If you get the superpower to choose whatever career you desire, what will you choose? For some, it might be doctors, for others, it might be influencers, and the list is endless. Still, this should be an imaginary situation in reality. But will you be surprised if I tell you that this superpower is already existing in the human body and many other animals? In humans, this all starts with the fertilization of an egg by a sperm, resulting in the formation of a single-cell zygote. A fun fact here is that although a zygote is a product of 2 cells (sperm and ova/egg) joining together, it is a single cell in which the nucleus contains combined genetic material from two parents. Zygote divides continuously, resulting in an increasing number of undifferentiated cells inside the zygote.
4-5 days after fertilization, a normally developing zygote will contain about 50-150 cells and is called a blastocyst. A Blastocyst is itself a rapidly dividing ball of cells. Blastocysts start differentiating into inner cell mass and outer trophoblast. And thus, here comes the entry of STEM CELLS, the superpower that has the unique ability to develop into any cell in the human body. This specific characteristic of stem cells has many applications. One of them is replacing or repairing damaged cells or tissues in the body, known as tissue regeneration.
As mentioned earlier, since stem cells are unspecialized or undifferentiated cells (i.e., they can take up the form and function of any cell in the body after the process of differentiation), they have the potential to differentiate into any type of body cell based on certain factors. These factors can be from within the cell (intrinsic) or from outside the cell (extrinsic/ from neighboring cell). This potential of stem cells is called differential potential. Thus, unspecialized nature and differential potential are the 2 main characteristic features of stem cells.
There are various types of stem cells in the human body. But the question is where do these stem cells come from? The very first stem cells are found in the inner cell mass of the human blastocyst. These are known as Embryonic Stem Cells (ESCs). ESCs have the potential to develop into almost all types of cells and are obtained from an embryo before it implants in the uterus. But many people consider an embryo to be a living organism so there have been many moral and ethical disputes in the past and present regarding the use of ECMs in scientific studies and experiments.
Adult stem cells are also a source of stem cells. These are the cells that are present in different locations in the body. They perform the function of repairing and replenishing injured cells. Adult stem cells are found in the brain, skin, liver, bone marrow, skeletal muscles, blood, and blood vessels. Mesenchymal stem cells, neural stem cells, hematopoietic stem cells, and skin stem cells are some major types of adult stem cells. However, adult stem cells might be tough to come across. They can remain non-dividing and undifferentiated for years before being summoned by the body to repair or generate new tissue. Moreover, ESCs can evolve into more cells than adult stem cells.
Induced pluripotent stem cells (IPS) are created by scientists in labs using tissue-specific cells like skin, etc. And there are similarities in the working of IPS and ESCs. But growing stem cells in a lab require a scientist to first collect samples from embryos or adult tissue. These cells are then placed in a controlled culture, i.e., conditions appropriate for the growth of stem cells are selected and modified to reduce the chances of contamination. By doing this, cells will continue dividing but will not differentiate. However, extracting samples from adult tissue and embryo also raises an ethical issue. Thus, more research is needed for further and safer development of the IPS procedure.
Coming back to the concept of the differential potential of stem cells, it always varies in different organs of the body. For example, neural stem cells (stem cells present in the brain) give rise to nerve cells and their supporting cells, all kinds of blood cells are created by Haematopoietic stem cells, etc. Classification based on differential potential is called stem cell potency. It is of 5 types:
- Totipotent cells: Stem cells that can divide and specialize into any cell of the body are called totipotent cells. Zygote is the perfect example of totipotency as all the cells develop from the zygote.
- Pluripotent cells: Stem cells that can divide and specialize into almost all types of cells of the body are called pluripotent cells. Pluripotent cells form germ layers but cannot form extraembryonic structures like the placenta. Extraembryonic structures are part of 4 membranes surrounding the growing fetus and assisting in its future development. Although they generate from the embryo, they are not considered part of the embryo. That is why they are called “extraembryonic”. ESCs are pluripotent in nature.
- Multipotent cells: Stem cells that have a narrow spectrum of specialization than totipotent and pluripotent cells are known as multipotent cells. Hematopoietic stem cells fall under this category.
- Oligopotent cells: Oligopotent cells are like intermediate steps in the formation of unipotent cells from multipotent cells. These cells are very similar to multipotent cells but have less differentiation capacity than multipotent cells. Although Myeloblast stem cells differentiate from hematopoietic stem cells, they are responsible for the production of white blood cells but not red blood cells.
- Unipotent cells: Stem cells that develop into one type of cell are called unipotent cells. Adult muscle stem cells can develop into muscle cells only
Stem cell potency decreases with each step, i.e., oligopotent cells cannot specialize into many cell types in comparison to multipotent cells. Apart from all this classification, stem cells have been of great use for scientific studies. One such interesting case is of HeLa Cells. HeLa Cells are a type of cancer stem cells derived from an African American woman named Henrietta Lacks. These cells collectively referred to as the HeLa cell line, became the initial human cells to be effectively cultivated in a lab and have since been extensively employed in scientific studies, resulting in several significant medical advances. HeLa cells have been used in a variety of scientific research fields, including cancer, polio vaccine testing, genetics, tuberculosis, leukemia, herpes, in-vitro pioneering, AIDS, Parkinson's disease, chemotherapy, HPV testing, and many others. They have also contributed significantly to understanding human biology and disease.
Recently, researchers in China created embryo-like structures from the ESCs of monkey. They further successfully implanted these structures in the uteruses of female monkeys and are observing responses similar to pregnancy. Scientists have the mindset of advancing this model as it will lay down a foundation for future studies.
