As the most important repair method in more than a century, the autologous tissue transplantation has saved the lives of many patients; but in this method, the autologous healthy tissue with an appropriate size and even a larger range must be harvested, which causes new artificial damage and is called the repair mode “robbing Peter to pay Paul.” And because the donor tissue is limited, especially for some special cases (such as large area of trauma and unique organ defects), sometimes the corresponding autologous donor tissue simply cannot be obtained.

Although this repair method will not cause additional trauma to the body of the patient, the source of the donor is limited and tissue typing is difficult. Therefore, it is very difficult to obtain tissues or organs with exactly matched histocompatibility antigens, and more than half of the patients die when waiting for a transplant every year according to statistics. In addition, the long-term and wide application of immunosuppressive agents after transplantation will not only lead to serious complications, but the high medical cost also brings a heavy financial burden on the patients.

The artificial materials have achieved a good clinical effect in the repair of certain tissue (such as bone tissue) defects, but because they don’t have biological functions, it is difficult to achieve real sense of clinical repair. As society progresses, people’s requirements for quality of life continue to increase; to explore a repairing technique which causes a small wound and can also well recover the morphologies and functions of the tissues and organs has become one of the important research directions in reconstructive surgery; the tissue engineering technology is produced in such a background.

The seed cells are the basis for the tissue construction. As the seed cells for tissue engineering, the cells must meet the following conditions: (1) the cells have a wide range of sources and are easily obtained with a sufficient quantity; (2) they have strong in vitro proliferation abilities, and the large-scale amplification can be carried out; and (3) they have specific biological functions.

The original conceiving of the tissue engineering is to take a small piece of homologous normal tissue to obtain a large number of target cells through the method of in vitro culture and amplification. But the results of many previous studies indicate that in vitro amplification of mature cells is difficult; the cells under culture conditions age quickly and lose the proliferation abilities and thus are unable to meet the requirement for tissue construction. For example, in the construction of cartilage tissue, most cells contained in the harvested cartilage tissue are mature chondrocytes; after digestion and culture, the cells after four or five generations of passage culture will be aging, the cell proliferation ability is decreased, and ultimately the enough amount of cells cannot be obtained to construct a cartilage tissue which is greater than the volume of the original cartilage. As we all know, the normal tissue has a physical ability of self-renewal and repair, and studies have confirmed that the main component participating in the renewal and repair are the tissue-specific stem cells. For example, the blood cell renewal is mainly participated by hematopoietic stem cells, the skin epithelial cell renewal is participated by the epidermal stem cells, and intestinal epithelial cell renewal is participated by the intestinal gland stem cells; even in some tissues with slow cell renewal, such as nerve tissue, the existence of tissue-specific stem cells is also confirmed. In recent years, due to the rapid development of stem cell research, a variety of tissue-specific stem cells have been found and successfully separated and cultured, which develops a new source for the seed cells for tissue engineering.

In general, the stem cells can be divided into embryonic stem cells and adult stem cells (namely, the tissue-specific stem cells) according to the different differentiation stages. The adult stem cells not only have certain abilities of in vitro amplification and differentiating into specific cells but also have the advantages of the autologous materials, thus avoiding the problem of immune rejection, and they have become the current research focus on seed cells for tissue engineering research. At present, the technologies including separation, culture, amplification, and induced differentiation of a variety of adult stem cells such as bone marrow stromal stem cells, adipose stem cells, epidermal stem cells, hair follicle stem cells, and limbal stem cells have been initially established. The bone marrow stromal stem cells come from the mesenchymal stem cells in the bone marrow, and they are involved in the bone metabolism and hematopoietic supporting functions under the physiological state. Some studies confirm that the bone marrow stromal stem cells can be differentiated into bone, cartilage, and fat cells under different conditions for induced differentiation; the cells after induced differentiation can be compounded with the biodegradable materials and then are implanted into the body to form the bone, cartilage, and fat tissue, respectively. The bone marrow stromal stem cells after induced differentiation and in vitro amplification not only have successfully repaired the defects of tissues such as bone and cartilage within the animal body but also have achieved stable and reliable curative effects in clinical application. The adipose stem cells are the mesenchymal stem cells present in the adipose tissue; they have the cell phenotypes which are mostly the same as those of bone marrow stromal stem cells; meanwhile, they have the abilities to differentiate into bone, cartilage, and fat cells; and in vivo experiments confirm that they can also be used as seed cells participating in repair of bone and cartilage defects. The epidermal stem cells are the stem cells present in the epidermal tissue, and they participate in the renewal and repair of epidermal cells. The tissue-engineered skin tissue constructed with the epidermal stem cells has been used to repair the skin defects in clinic. The hair follicle stem cells can repair the skin defects and can also participate in reconstruction of skin appendages such as the hair, sebaceous glands, and sweat glands. The limbal stem cells are the progenitor cells of the corneal epithelial cells and are distributed in the margin of the normal cornea, and they participate in the renewal and repair of the corneal epithelial cells under the physiological state. The tissue-engineered corneal epithelium constructed with limbal epithelial cells can repair the corneal epithelial defect in animal experiments. The successful separation and cultivation of various different tissue-specific stem cells create the conditions for reconstruction of a variety of tissues and organs.

However, at this stage, the method using tissue engineering technology to repair defects is to use autogenous tissue stem cells, which is a completely individualized treatment means. From the perspective of the long-term development trend of tissue engineering, realizing the large-scale treatment is the development direction of the tissue engineering. How to develop from the individualized treatment to the large-scale treatment and realize the industrialization of the tissue engineering technology has raised a higher requirement to the seed cells. The development of the allogeneic stem cells and the seed cells of general type will be the main research direction of seed cells for tissue engineering. Some successful applications of allogeneic stem cells, such as the successful use of the allogeneic bone marrow stem cells to repair tissue defects, suggest the feasibility of allogeneic stem cell applications. The embryonic stem cells have become the most promising new seed cells because of their unlimited proliferation and totipotent differentiation capabilities. The embryonic stem cells are derived from the inner cell mass of the early blastocysts, can be amplified unlimitedly under appropriate culture conditions in vitro, and thus maintain an undifferentiated state; after the removal of the factors inhibiting cell differentiation, the embryonic stem cells can spontaneously differentiate to three germ layer cells. When the cells are injected into the mice with immune dysfunction, the teratoma comprising three germ layer cells can be formed, which demonstrates that the embryonic stem cells have the ability to differentiate into all somatic cells. Currently, there have been many reports on human embryonic stem cell lines, especially the successful clone of the human embryonic stem cell lines making it possible to establish individualized embryonic stem cell lines. And if the parthenogenesis technology can be used to successfully establish the homologous diploid embryonic stem cell bank of general type, it will make the tissue matching become simple and convenient just like the blood matching; it will completely solve the problem of seed cell source for tissue engineering and lay a foundation for the industrialization of tissue engineering.

In the process of tissue engineering material preparation, if the tissues are difficult to be drawn materials from or have only a small amount of stem cells, selecting and developing the homologous cells as the sources of seed cells is alternative to researching stem cell. For example, in the tendon tissue construction, the number of tendon cells is small, the amplification ability is poor, and there are no reports on successful separation of tendon stem cells. At this time, the homologous skin fibroblasts which are developed and obtained easily with a strong amplification ability can be used to replace the tendon cells to successfully repair the tendon defects in the animals. In the urothelium construction, the replacement of the urethral transitional epithelium with epidermal stem cells can successfully repair the urethral defects. The successes of these studies suggest the feasibility of application of developed homologous cells and find a new way for the source of seed cells.

The biological material is another core of the tissue engineering research. It is the three-dimensional scaffold the seed cells must rely on for survival and attachment prior to forming into tissues and provide spaces for physiological activities such as cell proliferation, differentiation, nutritional exchange, metabolism, and extracellular matrix secretion. The tissue-engineered biological materials, in addition to requiring the characteristics of general biological materials such as no toxicity, no adverse reactions, adequate sources, stable nature, easy storage, and easy disinfection, also must meet the following basic requirements:

There are a wide variety of tissue-engineered biological materials. The tissue-engineered biological materials are generally divided into two categories of natural materials and synthetic materials based on their origins, and both have their own advantages and disadvantages. The natural materials such as collagen, chitosan, coral, and acellular matrix have good cellular affinities and tissue compatibilities, but the properties are unstable. There are big differences in pore diameter, porosity, degradation rate, and mechanical strength among the same natural materials of different species and individual sources, and it is more difficult to form a standardized product. The artificial synthetic material, such as polylactic acid (PLA), polyglycolic acid (PGA), and their composite (PLA-PGA), has uniform and stable properties and good plasticity and reproducibility and can form into the standardized products, but both their cellular affinity and tissue compatibility are poor, and their implantation into the body can cause severe inflammation reaction. The organic combination of synthetic materials and natural materials can play the effect of learning from others’ strong points to offset one’s weakness, which has become a new development trend of tissue engineering materials in the future.

These mentioned above are only the most basic requirements for tissue engineering materials. For the material researches, in addition to developing new materials, carrying out the material surface modification processing to improve the cellular affinity, and using different manufacturing processes to produce the scaffold structures with different spatial structure, destination, and microstructure, the more important thing is to study the relationships between materials and cells and the effects of the researched materials on the tissue formation and their mechanisms. Because different tissues have different compositions and spatial structures which assume different physiological functions within the body, there are different requirements for the materials; only the relationship between the materials and the tissues is to be clarified; the tissue-specific scaffold materials with a certain biological activity which really meet the requirements of tissue engineering can be developed. For example, in the construction of the tissue-engineered bone, the spatial structure, pore diameter, and porosity of the material must conform to the normal bone structure, while the material should have a certain osteoinductive effect and can induce differentiation of bone marrow stromal cells and promote the formation of bone tissue; in addition, the degradation of the material must match the speed of new bone formation; both too fast and too slow degradations are not conducive to the formation of bone tissue; it is required that the formation of new blood vessels can also be promoted in the process of osteogenesis. As for the cartilage tissue construction, the bone marrow stromal stem cells are taken as seed cells; in the same way, the materials must have the chondrogenic inducibility and can promote the differentiation of bone marrow stromal stem cells into chondrocytes, while the structure of the material should be in accordance with the structural characteristics and can promote secretion of cartilage matrix in cells and inhibit the neovascularization. At present, although some biological materials which are suitable for construction of tissues such as the bone, cartilage, and skin have been developed, there is a considerable gap from the requirements of perfect tissue engineering materials.