While many diseases such as bacterial infections and injuries can be cured by drugs and surgery, many others remain incurable. When such incurable diseases become severe, they are often treated by transplantation of tissues and organs from a donor. For example, treatment by transplantation of organs such as the kidney, heart, liver, cornea, and bone marrow are already common. However, such cases pose the problems of preservation of the organs to be transplanted and dealing with rejection of the transplanted organs.
Regenerative medicine was devised as a revolutionary treatment method for solving such problems. This method artificially produces various types of tissue and organs from stem cells that have been made to proliferate, and transplants them into the patient (Fig. 5-8). It is basically the same as the conventional tissue and organ transplantation treatment, except that the transplanted tissue and organs are artificial. If large amounts of tissue and organs can be produced in vitro, the present concerns of a shortage of tissue and organs to be transplanted will disappear.
Fig. 5-8. Formation of Tissue from ES Cells
The blastocyst inner cell mass of mammals has the potential to develop into various kinds of cells and tissues. When cells of a blastocyst inner cell mass removed from an embryo are cultured and allowed to proliferate, and if differentiation is induced in them, they differentiate into various tissues.
However, regenerative medicine that uses ES cells poses ethical problems because it destroys embryos that otherwise could have grown into humans, and because it removes embryonic cells from these embryos (see Chapter 11). This method also has the problem of tissue rejection. Therefore, a new method is being researched in order to overcome these problems. By this method, tissues and organs necessary for the treatment are produced using somatic stem cells collected from the patient and are then transplanted back into the same patient (Fig. 5-9). If this method is successful, it will be possible to solve both the ethical problems and the rejection problem. As further developments of this method, experiments are being conducted in which normal somatic cells are artificially transformed into stem cells, which will be used to develop tissues and organs (see Chapter 11).
In any case, many technical and ethical problems remain to be solved. For this purpose, it seems that these methods will take some time to reach the stage where then can be generally used for human treatment. Nonetheless, this technology is advancing rapidly, and if the ethical problems are solved, the day when these treatment methods would be generally used to treat diseases is probably not so far away.
Fig. 5-9. Methods of Regenerative Medicine
There are mainly two methods of regenerative medicine. One method uses ES cells, and in some cases, embryos of both the patient and another person are used. However, this method uses human embryos, and thus, involves ethical problems. Another method uses somatic stem cells taken from the patient. The method does not involve the above ethical problems.
Biology and Regenerative Medicine
In biology, the word "regeneration" generally refers to the restoration of a lost portion of the body to its original structure. Newts are typical examples of vertebrates that have a high regenerative capacity. They can regenerate not only their four limbs, but even their entire eyeballs. However, except for the exchange of cells in normal metabolism and except for the high regenerative capacity of the liver, the regenerative capacity of humans is limited to the repair of bone fractures and wounds on the body surface, as much as we may desire the high regenerative capacity of newts. This difference in regenerative capacity is thought to be based on the differences in the kind and quantity of stem cells in the tissues of adults, but the cause remains unknown.
From the perspective of comparative embryology, it is thought that a mechanism common to all species functions in the development of vertebrates. How the many abilities of various animals develop from their development mechanisms and how this development is maintained as a dynamic system are fundamental questions of biology. Organisms are a type of dynamic system, and each local modification in the body probably affects the entire body. Therefore, adequate knowledge of basic biology is necessary as a basis for sufficient understanding of the human body as a system, and for administration of appropriate medical treatment. Basic research on the mechanisms of development and applied research on regeneration and regenerative medicine are both essential for continued development of regenerative medicine that does not cause ethical problems or adverse effects.
The possibility of healing injuries that greatly impact normal human life, such as injuries to limbs, eyes, and nerves, would improve our so-called quality of life (QOL) and would also be beneficial in terms of social welfare. Furthermore, complete regeneration of malfunctioning principal organs, such as the heart, kidney, liver, and pancreas, would probably reduce human suffering and the medical cost burden on the society. In addition, the development of technology for regenerating organs would benefit public welfare by preventing illegal activities such organ sales.