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Tissue engineering is reviving itself, adopting the concept of biomimetics of in vivo tissue development. A basic concept of developmental biology is the modularity of the tissue architecture according to which intermediates in tissue development constitute semiautonomous entities. Both engineering and nature have chosen the modular architecture to optimize the product or organism development and evolution. Bioartificial tissues do not have a modular architecture. On the contrary, artificial organs of modular architecture have been already developed in the field of artificial organs. Therefore the conceptual support of tissue engineering by the field of artificial organs becomes critical in its new endeavor of recapitulating in vitro the in vivo tissue development., (© 2011, Copyright the Authors. Artificial Organs © 2011, International Center for Artificial Organs and Transplantation and Wiley Periodicals, Inc.)

Although tissue engineering uses powerful biological tools, it still has a weak conceptual foundation, which is restricted at the cell level. The design criteria at the cell level are not directly related with the tissue functions, and consequently, such functions cannot be implemented in bioartificial tissues with the currently used methods. On the contrary, the field of artificial organs focuses on the function of the artificial organs that are treated in the design as integral entities, instead of the optimization of the artificial organ components. The field of artificial organs has already developed and tested methodologies that are based on system concepts and mathematical-computational methods that connect the component properties with the desired global organ function. Such methodologies are needed in tissue engineering for the design of bioartificial tissues with tissue functions. Under the framework of biomedical engineering, artificial organs and tissue engineering do not present competitive approaches, but are rather complementary and should therefore design a common future for the benefit of patients.

Adipocytes forming fat pad in vivo are surrounded by well developed basement membranes. Synthesis of basement membrane is enhanced during in vitro differentiation of preadipocyte line. In order to know the role of basement membrane in adipogenesis in vivo, we injected 3T3-F442A preadipocytes subcutaneously into nude mice together with or without the reconstituted basement membrane, Matrigel. Histological sections of the fat pads newly formed by injecting the cell alone showed dense population of immature adipocytes and microvessels within 2 weeks and they matured rapidly. In contrast, injection of the cells together with Matrigel showed sparse adipocytes after 2 weeks and they matured slowly over the period of 6 weeks. Quantification of the process by measuring the weight, DNA content, triglyceride content and glycerophosphate dehydrogenase (GPDH) activity of the fat pads showed that injection of the cell alone resulted in early maturation of adipose tissue with fewer adipocytes while the presence of Matrigel decelerated but potentiated the maturation of adipose tissue with 2 fold contents of DNA, triglyceride and GPDH activity. We thus showed that reconstituted basement membrane (Matrigel) supported the survival and maturation of adipocytes.

Autografting of fat pads has a long history in plastic and reconstructive surgery for augmentation of lost soft tissue. However, the results are disappointing because of absorption of the grafts with time. The fate of transplanted fat is linked to adipose precursor cells distributed widely in connective tissues. Adipocyte precursor cells can proliferate and mature into adipocytes even in the adult body depending on microenvironment. When reconstituted basement membrane, Matrigel, supplemented with more than 1 ng/ml bFGF was injected s.c. into 6-week-old mice, the neovascularization induced within 1 week was followed by migration of endogenous adipose precursor cells, and a clearly visible fat pad was formed. The pad grew until 3 weeks after the injection and persisted for at least 10 weeks. Such de novo adipogenesis was induced reproducibly by s.c. injection of Matrigel and bFGF over the chest, lateral abdomen, or head. Adipogenesis could be induced even in ear cartilage or in muscle. Thus, our results demonstrated that an abundant population of adipose precursor cells is distributed widely in connective tissues of the adult body and that they migrate into the neovascularized plug of Matrigel for proliferation and maturation. These results suggest a technique of augmenting lost soft tissue in plastic and reconstructive surgery.