In this blog post, we’ll explore the different types of stem cells, their potential, and the balance between these possibilities and ethical concerns.
Stem cells are a term that anyone interested in life sciences has likely heard at least once. Stem cells are gaining attention as the key to solving numerous intractable diseases, and their importance is growing by the day. However, few people have a detailed understanding of stem cells, and even fewer can explain them clearly. Therefore, this article aims to cover what stem cells are, how they are classified, and what limitations they have.
Early-stage cells undergo various processes to develop into mature cells with distinct characteristics, one of which is differentiation. Cells that have undergone differentiation acquire the specific traits necessary to function within their respective tissues. Through this process, early-stage cells develop into cells that make up various tissues, such as bone, skin, and blood vessels. Among the countless cells in our bodies, there are undifferentiated cells that have not undergone the differentiation process, and these undifferentiated cells are stem cells. Because stem cells have not yet differentiated, they possess the ability to differentiate into various bodily tissues under the right conditions.
Stem cells can be broadly classified into adult stem cells, embryonic stem cells, and induced pluripotent stem cells (iPS cells). Adult stem cells are found in specific mammalian tissues; while they cannot differentiate into all types of tissues, they can differentiate into a limited range of tissue types. These stem cells are referred to as multipotent stem cells. Examples of adult stem cells include hematopoietic stem cells found in bone marrow and mesenchymal stem cells. Hematopoietic stem cells are responsible for producing red blood cells and various types of white blood cells, while mesenchymal stem cells are responsible for forming bone and the connective tissue that surrounds it, such as muscle. These multipotent stem cells differentiate as needed. In fact, blood cells differentiating in the bone marrow do so in response to signals from neighboring cells or the circulatory system.
Embryonic stem cells are stem cells found in early embryos and possess the ability to form a completely new organism. In humans, a cluster of cells called a blastocyst forms during the early stages of embryonic development, and the cells within this blastocyst have the ability to differentiate into all types of cells that make up the body. Embryonic stem cells are isolated from this blastocyst and can proliferate almost indefinitely under appropriate conditions; when given the right signals, they can be induced to differentiate in specific ways. These stem cells are called pluripotent stem cells. However, the process of obtaining embryonic stem cells raises ethical concerns. To obtain embryonic stem cells, the blastocyst must be destroyed, as it is at a stage with the potential to develop into a living being. This gives rise to ethical controversy. Additionally, there is a concern that embryonic stem cells obtained in this manner, and the tissues derived from them, may trigger an immune response in the recipient upon transplantation. However, this problem can be resolved by replacing the nucleus of the fertilized egg with the nucleus of a recipient’s somatic cell.
Fortunately, a new method for creating pluripotent stem cells has been developed that resolves both of these issues. Professor Shinya Yamanaka of Kyoto University in Japan and his research team created pluripotent stem cells using the patient’s skin cells instead of isolating embryonic stem cells from a blastocyst. These stem cells are called induced pluripotent stem cells (iPS cells). After investigating the differences in gene expression between embryonic stem cells and non-stem cells, the research team identified genes that are uniquely expressed at high levels only in embryonic stem cells. Believing these genes to be essential for the undifferentiated state of stem cells and the function of embryonic stem cells, the team introduced these genes into the patient’s skin cells to create induced pluripotent stem cells. Because these cells are derived from the patient’s own skin cells, they do not trigger an immune response. They have already been used for cell therapy in animals with diseases and are expected to be applied to humans soon.
As discussed earlier, stem cells are classified into three types: adult stem cells, embryonic stem cells, and induced pluripotent stem cells, each with its own advantages and disadvantages. Adult stem cells have the advantage of not triggering an immune response, but the disadvantage of having a limited range of differentiation. Embryonic stem cells, developed to address this issue, have the advantage of a broad range of differentiation, but they carry the disadvantage of potentially triggering an immune response, as well as ethical concerns. Induced pluripotent stem cells were created through research to resolve the disadvantages of embryonic stem cells while retaining their advantages. However, both embryonic stem cells and induced pluripotent stem cells are unstable and carry the risk of developing into cancer.
There are other issues that need to be resolved as well. During the stem cell culture process, it often happens that cells differentiate into other types of cells. To inject stem cells into the bloodstream and induce them to reach the desired target and differentiate, a significant number of stem cells are required. However, with current technology, even if a small number of stem cells are obtained, most of them differentiate into cells with different functions during the differentiation process, making it difficult to obtain the desired number of stem cells.
Despite these drawbacks, many people are paying attention to stem cells because they hold great potential and can offer hope for the treatment of incurable diseases. If we can find ways to overcome the specific and common limitations of different types of stem cells, they could become the key to solving intractable diseases. Looking at the journey from the discovery of stem cells to the development of induced pluripotent stem cells, stem cell research appears increasingly promising. If many people maintain their interest and continue this research, the day may come when stem cells solve intractable diseases.
