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Keywords:

  • Artificial organs;
  • Tissue engineering;
  • Biomedical engineering;
  • Complex systems

Abstract

  1. Top of page
  2. Abstract
  3. THE CONCEPT OF THE “THREE-DIMENSIONAL CELL GROWTH” IS NOT SUFFICIENT FOR THE DESIGN OF BIOARTIFICIAL TISSUES WITH TISSUE PROPERTIES
  4. FROM THE “BIOMIMETICS OF STRUCTURE” TO THE “MIMETICS OF FUNCTION
  5. THE MIMETICS OF FUNCTION IN THE METHODOLOGIES OF THE ARTIFICIAL ORGANS
  6. ARTIFICIAL ORGANS AND TISSUE ENGINEERING COMPLEMENT EACH OTHER WITHIN THE FRAMEWORK OF BIOMEDICAL ENGINEERING
  7. Acknowledgments
  8. REFERENCES

Abstract:  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.


THE CONCEPT OF THE “THREE-DIMENSIONAL CELL GROWTH” IS NOT SUFFICIENT FOR THE DESIGN OF BIOARTIFICIAL TISSUES WITH TISSUE PROPERTIES

  1. Top of page
  2. Abstract
  3. THE CONCEPT OF THE “THREE-DIMENSIONAL CELL GROWTH” IS NOT SUFFICIENT FOR THE DESIGN OF BIOARTIFICIAL TISSUES WITH TISSUE PROPERTIES
  4. FROM THE “BIOMIMETICS OF STRUCTURE” TO THE “MIMETICS OF FUNCTION
  5. THE MIMETICS OF FUNCTION IN THE METHODOLOGIES OF THE ARTIFICIAL ORGANS
  6. ARTIFICIAL ORGANS AND TISSUE ENGINEERING COMPLEMENT EACH OTHER WITHIN THE FRAMEWORK OF BIOMEDICAL ENGINEERING
  7. Acknowledgments
  8. REFERENCES

The concept of the “three-dimensional cell growth” introduced by Langer and Vacanti a few decades ago has initiated tissue engineering as a distinct research field (1). The restriction of tissue engineering under this concept, which should rather be perceived as a first approximation instead of a sufficient condition for in vitro tissue formation, and its application in research practice with the optimization of various indices at the cell level (e.g., cell viability, expression of differentiation markers), did not allow considerable progress in making functional bioartificial tissues, which rather requires optimized indices at the tissue level. It is not therefore surprising that profitable products are still expected to be made by tissue engineering despite the research and development efforts exceeding US$4.5 billion (2). While the design parameters referring to media, growth factors, cells, biomaterials, and bioreactors currently used in tissue engineering are directly related to the cell functions, they are only indirectly related to the tissue functions. A cell-populated scaffold, for example, with randomly dispersed hepatocytes is not an in vitro equivalent of liver tissue irrespective of hepatocyte viability or any other index at the cell level. According to the “metabolic zonation theory(3), liver functions arise from the spatiotemporal integration of heterogeneous and complementary hepatocyte functions. For example, the liver function of glucose homeostasis emerges from the metabolic cooperation of glycolytic (perivenous) hepatocytes, which take up glucose during the absorptive phase, and gluconeogenic (periportal) hepatocytes, which release glucose during the postabsorptive phase (4). The different metabolic character of the hepatocytes is due to the oxygen gradient along the sinusoids which induces differential expression of the metabolic enzymes (5). Chan et al. have already discussed the importance of the metabolic zonation in the bioartificial liver in regard to ammonia and toxin metabolism, and have suggested that these metabolic functions can be optimized at the tissue but not at the cell level (6). The complex way the tissue functions emerge from the cooperation of functionally heterogeneous cells is a general characteristic of tissues. In the developing cartilage, the operation of Ihh/PTHrP negative loop between chondrocytes determines the columnar pattern of the growth plate and the cartilage elongation rate, both of which are tissue and not cell properties (7). Similarly, the function of controlled release of insulin is not a function of beta cells, but a function of the organized beta cells in the structure of the islets where again the heterogeneity of beta cells (8,9), as well as their cooperation (10), are the factors involved in the establishment of this function.

FROM THE “BIOMIMETICS OF STRUCTURE” TO THE “MIMETICS OF FUNCTION

  1. Top of page
  2. Abstract
  3. THE CONCEPT OF THE “THREE-DIMENSIONAL CELL GROWTH” IS NOT SUFFICIENT FOR THE DESIGN OF BIOARTIFICIAL TISSUES WITH TISSUE PROPERTIES
  4. FROM THE “BIOMIMETICS OF STRUCTURE” TO THE “MIMETICS OF FUNCTION
  5. THE MIMETICS OF FUNCTION IN THE METHODOLOGIES OF THE ARTIFICIAL ORGANS
  6. ARTIFICIAL ORGANS AND TISSUE ENGINEERING COMPLEMENT EACH OTHER WITHIN THE FRAMEWORK OF BIOMEDICAL ENGINEERING
  7. Acknowledgments
  8. REFERENCES

The elaborated extension of the concept of “three-dimensional cell growth” to that of “structural biomimetics” proved to be insufficient for the design of bioartificial tissues with tissue properties. The randomization of cell functions inside the scaffold instead of their integration to tissue functions, a general problem in tissue engineering that makes the scaffold design the major problem (11), is the most probable scenario if the design relies only on a simple geometrical mimicry. If, for example, periportal-gluconeogenic and perivenous-glycolytic hepatocytes isolated from the liver will be precisely placed in a porous scaffold that resembles the liver structure, a randomization of hepatocyte functions instead of their spatial integration to the liver function of glucose homeostasis will take place. This is because the hepatocyte metabolic functions do not depend simply on the geometry of the tissue or scaffold, but instead on the geometry of the gradients of the oxygen concentration, which are established through an integrated action of biological and physical phenomena. If, for example, the fluidics and transport phenomena in a scaffold are different from those in the liver sinusoids, the oxygen gradients will also be different from the in vivo ones despite the geometrical mimicry. The metabolic character of hepatocytes will be adapted to the new distribution of the gradients with glycolytic hepatocytes transformed to gluconeogenic (or vice versa) if in their microenvironment the oxygen concentration will rise (or fall), irrespective of the initial metabolic character they had when they were placed in the scaffold. A similar scenario is expected in the bioartificial cartilage in which the operation of the negative feedback loop of Ihh/PTHrP does not simply depend on the distances between chondrocytes, but also on physical phenomena such as the diffusion of the protein signals to reach their target chondrocytes. If biological, such as protein secretion, as well as physical phenomena, such as protein diffusion, will not be quantitatively reproduced in a scaffold, the randomization of any initial arrangement of chondrocytes, as it happens in the in vivo experiments when any of the proteins participating in the negative feedback loop are over- or underexpressed (12), is again the most probable scenario.

THE MIMETICS OF FUNCTION IN THE METHODOLOGIES OF THE ARTIFICIAL ORGANS

  1. Top of page
  2. Abstract
  3. THE CONCEPT OF THE “THREE-DIMENSIONAL CELL GROWTH” IS NOT SUFFICIENT FOR THE DESIGN OF BIOARTIFICIAL TISSUES WITH TISSUE PROPERTIES
  4. FROM THE “BIOMIMETICS OF STRUCTURE” TO THE “MIMETICS OF FUNCTION
  5. THE MIMETICS OF FUNCTION IN THE METHODOLOGIES OF THE ARTIFICIAL ORGANS
  6. ARTIFICIAL ORGANS AND TISSUE ENGINEERING COMPLEMENT EACH OTHER WITHIN THE FRAMEWORK OF BIOMEDICAL ENGINEERING
  7. Acknowledgments
  8. REFERENCES

Finding the proper scaffold requires a design methodology in which the tissue-system functions are explicitly attributed to the spatial integration of the cell-components functions as these are induced by the integration of physical and biological phenomena inside the scaffold. Such an explicit reference to the way the organ function arises from the function of the components can be found in the methodology of artificial organs. The artificial heart or heart valves, for example, have to mimic the function, which in this case is the blood flow characteristics, instead of only mimicking structural features of the real organ. Mathematical and computational models of fluid dynamics have been extensively employed, connecting the design parameters with the desired global organ function (13). Similarly, the release of insulin is not a sufficient design criterion in artificial organs. Instead, the controlled release, which is a tissue-islet and not a beta cell function, is implemented with the use of mathematical methods based on the control theory (14).

It is interesting to mention that because of the lack of appropriate concepts and methodologies, tissue engineering applies the artificial organ methodology to implement tissue functions in bioartificial tissues. In a recent study, a hybrid scaffold-cell device has been developed implementing the function of the controlled insulin release with a totally artificial instead of bioartificial design (15). A glucose-responsive material scaffold that formed a gel at low and a sol at high glucose concentrations allowed the regulation of the insulin release from the device according to the surrounding glucose concentration. A bioartificial design would rather require the implementation of the control function to an organized beta cell community as in the islets, establishing the beta cell heterogeneity and cooperativity. There are several other examples in which tissue engineering adopts the methodology of artificial organs to implement the concept of functional mimetics with an explicit design of the desired mechanical tissue properties in scaffolds seen as artificial tissues. In these cases, the scaffolds are designed to exhibit the required tissue properties before the cell seeding. As examples, the design of a scaffold for bioartificial articular cartilage, which could exhibit the mechanical responses of the natural cartilage that temporarily deforms upon loading with no structural collapse and recovers with unloading to the original geometry (16), or the design of a scaffold for bioartificial aortic valve, which could exhibit durability in repeated loading cycles, and the opening and closing behavior of the native tissue (17) could be mentioned. In these cases, the scaffold itself could temporarily provide to the patients the already designed tissue functions until the cells will grow, making the scaffold-artificial tissue a bioartificial one. Similarly, functional mimetic studies in tissue engineering have been undertaken for hybrid scaffold-cell constructs treating them as special cases of materials and implementing to them global tissue properties, for example, the controlled fibroblast alignment inside a collagen scaffold through the design of appropriate boundaries, in order to simulate the anisotropy of the heart muscle (18). Further to the experimental work with the implementation of tissue functionalities to scaffold or scaffold-cell constructs, computational design studies in line with similar studies in artificial organs have recently appeared, which connect the structural and mechanical scaffold design parameters, such as permeability and stiffness, with the biological processes of cell growth and differentiation inside the scaffold, so that the desired mechanical properties of the finally developed bioartificial tissue after the cell growth could be assured (19).

Tissue engineering studies, however, addressing the problem of functional mimetics of metabolic instead of mechanical tissue functions are rather limited and are still at a preliminary stage. Such studies have been restricted until now in the explicit design of the cellular heterogeneity on which the tissue functions rely. As examples, the design of a bioreactor system with one-dimensional oxygen gradient that induces glycolytic and gluconeogenic hepatocytes coexisting in the same system (20), the design of a bioartificial articular cartilage with the different chondrocyte types in successive layers through which cellular communication takes place as in the native tissue (21), or the design of a bioartificial bone with osteoblasts and osteoclasts that could exhibit the tissue function of bone remodeling (22) could be mentioned. However, although heterogeneity is one of the factors needed for the establishment of the tissue functions, the other factor, which is the cooperativity among the heterogeneous cell populations, has not yet been implemented and controlled in the above-mentioned studies, so that a bioartificial liver with a glucose homeostasis function, or a bioartificial articular cartilage with the mechanical properties of the native tissue, or a bioartificial bone with the tissue function of bone remodeling could result. These studies signify a departure from the cell level and an explicit design of cell heterogeneity as a necessary condition for the establishment of tissue functions, and they could be further elaborated to design conditions that will allow the cooperativity among the heterogeneous cell populations.

The delay of functional mimetics studies for tissues with metabolic properties in comparison with the more advanced studies for tissues with mechanical properties is rather due to the stronger ties of the latter with molecular biology than with material science. The central concept of molecular biology was, until recently, the genocentric paradigm that attributed the cell or organism function to particular genes setting the cell as a reference system (23) and consequently restricting the tissue engineering studies to indices at the cell level for the evaluation of bioartificial tissues. Of course, something like this could not happen with mechanical tissue properties because these properties by default refer to the whole tissue instead of individual cells.

The use of cell-components instead of tissue-system concepts in tissue engineering can sometimes lead to complicated and unpleasant situations when it comes to patient treatments. For instance, in a clinical trial for Parkinson's disease in which patients were treated with transplantation of embryonic dopamine neurons, dyskinesia was observed in some of the patients and was attributed to an excess of dopamine (24). Transplantation of fewer cells was suggested by the authors to avoid this problem. However, in a nonlinear system, no such proportionality exists; that is, the decrease of one variable (cell number and dopamine release) may not always lead to a decrease of the value of a physiological or pathological function, something that is very well known in artificial organs where the explicit use of nonlinearities is made in mathematical models for the design of organ functions. Indeed, in a subsequent study with transplantation of a lower number of cells, dyskinesia appeared again but that time was attributed to too little instead of too much dopamine release (25). The responsibility to the patients does not allow the overestimation of the cell abilities to restore tissue properties controlled within physiological limits without an explicit design of how the cell properties should be organized for the tissue properties to be established. Given that advanced mathematical tools have already been developed for diagnosis (26), or early identification of brain degenerative diseases (27), equally, the treatments based on bioartificial tissues should rely on such elaborated and precise instead of empirical methods.

ARTIFICIAL ORGANS AND TISSUE ENGINEERING COMPLEMENT EACH OTHER WITHIN THE FRAMEWORK OF BIOMEDICAL ENGINEERING

  1. Top of page
  2. Abstract
  3. THE CONCEPT OF THE “THREE-DIMENSIONAL CELL GROWTH” IS NOT SUFFICIENT FOR THE DESIGN OF BIOARTIFICIAL TISSUES WITH TISSUE PROPERTIES
  4. FROM THE “BIOMIMETICS OF STRUCTURE” TO THE “MIMETICS OF FUNCTION
  5. THE MIMETICS OF FUNCTION IN THE METHODOLOGIES OF THE ARTIFICIAL ORGANS
  6. ARTIFICIAL ORGANS AND TISSUE ENGINEERING COMPLEMENT EACH OTHER WITHIN THE FRAMEWORK OF BIOMEDICAL ENGINEERING
  7. Acknowledgments
  8. REFERENCES

It became clear from the previous examples that tissue engineering has to adopt the artificial organ concept of “functional biomimetics,” which requires the explicit design of the tissue function and the formulation of the design problem through the systems methodology approach involving concepts, such as complexity, control, and nonlinearities, that must be incorporated in mathematical models that integrate cell properties to tissue properties. The difficulty of tissue engineering to present a coherent methodology in which its tools and methods could be integrated in a rational methodology has already been mentioned (28), as well as the need for incorporation of the complexity in its methods (29). Therefore, among the several boundaries that will vanish (30), there is one that has to be crossed by tissue engineering to enter the methodologies of artificial organs, which are based on such concepts and have been successfully used for many years. Looking superficially at and underestimating the field of artificial organs because no stem cells are used, the methodological way to treat the cells as components of integrated tissue systems exhibiting global system properties is also missed along with the tissue functions. The conceptual and institutional framework where these two fields can meet and design a common future has already been established. Bioengineering or biomedical engineering, a new field with the goal to integrate biological methods with those of engineering and mathematics, has started to emerge as a major research direction in all biomedical fields (31), replacing gradually the old genocentric paradigm of biology (23). Similar efforts in Japan, for example, the activities of the Systems Biology Institute in Tokyo, have led to increased research efforts in deciphering the way gene and protein interactions are integrated to cell functions. From these activities, besides the theoretical foundation of the gene organization (32), useful tools, such as the CellDesigner, have been devised (33), which could constitute the basis for the determination of cell functions according to the cell microenvironment and further for the integration of the cell functions to tissue functions. Another notable example is the E-Cell software system developed in the Institute for Advanced Biosciences, Keio University, Japan, to simulate cellular processes, and has been used for the hepatocyte metabolism (34). The use of whole-cell simulations that predict the hepatocyte metabolic function according to local condition in the cell microenvironment can make clear that random dispersion of hepatocytes is not a rational way to establish tissue functions in bioartifical liver. Special attention was also given recently to the complexity in biological phenomena in Europe under the FP6 program (“Tackling Complexity in Science” of the activity area “New and Emerging Science and Technology,” NEST). The above-mentioned new research efforts that are based on a system approach bring artifical organs in terms of conceptual background in line with these developments.

In conclusion, we could say that although tissue engineering is familiar with the use of biological tools, it is the field of artificial organs that is conceptually compatible with the new synthetic approach of biomedical engineering. Biomedical engineering is the new challenge for tissue engineering and artificial organs, but it is also the opportunity for a common future as recent encouraging studies in which the methods of the two fields have been combined have shown.

Acknowledgments

  1. Top of page
  2. Abstract
  3. THE CONCEPT OF THE “THREE-DIMENSIONAL CELL GROWTH” IS NOT SUFFICIENT FOR THE DESIGN OF BIOARTIFICIAL TISSUES WITH TISSUE PROPERTIES
  4. FROM THE “BIOMIMETICS OF STRUCTURE” TO THE “MIMETICS OF FUNCTION
  5. THE MIMETICS OF FUNCTION IN THE METHODOLOGIES OF THE ARTIFICIAL ORGANS
  6. ARTIFICIAL ORGANS AND TISSUE ENGINEERING COMPLEMENT EACH OTHER WITHIN THE FRAMEWORK OF BIOMEDICAL ENGINEERING
  7. Acknowledgments
  8. REFERENCES

Acknowledgments:  The authors would like to thank the reviewers for critical comments that lead to the conclusion that artificial organs methodologies have been incorporated in the tissue engineering methods in a greater degree than initially presented in the article. The partial financial support from the European project LSHM-CT-2007-037862 is gratefully acknowledged.

REFERENCES

  1. Top of page
  2. Abstract
  3. THE CONCEPT OF THE “THREE-DIMENSIONAL CELL GROWTH” IS NOT SUFFICIENT FOR THE DESIGN OF BIOARTIFICIAL TISSUES WITH TISSUE PROPERTIES
  4. FROM THE “BIOMIMETICS OF STRUCTURE” TO THE “MIMETICS OF FUNCTION
  5. THE MIMETICS OF FUNCTION IN THE METHODOLOGIES OF THE ARTIFICIAL ORGANS
  6. ARTIFICIAL ORGANS AND TISSUE ENGINEERING COMPLEMENT EACH OTHER WITHIN THE FRAMEWORK OF BIOMEDICAL ENGINEERING
  7. Acknowledgments
  8. REFERENCES