organism on their own; but they are " pluripotent " cells that do not have the potential to differentiate into all cells in the body, but can still develop into many cells . Embryonic stem cells are pluripotent cells originating from cells in the inner layer of the blastocyst.The stem cells that are found in adults and are specialized to form only certain cell types are called " multipotent " stem cells. Stem cells that can transform into even fewer cell types are called " oligopotent ", and stem cells that can transform into a single cell type are called " unipotent ". As you can see, these names are given according to the ability of the stem cell to transform into different cells.
Embryonic stem cells have the ability to transform into all somatic cells and multiply indefinitely under laboratory conditions in the early embryonic period.
Stem Cell Types Used in Research
Stem cells are found in every organ in our body. These stem cells can be different from each other, or they can occur at any time in our lives, without knowing each other. For example, while embryonic stem cells only take part in the beginning of our development in the mother's womb, tissue-specific stem cells (or adult stem cells) begin with fetal development and continue for the rest of our lives.
All stem cells can renew themselves and differentiate. However, not all stem cells can do everything. That's why research usually uses four types of stem cells:
- embryonic stem cells,
- Tissue-specific stem cells,
- Mesenchymal stem cells,
- Induced pluripotent stem cells .
Mesenchymal Stem Cells
Mesenchymal stem cells are obtained from the connective tissue surrounding the tissues and organs. The first mesenchymal stem cell was discovered in the bone marrow and it was shown that the cells there can form bone, cartilage and fat cells. As a result of subsequent research, it was concluded that mesenchymal stem cells can also be obtained from other tissues.
Although they have been used for the treatment of many diseases, until recently, there was no definite information about the effectiveness of these treatments. Scientists still do not have precise information about whether mesenchymal stem cells are really stem cells and what type of cells they can turn into. However, what they are sure about is that not all mesenchymal stem cells are the same and their properties differ according to the tissue or organ from which they are obtained.
Induced Pluripotent Stem Cells
This stem cell type is stem cells produced in the laboratory and transformed into tissue-specific cells. These types of cells behave like embryonic stem cells. Induced pluripotent stem cells provide scientists with important opportunities to understand normal development and gain knowledge about disease processes.
Although these cells are very similar to embryonic stem cells, they are not exactly the same. Scientists continue to explore these differences and their meanings.
Embryonic Stem Cells
These cells are obtained from the "inner cell mass" of the blastocyst, which begins to form about five days after the egg is fertilized.
During normal development, the cells in the inner cell mass are specialized to form our entire body. However, scientists can prevent these cells from losing their embryonic stem cell properties by collecting these cells in a laboratory environment and growing them under special conditions.
Embryonic stem cells are pluripotent; that is, they have the ability to specialize in almost every organ in the human body (they cannot only specialize in the placenta and umbilical cord). These cells are very valuable; because they provide an important research resource both for developmental biology and for treatments of diseases. Human embryonic stem cells are obtained from blastocysts formed by fertilization in vitro.
Tissue-Specific Stem Cells
This stem cell type, also called "somatic stem cells" or "adult stem cells", is more specialized than embryonic stem cells. Tissue-specific stem cells can only specialize in cell types belonging to the tissue they are found in. For example, blood -forming stem cells (aka "hematopoietic stem cells") found in the bone marrow can only specialize into red blood cells, white blood cells, and platelets .
Some organs and tissues may contain small groups of tissue-specific stem cells. The purpose of these groups is to somehow regenerate the damaged tissue or organ. However, these stem cells are not easy to find in the human body. In addition, it is not as easy as embryonic stem cells to renew themselves in the laboratory environment. But research on these stem cells provides clues about the normal developmental process, factors behind aging, diseases and injuries.
An Abridged History of Stem Cells
The derivation of embryonic stem cells from mouse early embryos was carried out in 1981 by Gail Martin and his team. This development, which was revolutionary at the time, had a great impact. After a long time, in 1998, stem cells from human embryos were isolated and grown in the laboratory by James Thomson and his team. These cells were named " human embryonic stem cells ". The main purpose of the research conducted in 1998 was to use the stem cells to be produced in the field of "in vitro fertilization" . In 2006, however, a striking new development took place: Kazutoshi Takahashi of Kyoto University and his team had succeeded in turning specialized adult cells into stem cells. to these cellsThey were given the name " induced pluripotent stem cells " or "iPSC for short".
Conducting embryonic stem cell studies encounters resistance due to ethical, moral and legal debates. Embryonic stem cell studies in our country were stopped by the Ministry of Health in 2006. As induced pluripotent stem cells (iPSCs) can be obtained from somatic cells of very different origins, there are no legal and ethical barriers to their use. For this reason, we focused on adult stem cell studies.
Characteristics of Stem Cells
All stem cells have three common characteristics:
- It can renew itself and continue to divide over a long period of time.
- They are unspecialized, that is, they do not have the characteristics of cells found in any tissue or organ.
- They can be specialized in tissue and organ cells.
Let's take a closer look at these features.
Division and Renewal
Unlike muscle, blood and nerve cells, which do not renew and divide themselves under normal conditions, stem cells can divide many times. In an experiment with a limited population of stem cells , millions of stem cells were obtained in just a few months. In addition, the fact that these cells do not become specialized at this time proves their long-term dividing ability. After this experiment, some questions came to the minds of scientists and they began to seek answers to these questions:
- Why can embryonic stem cells divide in vitro for years without specialization, while adult stem cells cannot?
- What are the factors that pave the way for stem cell division in an organism ?
Answering these questions will likely shed light on how cell divisions occur during embryonic development, as well as the causes of abnormal cell divisions that can lead to cancer .
Non-privatization
One of the most important features of stem cells is that they do not belong to any tissue or organ. For example, they do not help heart muscle cells, which are neighbors of stem cells, to pump blood to the body or provide oxygen to the body along with red blood cells. However, when the time comes, they can transform into different cell types.
Being Customizable
The process of specialization of an unspecialized stem cell is called the " differentiation process ". During this process, the cell passes through many stages and becomes more specialized at each stage it passes. Scientists gradually began to learn about the intracellular and extracellular signals that contribute to this process. In the light of the information obtained as a result of the researches, we can say that intracellular signals originate from "genes", while extracellular signals originate from other cells in the environment and chemicals in the environment. The interaction of signals with each other during differentiation leads to epigenetic changes in the DNA of the cell , and the cell completes its differentiation with these changes.
