Stem cells are unspecialized cells found in mammalian body that can divide through mitosis and have the potential to differentiate into different specialized cell types in the body during early life and growth. In mammals, there are two main types of stem cells are present i.e. embryonic stem cells which are present in the inner cell mass of blastocyst or fetal gonadal tissue, and adult stem cells which are found in various tissues. Stem cells also serve as a internal repair system inside the tissues of an adult organism. The stem cells can divide and proliferate for the long time as long as the organism is alive. When stem cells divide they form two daughter cells, and each cell have the potential to remain stem cells or form to another type of cells which has a more specialized function such as, muscle cells, red blood cells or brain cells.

There are two main properties of stem cells which distinguish them from the other cells types. First, they are unspecialized cells which have the ability to renew themselves through cell division, including long term of inactivity. Second, they can be induced to become tissue or organ specific cells with special function under certain physiologic or experimental condition. In some organs stem cells regularly divide to repair the damaged tissue such as the gut and bone marrow. While in some organs such as pancreas and heart, they can divide only under special conditions. (www.ncbi.nlm.nih.gov)   

Fig 1: stem cells differentiation (www.kumc.edu/stemcell/intro).

PROPERTIES:

A)    SELF RENEWAL: stem cells have the ability to divide several times through cell division, while maintaining their undifferentiated state for a long period or say long term self renewal. They are unspecialized cells but they give rise to specialized cells types. When unspecialized cells give rise to specialized cells the process is called differentiation. The stem cells renew themselves with two main processes which help them to maintain their population. First, Obligatory asymmetric replication in this a stem cell give rise to one father cell which is identical to the original stem cell and the other daughter cell which is differentiated. Second, stochastic differentiation in this the stem cells divide into two daughter cells and another stem cell undergoes through cell division and give rise to two stem cells which are identical to the original.

B)    POTENCY: potency referred the ability of stem cells to differentiate into different specialized cell types. The stem cells has to be totipotent or pluripotent to give rise to any mature cell types. However, multipotent or unipotent cells are referred to stem cells sometimes. The characterization of stem cells by their ability to differentiate into other types of cells are as follows:

·        Totipotent stem cells have the ability to differentiate into all possible cell types. These can differentiate into embryonic and extraembryonic cell types. These cells can form a complete viable organism.

·        Pluripotent stem cells are nearly same to totipotent stem cells and can differentiate into almost all cell types. These stem cells can derived from any of the three germ layers which are formed in the early stage of embryonic stem cell differentiation.

·        Multipotent stem cells can differentiate into cells closely related to their families. Such as formation of blood cells or platelets.

·        Oligopotent stem cells can differentiate only into few cell types. Such as lymphoid or myeloid stem cells.

·        Unipotent stem cells only produce the cells of their own types. But they have the ability of self renewal which keeps them in the category of stem cells. Such as muscle stem cells. (www.ncbi.nlm.nih.gov)   

Fig 2: stem cell classification 

STEM CELL TYPES:

Embryonic stem cells: These stem cells are derived from the epiblast tissue of the inner cell mass of the blastocyst or from early morula stage embryo. Embryonic stem cells are pluripotent and give rise to all three germ layers: endoderm, mesoderm and endoderm. ES cells can develop into each of 200 cell types present in the human body and they can provide necessary and specific stimulation for a specific cell type.

Adult stem cells: These stem cells are also termed as somatic stem cells or germline stem cells. These are undifferentiated cells found among differentiated cells in a tissue or organ that can renew themselves into specialized cells of a tissue or organ. The major function of these stem cells is to maintain and repair the tissue in which they are present. These stem cells are rare and generally small in number inside the body and most of them are lineage restricted i.e. multipotent.

Fetal stem cells: These are the primitive stem cells found in the fetuses. They are the elements that form the blood cells. They are divided into two type pluripotent stem cells and hematopoietic stem cells. PSC are found in fetuses and HSC are found in cord blood. These cells stimulate body own repair mechanism.

Amniotic stem cell: Theses are multipotent stem cells which are found in amniotic fluid. These stem cells are non tumirogenic and are very active and expand extensively.

Induced pluripotent stem cells: These cells are programmed to function as adult stem cells by inducing pluripotent capabilities. By using genetic programming and protein transcription factors pluripotent stem cells equivalent to embryonic stem cells have been derived from the human adult skin tissue.

Meritocracy of stem cells for therapy:

Advance research in stem cell biology has shown high degree of plasticity in adult stem cells in many tissues in the body. Stem cell therapy can treat various degenerative disorders for which organ transplantation is difficult or there is a shortage of organ donors. Stem cells have the potential for the treatment of degenerative disorders and they can be obtained from the variety of sources. The main types of stem cells studied for this purpose are embryonic stem cells and adult stem cells. Bone marrow is the good source of taking mesenchymal and hematopoietic stem cells which can be used for the treatment of various tissues from damage and degeneration like Alzheimer’s disease and Parkinson disease, myocardial infarction, diabetes, heart failure and liver insufficiency. Both MSC and HSC have the therapeutic potential. Already, clinical trials showed the treatment of heart and liver disease with the help of bone marrow derived stem cells.

Adult stem cells are found in many tissues and organs and have the potential to regenerate the organs where they found. Researchers had the experience of clinical application of adult stem cells from bone marrow for the regeneration of haemopoietic system. Haemopoietic and mesenchymal stem cells are present in haemopoietic tissue. Haemopoietic stem cells are the precursors of blood cells and the mesenchymal stem cells are the precursor of stromal cells of bone marrow and other lineages i.e. chondrogenic, osteogenic and adipogenic lineages. There are number of important points that should be kept into consideration before the clinical application of the stem cells i.e. easily available, ethically acceptable, physiological normality, regulatory compliance and isolation area. Due to ethical consideration blood and bone marrow are the easily available source for the isolation of stem cells. Cells taken from these tissues and their ability to differentiate into specialized cell types will allow them for the clinical application of stem cell therapy.