Stem cells are unspecialized cells in the human body that possesses two prominent properties: a capability for self-renewal and potency, which is the efficiency of proliferation and differentiation to various cellular lineages under suitable conditions. Essentially, stem cell stays uncommitted until it is signaled to change into a specialized cell. Stem cells have the special properties of developing into an expansion of different cells in the human body. They function as a repair system by being able to divide without restriction to replenish other cells.

Potency of Stem Cells

The stem cell Potency indicates the differentiation capability which means the ability to specialize into various types of cell:

• Totipotent stem cells Can differentiate into embryonic and extraembryonic cell types. The fertilized egg and the cells generated by the first few divisions of the fertilized egg are totipotent.
• Pluripotent stem cells Are considered the progeny of totipotent cells and can specialize into almost all cells including cells derived from any of the three germ layers. Pluripotent cells are distinguished by self-renewal and a differentiation capability to all cell types of the adult organism. Embryonic Stem Cells are categorized under this group.
• Multipotent stem cells Can produce several types of cells, but limited to those of closely related types. For example, hematopoietic (adult) stem cells are multipotent stem cells that can generate all blood cells.

Types of stem cells

Stem cells are divided into 2 main forms. They are embryonic stem cells and adult stem cells.

1. Embryonic stem cells. The embryonic stem cells used in research today come from unused embryos. These result from an in vitro fertilization procedure. They are donated to science. These embryonic stem cells are pluripotent. This means that they can turn into more than one type of cell.
2. Adult stem cells. There are 2 types of adult stem cells.

• One type comes from fully developed tissues such as the brain, skin, and bone marrow. There are only small numbers of stem cells in these tissues. They are more likely to generate only certain types of cells. For example, a stem cell that comes from the liver will only make more liver cells.
• The second type is induced pluripotent stem cells. These are adult stem cells that have been changed in a lab to be more like embryonic stem cells. Scientists first reported that human stem cells could be changed in this way in 2006. Induced pluripotent stem cells don''t seem to be different from embryonic stem cells, but scientists have not yet found one that can develop every kind of cell and tissue. Induced pluripotent stem cells (iPSCs) are derived from skin or blood cells that have been reprogrammed back into an embryonic-like pluripotent state that enables the development of an unlimited source of any type of human cell needed for therapeutic purposes.

Merits

1. The utilization of iPSCs has resolved the ethical debate concerning the use of ESCs in research and overcome the imposed restrictions on either human ES cell research or production.
2. Minimized immunorejection options. iPSCs are produced from the somatic cells of the same individual, therefore there is no hazard of immunorejection of these autologous cells.
3. Detection and study of toxicity/therapeutic reactions of recently produced drugs.
4. Reduce the total cost and risk of clinical trials. iPSCs can provide information on drug intoxication through various cytotoxicity analyzes and reduce the budget associated with the use of animal models, ultimately reducing the cost of clinical trials.
5. The application of a personalized approach to drug delivery and individual modeling of disease by iPSCs allows the screening of disease development and pharmacological agents to offer the appropriate choice for each individual. 6. Gene targeting and Correction Techniques (Gene Therapy). Reprogramming of somatic cells with genetic mutation to iPS cells enabled the production of cell lines that have mutations that cause disease. The ability to alter specific sites in a genome to modify specific mutated genes is very important here.

 Demerits

• For putative regenerative medicine applications, patient safety is the foremost consideration. Standardized methods must be developed to characterize iPSCs and their derivatives. Furthermore, reprogramming has demonstrated a proof of-principle, yet the process is currently too inefficient for routine clinical application.

• Despite numerous technical advances in the derivation of human iPSCs, relatively little is known about their molecular and functional equivalence to Embryonic Stem Cells (ESCs), which could affect their potential therapeutic utility.
• There are also concerns regarding risks and experimental interventions which may be irreversible. Generation of iPSCs make use of retroviral or lentiviral systems, so, it needs to be concerned if the viral systems get incorporated with the host genome. The genetic material inserted via retroviral vectors may randomly integrate into the genome of the host which can cause genetic aberration and teratoma formation.
• The use of the Myc gene as a transcription factor of reprogramming and/or the reactivation of a silenced c-Myc gene in iPS cells could lead to cancer cells.

Addressing this question will require a careful analysis of the genomic integrity of human iPSCs, as well as the development of optimized differentiation protocols and reliable assays to evaluate the functionality of iPSC-derived specialized cells.