Stem cells & their source
Stem cells are basic cells which can be compared to a mass of clay which is wet. Like the clay which as long as it is wet can be molded to any shape, the stem cells as long as they retain their stemness can become any type of cell in the body of any organ.
In the process of reproduction, ovum is fertilized by a sperm and they form an embryo. The initial stages of embryo (5-6 day) have all the cells of similar nature and each one of these cells is called embryonic stem cell. Though all these cells at the initial stage of the embryo are of similar nature, they gradually multiply and differentiate into various organs which is a highly complex mechanism, least of which is understood as they take place with a coordination among millions of signals interacting among the growth factors, cytokines and other biological active compounds. Suppose there were 32 embryonic stem cells in the embryo at the initial stage each one multiplies into a separate organ like liver, heart, skin etc. This process of such baic naïve cells becoming a particular type of organ is called “Differentiation” These embryonic stem cells which can become any organ are called “Totipotent” (Fertilized ovum stage) and they differentiate to become "Pluripotent" stage and then slowly further get differentiated into organs.
Though the embryonic stem cells are pluripotent, after child birth as we grow into the adult stage, several organs retain some portion of the pluripotent stem cells within them which help those organs repair themselves when needed (Eg. Corneal limbal stem cells) or on a routine basis (Eg. Skin) or some pluripotent cells continue to proliferate to contribute to the daily metabolism (Eg Liver stem cells) or the day to day physiological functions (Eg. Bone marrow stem cells). Now that stem cells have been isolated from various organs like cornea, liver, bone marrow, skin, adipose tissue, hair root, etc, it is hypothesized that the stem cells could be present in every organ. Under normal conditions, they remain dormant and when necessity arises they multiply into the cells of the respective organ. In some organs like skin, due to everyday wear and tear, skin stem cells in the basal layer keep multiplying to form keratinocytes and other cells of the skin routinely, to replace the dead or falling-off cells from the surface. The liver stem cells which are called oval cells have been found to become either liver cells or cells of pancreas. These are called “Multipotent” stem cells and that of corneal limbal stem cells can become only corneal epithelial (as of now) cells are called “Unipotent” stem cells.
The bone marrow which is the central portion of mainly long bones has plenty of stem cells. As the components of blood like Red Blood Cells (That carry oxygen), White blood cells (That act as barriers and protectors from entry of disease causing bacteria, viruses etc), Platelets (That help clotting of blood in case of a wound) have a limited life span they should be produced routinely by the bone marrow. The stem cells present in the bone marrow are also found to be “Multipotent”, which have potential to treat several diseases and it has also been found that the bone marrow stem cells can differentiate into several kinds of tissues including cardiomyocytes, neural cells, liver cells, chondrocytes etc.,
Umbilical cord blood is another rich source of Stem cells. While in the womb, the foetus is supplied necessary nutrients for growth and development from the mothers womb through an interface organ called placenta. This placenta receives the nutrients from the mothers womb, process it in such a way that the nutrients are made acceptable to the foetus and supplies it to the foetus through the umbilical cord. After birth, when the umbilical cord is clamped, still about 50-100cc of blood remains inside the placenta, which is usually discarded. Now having been found that this 50-100cc of blood is rich in stem cells mostly of Hemopoietic origin, we save the same for it to be used in treatments of blood cancers and related disorder in future.
Therefore the source for stem cells are broadly : (i) Embryonic stem cells from the inner cell mass of the embryo, (ii) Umbilical cord blood, (iii) Aborted foetus, (iv) Adult Bone marrow, (v) Peripheral Blood (vi) Adult organs such as: Limbus of the cornea, Dentine of the teeth, Adipose tissue, Skin, Corneal endothelial cells, Ciliary epithelium of the eye, Olfactory cells, Gingival epithelium, Germ cells, Myocardium, Liver, Bone marrow etc., each of this source have their own advantages and disadvantages.
| Advantages & disadvantages with different source of stem cells
Advantages: Its pluripotent and can become ANY type of cells provided the total mechanism of differentiating them into ONLY one particular type of cells is feasible (Which is at the moment not feasible)
Disadvantage: The main issue is ethical as deriving the pluripotent stem cells from an embryo kills the embryo, a potential life. Moreover their pluripotency could lead them to form tumours (cancer) inside the body.
- Umbilical cord blood stem cells:
Advantages: Its otherwise discarded and could be stored to use as a substitute when a matched blood donor is not available. This has been in clinical use for the past 40 years or so in treating blood cancer. The antigenicity is lesser than that of the bone marrow.
Disadvantage: The contents stored (50-100cc) could be stored only for around 20 years and also the quantity stored would be sufficient only upto a child of weight 10-15kgs. Thereafter either it has to be expanded (which is very difficult at the moment) or another matched unit is to be used. The hematopoietic stem cells in cord blood is lowe than that of the bone marrow. The application of umbilical cord blood in diseases other than blood cancer or at research/clinical trial level.
Advantages: Several organs of the foetus such as liver, Eye tissues and neural tissues contain more stem cells than adults and thei rejection is lesser than the adult stem cells.
Disadvantage: The main issue is ethical as deriving the stem cells from a foetus may lead to several social and allied problems, though there are guidelines to follow such as Only when sponatanouesly aborted, the cells should be used etc (varies from country to country).
- Adult Bone marrow stem cells:
Advantages: Its multipotent and has been in clinical application since 1950 for treating blood cancers. There are possibilities to store this if processed properly for upto 10 years. This is the best source of hematopoietic and mesenchymal stem cells in an adult body. Recently several clinical trials have proven that Bone marrow stem cells of the patient could be used to treat spinal cord injury, liver cirrhosis, peripheral vascular disease etc.
Disadvantage: The quantity when to be taken, the patient has to be subjected to bone marrow harvesting which at time may require anaesthesia. If frequent tapping is necessary it may not be liked by the patient and with aging the quantity of hematopoietic and mesenchymal stem cells from bone marrow may decline.
- Peripheral blood stem cells:
Advantages: The stem cells can be withdrawn from peripheral blood using several equipments which can also be used for treating blood cancer to substitute the lost cells after chemotherapy or radiotherapy.
Disadvantage: The quantity of hematopoetic stem cells in peripheral blood is very very low compared to bone marrow and so to mobilize the hematopoietic stem cells from the bone marrow factors such as GCSF to be given for a few days, which has some contraindication in patients with recent inflammation such as stent implantation etc and also in breast cancer patients. The application of this in other diseases is at clinical trial and not very good outcomes reported due to very very low quantity of stem cells.
- Adult organ stem cells and progenitor cells:
Advantages: Though the cells are several times unipotent, if used for the patient hm/herself (Autologous) they are best suited for any clinical treatment. Fore xample corneal limbal stem cells taken from one eye can be used to treat the other eye epithelial damage.
Disadvantage: Several of the tissues have least number of stem cells and to procure them without significant damage to the function of the particular organ is difficult. Moreover expansion of the harvested cells to a quantity required for treating a disease is difficult in the lab.
|Obstacles in taking stem cells to clinical application:
Stem cells, when are in the body, they behave according to the influence and situation of the surrounding tissues/cells. But when they are taken out and grown in the laboratory they start differentiating into not only as the cell line of our choice but into various cell lines, which is beyond our full control at the moment. Moreover it is very difficult to keep them in their original undifferentiated condition while they multiply. In such case, if they are injected into any organ for ex. Liver, not only liver cells would grow, but there are chances that in the area of application (within liver as in this example), cells of other organs like bone etc may grow, which is dangerous. This facts are true mainly with embryonic stem cells. Another obstacle for expansion of the embryonic stem cells in the laboratory being the necessity for biological materials like mouse fibroblasts(called as 3T3 Feeder cells) to grow them in the laboratory for any application. When biological materials are used for growing human stem cells in the laboratory, many genomes and proteomes expressed by the animal cells, get transmitted into the human stem cells and future behavior and function of such stem cells are not clear. Recently it has also been reported that a non-human component is transmitted when animal cells are used for growing stem cells which can develop antibody in humans when implanted and lead to rejection.
Umbilical cord blood stem cells have been found to be good for hematological diseases, but though they are multipotent, undifferentiated and pure expansion is difficult at the moment. The bone marrow stem cells are considered to be multipotent, but in vitro expansion of them into a particular cell type/tissue and application into the bdy is difficult and so application of them after isolation of the stem cells alone or specific mesenchymal or hematopoietic stem cells is undertaken as clinical treatment for various diseases as of now.
Apart from the above two major obstacles, minor problems like isolation of them without damage to the organ (in adult stem cells), an appropriate vehicle for applying them into the organ of choice, prolonging their viability in situ for a longer period of time are yet to be cleared fully. The dosage of the stem cells required for any kind of treatment is also not yet fully standardized except in autologous stem cell transplantation of bone marrow for blood cancers.