Tissue engineering is the process whereby you use a combination of cells, such as stem cells and differentiated somatic cells (3). It is used to create natural or synthetic tissues and organs. These then can be implanted as fully functional organs or tissues or they may develop to perform necessary functions following from an implantation. For example a damaged heart can later be replaced to a fully functioning normal heart. Somatic cells are any cells forming the body of an organism and they can be defined as cells other then sex cells (4). The picture below is of a somatic cell and it shows the nucleus and the other parts of the cell.
Somatic Cells (4)
Tissue engineering is very useful for many reasons. One of the reasons is that to replace and/or repair the damaged tissue you need a donor (1). There are some implications as the donor replacement has to be an exact match for it to be used, also they may be recognised as a 'foreign' object by the immune system. This means that steps would need to be taken to suppress the immune system so that doesn't attack the donor replacement.
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Tissue engineering has also got the opportunity to provide an unlimited stock of organs/tissues (1). This would be a problem for a person looking for a donor as nowadays donor availability is low. This is where tissue engineering can become very useful.
There are 3 steps of tissue engineering
First step- cell isolation
Firstly a small sample of tissue is taken from an individual person. However not just any part of the tissue taken, the part of the tissue that is taken is where cell don't normally line the small capillaries and these are therefore omitted (1). This would mean that when the organ/tissue is reconstructed from the cells, the immune system would not recognise it as a foreign object. This is a bonus as the organ/tissue can be implanted into anyone. In the future tissue matching would not be necessary because of this. These implants are called allografts.
The cells can also be taken from animals as animal cells. These are called xenografts (1). However the downside of using the animal cells is that they would be attacked very aggressively by the immune system.
The tissues that provide the cells for tissue engineering can come from a variety of sources. For example liver replacements are made from pig's liver (1). Once the tissue has been collected it is quickly transported in ice to a laboratory and then special enzymes are used to dissolve the tissue and to release the cells.
Second step- selection and cultivation of cells
The cells are now separated into their various cell types and the required cells are then carefully grown in tissue culture dishes to increase their number.
By expanding the number of cells that are available, a great number of implants and replacements can be made available from a small sample of original tissue. For example in producing skin, a small sample of skin can be increased in numbers to form 4 acres of tissue engineered skin (1). This would be enough to replace more then 100,000 individual skin grafts. The step is illustrated in appendix on the last page
Third step- Tissue Assembly
The large numbers of cultured cells from the second step are stored in liquid nitrogen. When the cells are needed the cells are thawed and assembled into the replacement tissue/organ
When the cells are being assembled, the tissue should have sufficient strength so that it can be implanted by a surgeon. In terms of the strength the tissue or organ that is going to be replaced should be able to withstand the pressure when being used, e.g. bending knees after being replaced, it also should be able to be handled by the surgeon. The strength of the tissue would enable the replacement product to function in the patient after the surgery (1). It also has to be strong as it has to work for a lifetime or a couple of years.
However the method of strengthening the tissues/organs is yet to be developed. One method of achieving the strength that is needed is by using artificial extra cellular matrix (1). This is an extensive network of collagen fibres that surrounds the cells.
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They can be either formed form natural material (e.g. collagen) or a synthetic, biodegradable material (e.g. polyglycolic acid). This matrix is then formed into an appropriate shape and 'seeded' with the living cells.
When the cells are established into their new environment, they begin to degrade away the matrix and replace it with a typical connective tissue. This is then compromised of a complex fibrous extra cellular matrix formed from collagen, proteoglycans and glycoproteins.
The replacement begins to take place in the laboratory then continues after the material is implanted into the person's body that needs the tissue engineered replacement. For example cartilage replacements (1) are produced from tiny samples taken from the cartilage that makes the sternum and then the cells multiply and are then assembled into tissue engineered cartilage.
The implications of tissue engineering
There are positive and negative implications with the procedure and the environment in which the replacement tissues/organs are grown.
The first implication is that when freezing the cells there should be great care taken to ensure that the water that is still inside the cells when frozen does not expand (1). If the water was to expand the cells that would be needed for the tissue engineering replacement would rip apart and would not be of use. In this situation great care should be taken into account. This implication could be possibly occurring, however most of the time this doesn't occur (5).
The second implication is that the xenograft has to be placed in special chambers that would isolate the animal cells from the immune system and it would prevent the immune system from aggressively attacking the animal cells that may be used for the replacement of a damaged tissue or a damaged part of an organ. For example for a heart transplant made from tissue engineering, the immune system detects the transplant as foreign object so it tries's to kill of the heart transplant.
The third implication of tissue engineering is that of sterilisation, the following procedures are not allowed: the use of gamma radiation, use of high pressure steam and use of ethylene oxide (1). These procedures can kill the cells that are needed for the tissue engineered replacement and it may also kill any vital micro-organisms.
The fourth implication is that before the culture cells can be used they must undergo a rigorous safety testing to ensure that they don not carry any viruses (1) such as hepatitis and HIV.
The ethical issues of tissue engineering are that sometimes tissue engineering may use human embryonic stem cells (2). The ethical issue here is that because most embryonic cells are taken from a foetus, it would lead to terminating a pregnancy in most cases. So is it right to take away that life for the better of human kind.
One of the sources that I have used may not be valid as the webpage (3) did not have an author, this would make the source unreliable as we don't know who has published it and also there aren't dates in the article, and this again may make the source unreliable. This is because it would mean that article could be outdated. This could be a problem because we don not know the date there could be more updated information on tissue engineering, it would therefore be hard to judge if the article has updated information or if it is outdated.
One of the sources that were very valid is the article from the biological sciences review text (1). It is reliable but as it has any author and the author is reliable as the person clearly knows about the subject of the article. There is a date when it was published, however the date suggests that the article is outdated and new techniques could have been introduced since then.
In conclusion I have found out that their are many steps involved in making a tissue or part of a damaged organ by using tissue engineering. However with the procedures that are used, involve quite a lot of implications such as the rigorous testing on the culture cells to check for any viruses before being assembled. In my research I had also found that in order to assemble the tissue/organ, the extra cellular matrix has to be strong because if it was torn then it would of no use to the person who is getting this tissue/organ, it also has to last a long time without any replacement.
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(1) Dr Paul Kemp, (1998), biological science review, tissue engineering pages 11-13
(2) Rob B.M. de Vries, (2008) Ethical Aspects of Tissue Engineering: A Review, page http://www.liebertonline.com/doi/abs/10.1089/ten.teb.2008.0199
(3) http://www.buildingbiotechnology.com/glossary6.php Tissue engineering page 6
(4) http://www.cgm.northwestern.edu/glossary.htm somatic cells page 1
(5) http://www.xcell-center.de/treatments/glossary.aspx Tissue Engineering