A New Age of Regenerative Medicine: Fusion of Tissue Engineering and Stem Cell Research

After the biotech medicine era, symbolized by the progress of genetic engineering and the developments of physiologically active substances such as proteins and cytokines as medicine, the regenerative medicine era, when cells and tissues become medicine, is going to start and getting popular in recent years in the world. In particular, regenerative medicine is highly expected as curative-therapeutic treatments that can cure patients with obstinate diseases and physically impaired function, while the conventional symptomatic treatments are unable to do so. For supporting and enhancing the progress of regenerative medicine, remarkable advancements in regenerative medical sciences, and cell and tissue engineering are desired worldwide. Furthermore, to achieve new treatments based on regenerative medicine, multidisciplinary approaches from various fields such as molecular biology, cell biology, science and engineering, and pharmaceutical and medical sciences are essential. Regenerative medicine should be recognized as a completely new interdisciplinary academic discipline, which is unable to be obtained from the extrapolations of vertically connected and conventional academic disciplines. Therefore, for realizing regenerative medicine, not only the reconstruction of conventional medical science but also the establishment of new academic field, which integrates and assimilates scientific and engineering technologies, and biotechnology, become important. For example, technology controlling the expansion and differentiations of embryonic stem (ES) cells and induced pluripotent stem (iPS) cells is essential for obtaining the sufficient numbers of the cells allowing desired medical treatments to be realized. For separating remaining small amounts of undifferentiated ES and iPS cells from the differentiated cells with a high accuracy, an interdisciplinary research project having the horizontally integration of science, engineering, medicine should be considered. Further, the developments of technologies, which can efficiently transplant somatic cells in the body, are more important than those of cell sources, and new cooperative and integrative systems having various academic disciplines including medicine, science, engineering, and pharmacy are extremely important. For example, harvested cells, which are obtained from culture dishes by treating them with enzymes including trypsin or dispase after being cultured and expanded, are damaged in their structures and functions by enzymatically cleaving their cell-membrane proteins. Upon the direct injection of suspension containing damaged cells following the enzyme treatments to the tissue or organ, approximately 95% of the cells fail to stay in the target, and only less than several percent of the cells transplanted in the organ is speculated to be effective (Hofmann et al., 2005). The unwelcome fact that possible therapeutic effects are expected with a small number of transplanted cells should be noticed seriously. Tissue engineering is extremely important in terms of the more efficient engraftment of cell transplantation and is expected to proceed further with the integration of the technology of biomaterials contacting cell directly and bioand medical-technologies. One of the tissue engineering approaches is biodegradable polymer scaffold-based methods (Langer and Vacanti, 1993). It is important to promote research that investigates how the shape and function of cells cultured in the scaffolds are maintained after transplantation. In this special issue, the recent progress of tissue engineering approaches using biodegradable polymer scaffolds are reviewed by Rui Reis and his colleagues on periodontal tissue, Charles Vacanti and Koji Kojima on trachea, Toshiharu Shinoka and his colleagues on vasculature, Dietmar Hutmacher and his colleagues on osteochondral tissue, and Stephan Badylak and his colleagues on skeletal muscle. For maintaining cell-dense thicker tissue viability, sufficient amounts of oxygen and nutrients should be supplied into the tissue. However, scaffoldbased tissue has a heavy burden to have supplies of oxygen and nutrients from the scaffold surface by diffusion, which is hampered with increasing cell density. The developments of new technologies to break through this problem would be desired. Another approach for tissue engineering is cell sheet-based methods (Yang et al., 2007). Dense monolayered cell sheets that are harvested from temperature responsive culture dishes are fully viable and almost all of the cells are engrafted and keep their function after transplantation. In this special issue, the cutting edge research using cell sheet-based tissue engineering methods are reviewed by Teruo Okano and Kazuo Ohashi on liver and islets, Tatsuya Shimizu and Katsuhisa Matsuura on cardiac tissue, Masayuki Yamato and his colleagues on periodontal tissue, and Masato Sato and his colleagues on articular cartilage. In the cornea,