Although these cells meet the defining criteria for pluripotent stem cells, it is not known if iPSCs and embryonic stem cells differ in clinically significant ways.

Mouse iPSCs were first reported in 2006, and human iPSCs were first reported in late 2007.

Mouse iPSCs demonstrate important characteristics of pluripotent stem cells, including expressing stem cell markers, forming tumors containing cells from all three germ layers, and being able to contribute to many different tissues when injected into mouse embryos at a very early stage in development.

Human iPSCs also express stem cell markers and are capable of generating cells characteristic of all three germ layers.

Although additional research is needed, iPSCs are already useful tools for drug development and modeling of diseases, and scientists hope to use them in transplantation medicine.

Viruses are currently used to introduce the reprogramming factors into adult cells, and this process must be carefully controlled and tested before the technique can lead to useful treatments for humans.

In animal studies, the virus used to introduce the stem cell factors sometimes causes cancers. Researchers are currently investigating non-viral delivery strategies.

In any case, this breakthrough discovery has created a powerful new way to “de-differentiate” cells whose developmental fates had been previously assumed to be determined. In addition, tissues derived from iPSCs will be a nearly identical match to the cell donor and thus probably avoid rejection by the immune system.

The iPSC strategy creates pluripotent stem cells that, together with studies of other types of pluripotent stem cells, will help researchers learn how to reprogram cells to repair damaged tissues in the human body.

Source: NIH

A cryopreserved bank of cells is a kind of freezed culture. The cells can be characterised, qualified
and shared.

Otherwise , the cells stored are highly susceptible to the adverse effects of laboratory accidents and microbial
contamination. They are also prone to genetic change which may not be recognized on microscopic examination but can have critical effects on their properties and characteristics. Thus cell banks also provide vital safe depositories of
precious material that would otherwise be lost over time.

Using stem cells banks as distribution centers also promotes standardisation of experimental research. A stem cell bank subjects all the stem cells to the same system of quality control and safety testing. This would provide a central resource with expertise in appropriate quality standards.

This would also influence more efficient international transfer of cells, by providing a central point for information on regulation.

Tissue injuries are normally repaired by formation of fibrous scar tissue in humans. But a number of tissues in mammals including epithelia, bone marrow, and muscle undergo regeneration to keep tissue integrity after cell loss .

Some lower animals can undertake full organ regeneration.Though not entirely clear, two  mechanisms play roles in regeneration.

• Stem cells mediation
• Dedifferentiation of lineage specified cells

In the freshwater planarian,  pluripotent, stem-like cells called neoblasts distributed throughout the body divide and differentiate into the approximately 40 cell types necessary to regenerate the planarian body from small fragments.

Activation of preexisting stem cells in mammals is responsible for the limited regeneration seen in tissues such as muscle and blood

The extracts of the newt regenerating limb  render dedifferentiation of multinucleated mouse myotubes into mononucleated myoblasts in vitro.  This suggests that some of the pathways involved in regeneration are conserved. This area of dedifferentiation is being actively pursued. Aparticular strain of mice  confers the ability to regenerate, resembling that of amphibians. This raises the hope that humans may also have untapped regeneration capacity.

Stem cell biologists study the molecular pathways involved in self-renewal and lineage-specific differentiation to generate specific cell types required for clinical applications.

Human embryonic stem cells that have been propagated for more than two  years also demonstrate a stable and normal complement of chromosomes. This is in contrast to the unstable and abnormal complement of embryonic cancer cell lines used in the past.

Research on human embryonic stem cells is very recent. The data about properties of stem cells comes from mice.

The embryonic stem cells in mice  have been studied for  more than two decades  and provide a critical basic knowledge.

Types of cells derived from cultured mouse embryonic stem cells are

• Fat cells
• Brain and nervous system cell
• Insulin-producing cells of the pancreas
• Bone cells
• Hematopoietic cells
• Yolk sac
• Endothelial cells
• Primitive endodermal cells
• Smooth and striated muscle cells including  heart muscle cells.

Mouse embryonic stem cells, to proliferate in an undifferentiated state require

• The presence of  leukemia inhibitory factor
• A layer of mouse embryonic fibroblasts (feeder cells) in a medium containing serum from cows.

The feeder cells produce growth factors that sustain the embryonic stem cells.

if feeder cells are not present,  human embryonic stem cells form aggregated balls of cells called embryonic bodies  that has cells from all the three layers.

Human embryonic stem cells injected into mice form a type of benign tumor called a teratoma that is made up of tissues from all three embryonic layers forms tissues like teeth, gut, hair follicles, skin, epithelium, muscle, bone, cartilage, lung tissue, and neural cells.

The experiments showed the capability of ESCs to produce a variety of tissues but also  also highlight danger of causing malignancy. It has been possible to create a lineage of mouse embryonic stem cells that generate neural cell precursors.

Embryonic stem cells have a definite role in regenerating the tissues. We would know better as the time unfolds.

Hematopoietic stem cells are used in bone marrow transplant and have the ability to self renew in the marrow and to differentiate into the full complement of cell types found in blood.

They were first adult stem cells to be discovered.

Heamatopoietic cells are found in the bone marrow a, fetal liver, fetal spleen, umbilical cord and placental blood.

The cells are discovered nearly four decades ago but their ability to grow and  generate tissues other than those of the blood system has recently come into light. this property is called plasticity. therefore it is hoped that we might be able to grow many lines of stem cells from these hematopoietic cells.

There are encouraging studies that report this.

If possible this would avoid the risk graft versus host disease associated with embronic stem cell tissues.

But  there are a  lot of possible  problems. First is that human adult stem cells are rare, of few types  and difficult to isolate.

Most of these are difficult to grow in culture. Much of the work on human adult cells is of hematopoietic stem cells an results of this work can not be generalized to other types.

However, adult stem cells if they can be harnesses have potential to produce  histocompatible transplant which would not trigger immunological responses.

Stem cells are surrounded by controversies

Embryonic stem cells have been center of controversies. The issue s involved are legal, ethical, religious and moral. As it involves area surrounding human reproduction, it is natural to get much attention and opinion from all authorities and fields just like contraception, abortion, and in vitro fertilization.

Actually it is the way society takes this issue of human life and the moral and legal status of the human embryo.

Greatest objection to embryonic stem cell research comes from the fact that research deprives a human embryo of any further potential to develop into a complete human being.

It amounts to killing of  life  of  a human being and killing humans for treating humans is not justified. But even society does not universally agrees when an embryo can be called live.

Let us look at this

Eggs and sperm mixed together in a petri dish have no legal status, because they are not even part of a human being unless implanted in a woman’s womb.

In Jewish law and tradition the embryo has no moral status until 40 days after implantation.

Muslim tradition accords legal and moral status to the fetus only after ensoulment  at the end of the fourth month of pregnancy.

Catholic Church, requires that human life be protected at the earliest possible time starting from conception

Most accept embryonic stem cell research.

Thus we have a spectrum of people from complete ban to selective use of embryonic stem cells. Some maintain that laboratory cultures can be used but embryos from womb should not be taken while  other attach a time before which fetus can be killed for a noble purpose.

No doubt he issue is complex and demands evolution of science and society as well.

TIll now two major risks which have been identified are

• Tumor formation
• Immune rejection.

Tumor Formation

It has been found that human embryonic stem cells when injected into mice can produce a benign tumor made up of various tissues. This response is surely due to ability of stem cells to divide  this response is believed to be related to the multipotency of the undifferentiated cells in an in vivo environment. But the problem is not found with the cells which have already begun differentiation before transplantation.

Nevertheless, this problem must be understood and checked. It is very important to understand how body regulates the  differentiation of stem cells. There is a long way to go in this matter. Somehow stem cells need to be told that they would be transforming into a particular tissue type rather than going haywire and produce whatever they feel like.

These growth factors or inducer cells are stil being experimented.

Immune Rejection

Rejection is a serious obstacle to successful transplantation of stem cells. Embryonic stem cells provoke an immune rejection by the recipient which might result in failure of the cells. though the degree is less  but it is still there.

Removal of the immune reaction triggering antigens is the major concern and various options are being considered like matching histocompatibility antigens of the recipient , produce stems cells from less antigenic organs or  using  genetic engineering to eliminate  surface antigens on them.

An understanding of how to prevent rejection of transplanted cells is a great challenge to the research. Only then stem cells could be used in  medicine.

Neural stem cells, which are numerous in the fetal brain, can be isolated and grown in an undifferentiated form in culture, and they have been shown to differentiate into the three main types of brain cells.

These cells have been used in rodent models of Parkinson’s disease.

Neural crest cells arise from the neural tube and migrate from it throughout the developing fetus. They are able to develop into multiple cell types, including the nerves that innervate the heart and the gut, non-neural cells of hormone-secreting glands, pigment cells of the skin, cartilage and bone in the face and skull, and connective tissue in many parts of the body. Neural crest cells from mice have been cultured in the laboratory.

The fetal liver and blood are rich sources of hematopoietic stem cells, which are responsible for generating multiple cell types in blood, but their properties have not been extensively investigated. The umbilical cord and placenta are also rich sources of hematopoietic stem cells.

Multipotent cells called primordial germ cells have been isolated from the gonadal ridge.

It must be understood that we are still in infancy as far as stem cell research is considered. At the same time it is the need of time to monitor the research so that it does not go astray. Every research is a promise or threat. We must make sure that it does not become a threat.

Evaluation of potential risks to research subjects and ensuring compliance with all legal requirements must be done.

We must work for betterment of society. Stem cell is cutting edge and very promising. But it needs to be watched.