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Challenges and Limitations of Tissue Engineering
Organ and tissue transplantation in developed countries is in steady increase. There are whole networks in some parts of the world, and these methods have significantly prolonged lives of many people. However, there are some difficulties in simple organ and tissue transplantations. The main difficulty is shortage of matched organs for people who are waiting for transplantation. Next disadvantage is constant immunosuppressive therapy which can damage health of the patients. Impossibility of nerve transplantation and limited life of transplanted organs are also big disadvantages of this technique.

Most of these problems can be solved in future by constructing whole organs and tissues in laboratories. Tissue engineering has a lot of hard but not impossible tasks. Tissue engineering's objectives are repairing and replacing the malfunctioned organ or preserving and improving of the function in vital organs. Unfortunately, in presence, only repairing of a lost function is possible, other objectives are about to be achieved.

Tissue engineering is based on creating tissues of cells and scaffold (extracellular matrices that provide support to cells)and suitable conditions under which cells are kept.

Cells in Artificial Tissue

Every organ and tissue in human organism is consisted of cells and scaffold. For laboratory cell growing, the ideal cell would be easily accessible, huge proliferation ability and property to differentiate into desired cell type. Many types of cells are available today, and we can divide them in many ways, but the simplest way is to divide them in 1. Autologous cells (cells from the same person) 2. Allogenic cells (cells from same species) 3. Xenogenic cells (cells from another species individuals). Some authors are using stem cells separately in this classification, and some include them in first or second category. Another, maybe better classification is in five types of cells are available for tissue engineering.

First type of cells are adult cells taken from the same person. This way of tissue production is very good, because the donor is the recipient. Thus infection with viruses if avoided. Also good side of this method is complementary tissue, because immune response is not activated after this way of transplantation. Second type of cells are adult stem cells (ASC). ASC of the different tissues are responsible for reparation of that tissue by modifying themselves into type of cells which are lacking. Also mesenchymal stem cells are good way of replacing adult stem cells. Third type of cells are embryonic stem cells. There are opinions that embryonic stem cells are not that reliable because of their potential tumorigenic properties. Fourth type are induced pluripotent stem cells (iPSC). Scientists have managed to build these cells by reprogramming adult cells with retroviral activity. This is excellent concept providing us plenty of autologous stem cells. However, these cells are potentially oncogenic because this type of stem cells relies on retroviral vector that integrates in hosts genom. Fifth type are cells from placental tissue, umbilical cord and amniotic fluid. This type can be considered as part of the third type or embryonic stem cells, but it has no tumorigenic properties.

Scaffold in Artificial Tissue

Scaffold is artificial three-dimensional structure for cell implantation. It is made of glycoproteins and proteoglycans. It presents simulation of extracellular matrix in human tissue. Scaffold structures should be made of bioresorbable materials like collagen and some polyesters. Commonly used material is polylactic acid.

Cells are carried out in two dimensions and if scientists want to have third dimension they use scaffold structure. Scaffold must have micro pores for diffusion of nutrients, growth factors and diffusion of the waste. Tiny pores suitable for blood vessels are also expected from scaffolds.

Today, new technologies give possibility of organ-printing. Organ-printing is process of making scaffold from small fragments with complete control of making structure. This way of making scaffold is more complex, but it can provide space for future vascular tree. Providing space for vascular elements is crucial and limiting factor in making various kinds of tissues.

Instead of making scaffolds, there are some alternatives in tissue bioengineering. The important sources for cell implantation are native decellularized tissue and organs. This kind of scaffold is better than artificial scaffolds because they already have developed structure of blood vessels. This technique has provided production of whole organs such as heart and liver. This scaffolds are most likely the future of laboratory organ-making process.

Tissue Engineering Problems

Beside cells and scaffolds, external conditions are very important factors. There are some basic conditions that have to be provided for tissue growth. Temperature of 37 °C with 5%-10%, isolation and growth and differentiation factors are required. In many cell cultures these basic factors are not sufficient. Physical and chemical factors, hormones, specific metabolites are necessary for some tissues. Thus chondrocytes should be used to hypoxia, shear stress is involved in normal differentiation of endothelial cells and others. All these additional factor are obligatory for functional development of tissues.


Our organs are receiving constant stimulations from electrical, biochemical, mechanical and other stimuli. These stimuli help our organs to work properly. Also tissue engineered tissues should receive these stimuli. This can be obtained with bioreactors. Bioreactor is device that provides mechanical and biological stimuli. There are many bioreactors for various tissues. Every bioreactor provides different stimuli for specific tissue.

Angiogenesis in TE Tissues

The limitations in production of more complex tissues and organs for sure is nutrition of tissues. Tissues thicker than few millimeters are vulnerable because diffusion is not effective for supplying central parts of the tissue. This is the biggest limitation in TE and scientists are struggling with this obstacle.

Tissue Bioengineering is for sure the future of transplantation. It has many advantages in comparison with traditional organ transplantation. However, there are some limitations and challenges for scientists. Major challenges for TE is the need for more complex functionality, providing additional factors for tissue and organ development and development of ideal scaffolds.
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