Stem cells are biological cells found in most, if not all multicellular animals. They are characterised by the ability to go through numerous mitotic cycles while maintaining the undifferentiated state and differentiate into diverse specialized cell types. Stem cells are responsible for development, repair of adult tissues and cancer, when the control over them is lost. Stem cell research has been hailed for the potential to revolutionize the future of medicine with the ability to regenerate damaged and diseased organs. In addition to self-renewal, stem cells also exhibit 'cellular potency'.
Potency specifies the differentiation potential of the stem cells. Totipotent (Latin, 'totus'means entire)/ omnipotent; These stem cells can differentiate into embryonic and extra embryonic cell types. Such cells are capable construct a viable ,complete, organism. Cells produced by the first few divisions of the zygote and even those from the morula are totipotent. Pluripotent (Latin,plures means several or many) stem cells are the descendants of totipotent cells and can differentiate into nearly all types of cells, i.e. cells derived from any of the three germ layers, endoderm ,ectoderm, and mesoderm, e.g:-embryonic stem cells. Multipotent stem cells can differentiate into a number of cells, but the cells are only those of a closely related family of cells,e.g.haemopoietic stem cells. Unipotent (Latin, unus means one) cells can produce only one cell type, their own, but have that the property of self-renewal, which distinguishes of them from non-stem cells (e.g. muscle stem cells).
In mammals we find there are two broad types of stem cells: embryonic stem cells and adult stem cells.
Embryonic Stem Cells (ES CELLS)
They are isolated from the epiblast tissue of the inner cell mass of a blastocyst. ES cells are pluripotent and can give rise to the three primary germ layers-ectoderm, endoderm and mesoderm. When given the necessary stimulation, they can develop into more than 200 cell types of the adult body. ES cells are immortal cells i.e. they can proliferate in a culture medium and they are a source of a large number of cells in the undifferentiated state.
Adult stem cells
They are found in various tissues of children as well as adults. Any cell which is found in the body of a developed animal that has the ability to divide and create another cell like it is called an adult stem cell or somatic stem cell. These cells act as a repair system for the body, replenishing adult tissues. Most of the adult stem cells are multipotent.
The red bone marrow is a rich source of adult stem cells. The examples include mesenchymal stem cells, endothelial stem cells, adipocyte stem cells, etc.
Pluripotent adult stem cells are to be rare and generally in small number; occur in umbilical cord.
Haemopoietic stem cells (HSCs) of red bone marrow are multipotent stem cells. HSCs are of two types namely myeloid stem cells and lymphoid stem cells that give rise to all the blood cell types.
lymphoid stem cells And Myeloid stem cells are together called secondary stem cells.
Myeloid stem cells: These are non-renewing cells and give rise to erythroid committed progenitor, basophil committed progenitor, eosinophil committed progenitor, granulocyte-monocyte progenitor and Megakaryoblast. Each of these cells give rise to different but specific cell(s).
Erythroid committed progenitor (proerythroblast): Erythrocyte is formed through stages like proerythroblast, erythroblast and reticulocyte. Basophil committed progenitor: Gives rise to basophil. Eosinophil committed progenitor: Gives rise to eosinophil. Granulocyte-monocyte progenitor: it gives rise to myeloblast and monocyte committed progenitors. Neutrophil is formed from myeloblast committed progenitor and monocyte is formed from monocyte committed progenitor.
Megakaryoblast: it forms megakaryocyte which by fragmentation gives blood platelets.
Lymphoid stem cells: It gives rise to T-cell committed progenitor and B-cell committed progenitor. T-cell committed progenitor differentiates into T-lymphocyte in Thymus. T-cells may be T-helper or T-cytotoxic cell. B-cell committed progenitor produces B-lymphocyte in the bone marrow.
Note: Natural killer cells originate directly from lymphoid stem cells whereas dendritic cells originate from both myeloid stem cells and lymphoid stem cells directly.
Applications Of Stem Cell Therapy
Stem cells, taken from umbilical cord, can be transform into specialized cells to replace myocytes or neurons or perhaps the entire organism through cell culture. A number of adult stem cell therapies already exist.
