Stem cells

What is cell?


Cell is a structural, functional and biological unit of all living organisms. Anton Von Leeuwenhoek was the first scientist to see and describe a living cell. Cells
vary in their shapes, sizes and functions. Human body is made up of several different types of cell.  Most of the cells are specialised to perform particular functions. For example red blood cells carry oxygen in the blood of human body, white blood cell protect the human body from infectious agents but both these cells are not able to divide.

What are stem cells?


Stem cells are special human cells that have the ability to develop into many different cell types e.g. nerve cells, heart cells, etc. during early life and growth. They can also act as an internal repair system in many tissues of the body. When a stem cell divides, each new cell which is formed has the potential to either remain a stem cell or become another type of cell (which can have more specialised function).


Properties of stem cells

Due to following two important properties stem cells are different from other cell types:
  1. Stem cells are unspecialised: They are unspecialised cells which are capable of renewing themselves by cell division. One of the unique feature of stem cells is that they do not have any tissue specific structure, thus they are not allowed to perform specialised functions. For instance, a stem cell cannot carry oxygen in the blood as red blood cells nor they can pump blood through the body like heart cells. But unspecialised stem cells can give rise to specialised cells (like heart muscle cells, red blood cells, etc)
  1. Stem cells can give rise to specialised cells: Under certain normal or experimental conditions, stem cells can be induced to become tissue or organ specific cells which can perform special and specific functions. The process of unspecialised stem cells giving rise to specialised cells, is called differentiation. In the process of differentiation, the cell goes through several stages and becomes more specialised at each step. Stem cells divide differently in different organs. For instance in organ like bone marrow, the stem cells regularly divide and replace the damaged tissue where as in organs like heart, the stem cells divide under specific conditions.

Sources for Stem Cells

The three main sources of stem cells are:
  1. Embryos formed during the blastocyst phase of embryological development lead to the formation of embryonic type of stem cell.
  2. Bone morrow and Peripheral blood tissue are the source of Adult type of stem cells.
  3. Umbilical cord, placenta, aborted foetal tissue are also sources of stem cells

Types of Stem cells

Till now scientists have primarily worked on two kinds of stem cells (both from animals and humans).
  1. Embryonic Stem cells
  2. Non-embryonic somatic or adult stem cells.
In 1998, the method to derive stem cells from human embryos and growing them in laboratory was discovered by scientists. These cells are called as human embryonic stem cells.
In 2006, scientists identified the conditions that could allow specialised adult stem cell to  genetically rearrange to assume a stem cell like state. These newly type formed stem cells are knwown as induced pluripotent stem cells.


Embryonic stem cells

These stem cells are derived from embryos that have developed from eggs that are fertilised in vitro in an IVF (in-vitro fertilisation) clinic and are given for research purposes with the consent of the donors. They are not taken from eggs fertilised in the human body.
Embryonic stem cells remain undifferentiated or unspecialised till their culture is grown under specific conditions. Once cells are allowed to form embryoid bodies, they begin to differentiate spontaneously. They can form many types of cells like nerves cell, heart muscle cell etc.


Adult stem cells

    They are considered as undifferentiated cells which are found among differentiated cells in a tissue or organ. The adult stem cell can differentiate to produce some or all of the major specialised cell types of the tissue or organ.
    The main role of adult stem cells in a living organism is to maintain and repair the tissue in which they are found. They are also known as somatic stem cells. The term 'Somatic' refers to cells of the body and not to the germ cells like sperms or eggs.
    Adult stem cells are found in a specific area of each tissue. They are found in many organs and tissues like skeletal muscle, bone marrow, brain, skin, teeth, heart, gut, liver, peripheral blood, blood vessels, ovaries epithelium and testis.

    Similarities and differences between embryonic and adult stem cells


    1. Cell types: One major difference between these two types of stem cells lies in their ability to be differentiated in different types of cells. Embryonic stem cells can be differentiated into all cell types of the body because they are pluripotent. Whereas, adult stem cells have limited differentiating ability. They can differentiate into limited types of cells.
    2. Processing: In comparison to adult stem cells, the embryonic stem cells can be grown more easily in culture. Whereas, isolating adult stem cells is challenging as they are found rarely in mature tissues.
    3. Transplantation: The tissues which are developed from embryonic and adult stem cells may differ in the likelihood of being rejected after transplantation.
    Since, tissues taken from embryonic stem cells have been tested few times in clinical trials for transplantation so it is not yet known for certain whether these tissue will cause transplant rejections or not.
    Whereas the tissue developed from adult stem cells is less likely to cause transplant rejection as the patient's own cells are expanded in culture and persuaded into specific cell type and then reintroduced into the patient. With the use of patient's own adult stem cells,  the chances of getting rejected by the immune system are less.
    This is a significant advantage as immune system rejection can be avoided only by continuous administration of immunosuppressive drugs and these drugs have major side effects.


    Induced pluripotent stem cells (iPSCs)

    The adult stem cells are genetically reprogrammed to give rise to an embryonic stem cell like state.
    It is forcibly made to express genes and factors that are crucial for maintaining the underlying properties of embryonic stem cells.
    In 2006, induced pluripotent stem cells in mouse were first reported and late in 2007 Human iPSCs were reported. Mouse iPSCs shows the significant characteristics of pluripotent stem cells such as  expressing stem cell markers, forming many different tissues when injected into the embryo of mouse at an early stage of development. Similarly, Human iPSCs also expressed the characteristics like stem cell markers and features of all three germ layers.


    Classification of Stem Cells on the basis of potency

    The stem cells can be classified into five categories based on their differentiation potential.
    1. Totipotent Stem Cells: These types of stem cells can differentiate into embryonic and extra embryonic tissues such as yolk sac, chorion, amnion etc. In humans, these tissues form the placenta.
    The totipotent stem cells can give rise to fully functional living organism. The example of totipotent is a zygote (it is formed by the fusion of egg cell from a woman and the sperm cell from a man).
    1. Pluripotent Stem Cells: These stem cells are equally potent when compared to totipotent stem cells except for one thing i.e. creating an entire organism. These cells can self renew and differentiate into any of the three germ layers (ectoderm, endoderm, and mesoderm). These three germ layers further differentiate to form all tissues and organs within a human being. There are several known examples of natural pluripotent stem cells. The best example is embryonic stem cells. The human-made pluripotent stem cell also exists and they are known as induced pluripotent stem cell.                                                                                                     
    2. Multipotent Stem Cells: They can give rise to other cell types but their differentiating ability is limited e.g. mesenchymal stem cells.                                    
    3. Oligopotent Stem cells: They are similar to multipotent stem cells but their differentiating ability is more limited. They can self-renew and differentiate only in closely related types of cell e.g. haematopoietic cells.                                                     
    4. Unipotent Stem Cells: They have the least differentiation potential of all the stem cell types. They have the ability to differentiate only into one type of cell e.g. muscle stem cells.

    Applications of stem cells 

    The main aim of stem cell therapy is to repair a damaged tissue that can't heal on its own. The ongoing researches on stem cell therapies give a hope to cure the diseases whose treatment, otherwise, is not possible or not efficient. The stem cell therapies not only involve the process of transplantation of stem cells into the body but also to manoeuvre the stem cells already present in the body and produce new tissues.
    There are number of stem cell therapies existing today, but most of them are at experimental stages or costly. However, bone marrow transplantation is quite successful. The scientists have anticipated that the stem cells can be used to treat cancer, Type 1 diabetes mellitus, cardiac failure, neurological diseases, etc.                                                   
    Bone marrow transplantation (BMT)

    It is a known clinical application of stem cell transplantation. In chemotherapy or radiotherapy (process to eliminate endogenous cancer cells), the blood cells get damage. In bone marrow transplantation, different blood cell types are restored.
     
    Skin replacement

    The skin stem cells are found in the hair follicle and can be removed by plucking the hair. By using these skin stem cells, the scientists can grow skin from the plucked hair of a person. Skin stem cells are cultured to form an epidermal equivalent of the person's own skin and provide tissue for autologous graft.

    Brain cell transplantation

    Stem cells can be used to treat Parkinson's disease and Alzheimer's disease by replenishing the damaged tissue.

    Organ and tissue regeneration

    Tissue regeneration is one of the important application of stem cell therapy. Stem cells can be used to grow a particular tissue or the organ.

    Treatment for diabetes

    Diabetes is caused by abnormal metabolism of insulin (it is produced by islets of Langerhans located in pancreas). It affects people all around the world. Recently, insulin expressing cells have been derived from mouse stem cells. Thus, future research might aim at providing a stem cell-based therapy to treat diabetes and replace the constant need for insulin injections.

    Stem Cell Treatment in India

    Stem Cell Treatment is still under research mode in India. Indian Government is supporting various basic pre-clinical and clinical researches. Stem cell treatment is provided to cancer and thalessemia patients at All India Institute of Medical Sciences, New Delhi. Tata Memorial Centre located in Mumbai is also leading in Stem Cell treatment.


    Guidelines related to Stem cells in India

    1. In 2002, Indian Council of Medical Research had released Draft Guidelines for Stem Cell Research/Regulation.
    2. In 2007, Department of Biotechnology had released Guidelines for Stem Cell Research and Therapy.
    3. In 2013, National Guidelines for Stem Cell Research were released. These guidelines help the clinicians and scientists working in the field to conduct research in scientific and ethical manner.
    4. In 2017, National Guidelines for Stem Cell Research were released. According to these guidelines, only bone marrow stem cell transplant for blood disorders is permitted.

    Conclusion

    It is believed that stem cell research can pave a way in future to cure many life taking chronic diseases. Antagonists view the usage cells from embryos as immoral.
    But stem cell therapy has the vast potential to be unleashed to transform the human life by providing new milestones in the field of therapeutics such as skin replacement, bone marrow transplant, organ development and tissue replacement.


    Associated Terms


    Stem cell culture
    It is the process of growing cells in the laboratory. Cells from pre implantation stage embryo are transferred into culture dish containing nutrient broth called as culture medium.
    Stem cell markers are genes and the related protein structures, used to isolate and identify stem cells.

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