The undifferentiated cells in multicellular organisms which have the potential to differentiate into specialized cells and can divide to form more are called stem cells. The main characteristic features of stem cells are:
The stem cells are classified into four types:
Embryonic stem cells: The embryonic stem (ES) cells are derived from 4 or 5 days old human embryos that are in the blastocyst phase of development by in vitro fertilization (IVF) for assisted reproduction that are no longer needed. Generally, the embryos are extras as several eggs are fertilized in a test tube, but only one is implanted into a woman. These cells are pluripotent, that is, they can give rise to any cell type except placenta and umbilical cord. They provide a replaceable resource for studying normal development and disease, and for testing drugs and other treatments.
Adult stem cells: These are tissue-specific stem cells and can generate various cell types for the specific tissue or organ in which they reside. Hematopoietic stem cells in the bone marrow can generate RBCs, WBCs and platelets but can’t generate lung, liver or brain cells. The function of these stem cells is to replace cells which are lost in normal day-to-day living or in injuries, such as in the skin, blood, and the lining of the gut. The adult stem cells can’t renew during culturing as easily as the embryonic stem cells but these cells have enabled the researchers to study the events during the course of aging, injury and disease.
Mesenchymal stem cells: These are the stromal (the connective tissue layer that surrounds the tissue) cells that can differentiate into a variety of cell types such as cells of the bone, cartilage, muscle and fat tissue. The first MSCs were discovered in the bone marrow but now they are shown to be derived from placenta, adipose tissue, umbilical cord blood, corneal stroma, adult muscle, or the dental pulp of deciduous baby teeth and so they are called as multipotent stromal cells.
Induced pluripotent stem cells: There are some specialized adult cells which can be "reprogrammed" genetically to assume a stem cell-like state and are called as induced pluripotent stem cells (iPSCs). These cells have been generated in the lab by converting tissue-specific cells, such as skin cells, into cells that behave like embryonic stem cells. Both ES and iPSCs have the same use and characteristics except the fact that the first iPSC cells were produced by using viruses to insert extra copies of genes into tissue-specific cells.
The future: For sure, the most significant and potential application of stem cells is to generate cells and tissues that could be used for cell-based therapies. Generally, organs and tissue donations are used to replace ailing or destroyed tissue, but the lack of adequate supply of these hampers the treatment. When allowed to differentiate into specific cell types, stem cells offer the possibility of replacing cells and tissues to treat diseases such as diabetes, cardiovascular disease, spinal cord injury, burns, osteoarthritis and rheumatoid arthritis.
In type 1 diabetic individuals, the insulin-producing pancreatic cells are destroyed by the patient's own immune system. Interestingly, recent studies have indicated that human ES cells can be directed to differentiate into insulin-producing cells that could be later transplanted in diabetic patients.
Recent studies in cell culture systems showed the direct differentiation of embryonic stem cells and adult bone marrow cells into heart muscle cells and then these cells can be transplanted into patients with chronic heart disease. Further, it was observed that injection of stem cells directly into the injured heart tissue improved cardiac function and induced the formation of new capillaries. The mechanism for this repair is not fully understood, however, these preliminary experiments give us a new hope of using the stem cells in repairing the damaged heart tissue and treating the cardiovascular diseases. To treat the prevalent and weakening diseases, scientists should manipulate stem cells in such a way that they acquire the necessary characteristics for successful differentiation, transplantation, and engraftment.
Generally, physicians and patients believe that stem cells are the future of medicine and one can have a patient specific treatment using stem cells. In fact, some researchers assume that very soon the stem cells can be used to replace drugs. However, stem cells are alive and can replicate inside the patient body and may be they can work in a more powerful and flexible way than we imagined.
In summary, though there are significant technical obstacles associated with the use of stem cells, the intensive research on stem cells can remove those hurdles and can offer exciting promises for future therapies.