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Abstract

  1. Top of page
  2. Abstract
  3. GENERAL OVERVIEW
  4. ECM COMPOSITION
  5. ECM RECEPTORS
  6. CELL–ECM INTERACTIONS
  7. GROWTH FACTORS–ECM INTERACTIONS
  8. TISSUE ENGINEERING
  9. CONCLUSIONS AND FUTURE PERSPECTIVES
  10. LITERATURE CITED

The extracellular matrix (ECM) consists of a complex mixture of structural and functional macromolecules and serves an important role in tissue and organ morphogenesis and in the maintenance of cell and tissue structure and function. The great diversity observed in the morphology and composition of the ECM contributes enormously to the properties and function of each organ and tissue. The ECM is also important during growth, development, and wound repair: its own dynamic composition acts as a reservoir for soluble signaling molecules and mediates signals from other sources to migrating, proliferating, and differentiating cells. Approaches to tissue engineering center on the need to provide signals to cell populations to promote cell proliferation and differentiation. These “external signals” are generated from growth factors, cell–ECM, and cell–cell interactions, as well as from physical-chemical and mechanical stimuli. This review considers recent advances in knowledge about cell–ECM interactions. A description of the main ECM molecules and cellular receptors with particular care to integrins and their role in stimulation of specific types of signal transduction pathways is also explained. The general principles of biomaterial design for tissue engineering are considered, with same examples. J. Cell. Physiol. 199: 174–180, 2004© 2003 Wiley-Liss, Inc.


GENERAL OVERVIEW

  1. Top of page
  2. Abstract
  3. GENERAL OVERVIEW
  4. ECM COMPOSITION
  5. ECM RECEPTORS
  6. CELL–ECM INTERACTIONS
  7. GROWTH FACTORS–ECM INTERACTIONS
  8. TISSUE ENGINEERING
  9. CONCLUSIONS AND FUTURE PERSPECTIVES
  10. LITERATURE CITED

The main goal of tissue engineering is the reconstruction of living tissues to be used for the replacement of damaged or lost tissues/organs of living organisms. To achieve this, it is necessary to combine the use of cell together with natural or synthetic scaffolds in or onto which cells must develop, organize, and behave as if they are in their native tissue. Therefore, cells must receive signals from the environment to carry out in an orderly way proliferation and differentiation programs finalized to tissue/organ formation (Abatangelo et al., 2001).

With regard to these biological process cells need a continuous flow of signals from the surrounding extracellular environment that allow for specific genetic program fulfillment. It is tempting to generalize that in all tissues cells live in contact with matrices or scaffolds. Since the early developmental phases, embryonic cells produce their own extracellular scaffolds, by secreting many types of molecules in the surrounding space, according to a well defined program of differentiation (Adams and Watt, 1993; Vaino and Muller, 1997). The different spatial organization of these secreted molecules gives rise to a great variety of natural scaffolds in which cells continue to proliferate and to organize themselves in order to build tissues and to accomplish all their natural functions. The understanding of cell differentiation and functions means the understanding of cell–cell and cell–extracellular matrix (ECM) communication mechanisms. In this respect, if one looks considering the complexity of tissue ECM components, it is not surprising to find the same great variety and complexity of existing interactions between cells and ECM. Bearing these basic considerations in mind, our efforts in trying in vitro tissue reconstruction must be driven toward the exact knowledge of cell function on the one hand, and, on the other hand, toward the knowledge of interactions and signals that cells must receive from the environment to behave as in natural tissues (Abatangelo et al., 2001).

We know that ECM plays an instructive role for cellular activities and that the cell surface contains receptors to respond to extracellular signals. As soon as ligand–receptor interaction is established, the biochemical machinery involved in the control of gene expression starts.

ECM COMPOSITION

  1. Top of page
  2. Abstract
  3. GENERAL OVERVIEW
  4. ECM COMPOSITION
  5. ECM RECEPTORS
  6. CELL–ECM INTERACTIONS
  7. GROWTH FACTORS–ECM INTERACTIONS
  8. TISSUE ENGINEERING
  9. CONCLUSIONS AND FUTURE PERSPECTIVES
  10. LITERATURE CITED

The ECM is composed of a great variety of molecules and includes collagen family, elastic fibers, glycosoaminoglycans (GAG) and proteoglycans, and adhesive glycoproteins. The different combination, immobilization, and spatial organization of these secreted substances give rise to different types of scaffolds that characterize the different body tissues and organs.

Collagen is the most abundant protein in the vertebrate body which constitutes a heterogeneous class of proteins. Up to now about 20 different collagens have been characterized which exhibit different mechanical and functional properties. Some collagens are specific for a given tissue, such as type II collagen, which is found only in cartilage (Zhang et al., 2003). Types I, II, and III are the most abundant collagens of human body that form fibrils responsible for the tensile strength of the tissue. Other collagens, such as types IV, VII, IX, X, and XII are found associated with collagen fibrils or organized in the network as basal laminae. In addition to mechanical and structural functions, collagens play an important role in determining cell attachment and spreading (Prasad et al., 2002). In consequence of these properties, collagens influence cell differentiation and movement (Keely et al., 1995). GAG are linear polysaccharides formed by repeating disaccharide units that may contain sulfate groups. The most common GAGs are chondroitins, keratins, and dermatans, which are sulfated, and hyaluronan, which is not. Another class, the heparans, are mainly associated with cells and basement membranes. With the exception of hyaluronan, all GAGs can be associated to a protein backbone and give rise to the so-called proteoglycans.

Cartilage aggrecans are well characterized large proteoglycans whose structure consists of linear polypeptide on which several chondroitinsulfate (CS) and keratansulfate (KS) molecules are covalently attached. These aggrecans are in turn assembled in large aggregates by means of specific binding sites of their core protein to hyaluronan (Kiani et al., 2002). In this case a single molecule of hyaluronan is able to non-covalently link several aggrecan molecules to form huge water-containing dominions. The swelling pressure caused by these aggregates on collagen network allows cartilage tissue to withstand compression generated during the joint movement (Kiani et al., 2002). Similarly, in other tissues such as skin, proteoglycans in association with other structural molecules form specific complexes responsible for hydration and spatial organization of ECM. Proteoglycans are also present on a cell surface and act there as receptors by binding to collagens, fibronectin, and thrombospondin, and to some growth factors.

Among the GAG, hyaluronan represents a unique non-sulfated polysaccharide that can exist in free form, with no covalently bound protein. It is present in all living organisms and plays an important role in many biological processes, such as matrix structure, water balance, lubrication, cell movement, and differentiation (Turley et al., 2002). During embryonic development hyaluronan represents the major constituent of the early ECM, and is substituted during the late phases of the development by other structural ECM components. Because of these biological properties, proteoglycans and GAG are often used, in association with other ECM molecules such as collagen, to create supporting biomaterials utilized in tissue engineering (Orgill and Yannas, 1997; Miralles et al., 2001).

Chemical modification of some GAG molecules has allowed the production of a new class of biomaterials suitable for cell culture and in vitro tissue reconstruction. Hyaluronan has also be modified by estrerification of the carboxyl groups along the backbone with aliphatic or aromatic alcohols. This modification lowers the water solubility of molecule and make possible the manufacture of various devices, such as spun fibers, woven and non-woven textiles, films, etc. (Giusti and Callegaro, 1994). These biomaterials can be used both for soft tissue augmentation and as scaffolds for cell culture and tissue engineering.

Adhesive glycoproteins are a class of ECM molecules including interactive glycoproteins that exsist in several variant forms and possess multiple binding domains capable of binding collagen and proteoglycans, as well as binding to the cell surface (Friedl and Brocker, 2000). Fibronectin, laminin, vitronectin, thrombospondin, tenascin, and some other glycoproteins are members of this class of ECM molecules.

Fibronectin plays an important role in the cell attachment to the substrate, in the cell movement and differentiation (Mostafavi-Pour et al., 2003). There are variant forms of fibronectins which arise from alternative splicing in its mRNA precursor. Along the backbone of the molecule there are present multiple RGD (Arg-Gly-Asp), RGDS (Arg-Gly-Asp-Ser), LDV (Leu-Asp-Val), and REDV (Arg-Glu-Asp-Val) sequences that are responsible for cell binding (Kao, 1999), while other domains represent binding sites for other ECM molecules such as collagen, fibrin, heparin sulfate, etc. Due to its broad binding properties, fibronectin is widely used in cell culture systems, in order to favor cell adhesion and spreading. Fibronectin also acts as a negative regulatory signal for preventing involucrin expression by keratinocytes, thus avoiding their terminal differentiation (Watt et al., 1993).

Laminin is found mainly associated with basement membranes, and exists in different forms, all of which are the product of closely related genes. These interactive proteins are characterized by high binding affinity for cell surface as well as for heparin and type IV collagen. Lamini-5 isoform is typically found associated with the basement membrane. RGD sequences are also present along the backbone of the molecule chains together with other specific sequences, such as PDSGR, YIGSR, and IKVAV sequences that are able to recognize and bind to cell-surface receptors (Timpl et al., 2000). Interestingly, laminin possesses some growth factor-like sequences that become available to cells only upon degradation. These domains are EGF-like peptides and can stimulate cell proliferation and differentiation (Schenk et al., 2003). Given its high cell binding affinity, laminin alone or in combination with other ECM molecules is widely used to coat cell culture dishes and plates to enhance cell attachment and spreading (El-Ghannam et al., 1998).

Thrombospondin backbone is provided with cell-binding domains, among which an RGD sequence is also present. Thrombospondin has binding sites for collagens and laminin, as well as for fibrinogen (Chen et al., 2000). This interactive glycoprotein can be found in ECM associated with fibronectin or heparin sulfate proteoglycan. Depending on cell type, thrombospondin can either facilitate or inhibit cell attachment and spreading (Murphy-Ullrich, 2001).

The adhesive glycoprotein vitronectin, can be found, like fibronectin, in soluble form in the blood, and in fibrillar form in the ECM of several tissues. Again in this molecule RGD sequences are present and recognize and bind to the integrin receptor of the cell surface. Vitronectin also binds to collagens, heparin, and plasminogen activator inhibitor. All these binding properties make this molecule a good candidate for tissue remodeling and cell attachment and migration (Preissner, 1991).

To the adhesive glycoproteins above mentioned that are the most common and well characterized ones, other molecules with similar biological function must be added. Tenascin, for example, is another important glycoprotein containing several EGF and fibronectin repeats (Swindle et al., 2001). The von Willebrand factor (vWf) is a large protein found in the vascular ECM and plays an important role in platelet adhesion and activation (Schmugge et al., 2003). Other glycoproteins of ECM have RGD sequences involved in cell recognition, such as chondronectin (Carsons and Horn, 1988), bone sialoprotein, osteopontin, and fibrillin. Given the heterogeneous and complex composition of ECM, it is not surprising that in future other components will be added to this interesting family of interactive glycoproteins.

The elastic fibers are composed of an amorphous protein, elastin, whose structural characteristic is elasticity. Arterial walls, dermis, and lungs are the sites where a large quantity of elastic fibers can be found. With regard to tissue engineering and cell culture systems, elastin has received little attention up to now. Cross-linking of these peptides by gamma irradiation give rise to hydrogels which can be used as substrate for tissue reconstruction (Wood et al., 1986).

ECM RECEPTORS

  1. Top of page
  2. Abstract
  3. GENERAL OVERVIEW
  4. ECM COMPOSITION
  5. ECM RECEPTORS
  6. CELL–ECM INTERACTIONS
  7. GROWTH FACTORS–ECM INTERACTIONS
  8. TISSUE ENGINEERING
  9. CONCLUSIONS AND FUTURE PERSPECTIVES
  10. LITERATURE CITED

Cell adhesion is crucial for tissue formation and integrity. For tissue engineering strategies it is essential to know how cells can interact with ECM and transduce the information received by the extracellular molecules into an intracellular event. The identification of cell binding sites within extracellular molecules is a key step toward identifying the mechanisms of cell–ECM interactions. The cell surface possesses two kinds of receptors: non integrin and integrin receptors.

Nonintegrin receptors

Proteoglycans, CD36, and some laminin-binding proteins belong to this class of cell surface receptors. The most studied, however, are the cell surface proteoglycans syndecan and CD44. Syndecan is a transmembrane proteoglycan comprising a cytoplasmic domain, a hydrophobic membrane region, and an extracellular domain. The GAG side chains are represented by chondroitin sulfate and heparin sulfate. Syndecan receptors are a family of related proteoglycans which differ mainly in the extracellular domain GAG composition (Couchman et al., 2001). These cell surface receptors, in addition to binding collagens, fibronectin, and thrombospondin, bind bFGF. The colocalization to the cell surface of both growth factors and ECM molecules makes syndecan a unique molecule capable of assembling signaling complexes in combination with other receptors (Yoneda and Couchman, 2003). During in vivo tissue development, the expression pattern of syndecan follows morphogenetic rather than histological boundaries (Kosher, 1998). For this reason, syndecan expression during the in vitro development of tissues may represent for cell biologists a particular marker of differentiation and development.

CD44 is a cell surface glycoprotein which carries N- and O-linked sugars and GAG side chains. Alternative splicing and post-transductional modification give rise to different tissue specific forms. The binding of CD44 to type I and IV collagens and hyaluronan plays a vital role in cell adhesion and movement (Cichy and Pure, 2003).

Integrins

Integrins represent a large group of a glycoprotein family, whose structure consists of the heterodimeric non-covalent association of α and β subunits. The family has been classified into two subgroups according to the identity of the β subunit to which different α subunits combine to give rise to several specific receptors (Hynes, 1992). However, the possibility that some α subunits can combine with several β subunits adds a further level of complexity to the binding capacity of these cell surface receptors. While β subunits seem to have a non-specific role in ligand binding activity, α subunits on the contrary confer high specificity of signal transduction. Molecular biology studies have demonstrated that the β1 cytoplasmic domain of integrins play an important role in cytoskeletal association (Schaffert et al., 2001). As soon as ECM molecules bind to their specific integrin or non-integrin receptors, a change in cytoplasmic domain of the receptor occurs, which associates with the cytoskeleton at focal adhesion sites. Consequently, an assembly of the focal contact proteins occurs with other intercellular components, such as phosphorylated proteins, happens. These changes can promote cytoskeleton rearrangement, which may determine differential interactions of chromatin and nuclear matrix at the nuclear level. The dynamic association of integrin receptors with the actin cytoskeleton may also induce changes in cell shape, which in turn alter the ability of cells to proliferate or differentiate (Stroker et al., 1990). Briefly, downstream of the binding of ECM molecules to integrins the following steps may be included: clustering of the receptors, activation of intracellular protein kinases and subsequent phosphorylation of cytoskeleton and other associated proteins, and transmission of the signals generated by receptor occupancy to the transcriptional machinery in the nucleus (Schlaepfer et al., 1994). Each of the major ECM components can be recognized by one or more integrins (Giancotti, 2003), but the significance of this apparent redundancy is not clear at present.

The expression of proper receptors during in vitro development of tissues may indicate that the environmental conditions in which cells are cultivated are permissive for cell differentiation. An example is represented by cultured human keratinocytes that are able to express β1 and β4 integrin subunits when in contact with a dermal-like support (Zacchi et al., 1998). Like in vivo, in vitro multipotent cells can also be induced to differentiate along a different lineage by a variety of agents that are able to induce specific expression of integrins (Bagutti et al., 2001). With regard to tissue engineering, it may be possible to induce specific integrin expression by altering the extracellular milieu in order to guide cultured cells toward specific phenotype expression (Boudreau and Bissell,1998). Finally, differentiation is not only associated with changes in integrin expression, but also with down-regulation mechanisms of receptor function, probably related to changes in receptor conformation.

If we consider that the ultimate goal of a cell culturist is to provide cells with appropriate environmental scaffolds and supports, then it is reasonable to think that the utilization of ECM molecules and suitable growth factors can facilitate the development of artificial tissues.

CELL–ECM INTERACTIONS

  1. Top of page
  2. Abstract
  3. GENERAL OVERVIEW
  4. ECM COMPOSITION
  5. ECM RECEPTORS
  6. CELL–ECM INTERACTIONS
  7. GROWTH FACTORS–ECM INTERACTIONS
  8. TISSUE ENGINEERING
  9. CONCLUSIONS AND FUTURE PERSPECTIVES
  10. LITERATURE CITED

There are at least two main ways by which the ECM can affect cell behavior. One of these is the cell–ECM interaction which may directly regulate cell functions through receptor-mediated signaling. The other is that ECM can control the mobilization of growth or differentiation factors, thus modulating cell proliferation and controlling cell phenotype (Taipale and Keski-Oja, 1997). In addition to generating their own signals, it is well known that molecules able to regulate cell adhesion to ECM can modulate the cellular response to other extracellular stimuli, such as soluble growth factors and differentiation-inducing agents (Schwartz and Baron, 1999). This means both that different ECM macromolecules may selectively stimulate specific types of signal transduction pathways, and that for an efficient response to growth or differentiation factors, the cell needs to interact with a matrix component in order that appropriate intracellular signal transduction cascades are triggered, as studies by Aplin and Juliano (1999) reveal. These studies indicated that a limited degree of adhesion-mediated cytoskeletal organization and focal adhesion complex formation are required for efficient EGF activation of p42 and p44-MAPKs.

The ability to influence the downstream signaling of receptors that are activated by soluble growth and differentiation factors may be one of the most vital biological functions of ECM molecules and their cognate ligands. Perhaps the clearest incarnation of this is anchorage-dependent growth, a phenomenon which has been studied for several years (Howe et al., 2002). By these studies it has been shown that the regulation of cellular events is through the coordinate effects of positive and negative signals, including those from soluble factors as well as those from ECM and from adjacent cells (Schwartz and Lechene, 1992). It is this summation of signals conveying both biochemical and positional information which tells a cell when the time and place is right to conduct a particular activity. For example, there is compelling evidence that ECM and soluble factors can synergize to regulate the intracellular ionic environment. While both basic fibroblast growth factors (bFGF) and adhesion to fibronectin can independently activate the Na+/H+ antiporter and raise intracellular pH (pHi) in endothelial cells, growth factor stimulation of adherent cells is more efficient in the process (Ingber et al., 1990). Similarly, in fibroblasts, protein kinase C (PKC)-dependent activation of the antiporter (and elevation of pHi) by platelet derived growth factor (PDGF) requires cell adhesion to ECM. PDGF-stimulated Ca2+ mobilization, another ion transient required for cell cycle progression in murine fibroblasts, does not occur in cells in suspension, but readily occurs in cells adherent to fibronectin (Turker et al., 1990).

The mechanism underlying this regulation is now well known. In adherent cells, PDGF stimulates tyrosine phosphorylation and activation of phospholipase C (PLCγ), which hydrolyzes phosphatidylinositol 4,5 bisphosphate PI(4,5)P2 into DAG and inositol triphosphate (IP3), which in turn activate PKC and increase Cai2+. In non-adherent cells, the activation of PLCγ by PDGF still occurs, but there is a dramatic decrease in the level of its substrate, PI(4,5)P2, and therefore no appreciable generation of diacylglycerol (DAG) and IP3 can occur (Mc Namee et al., 1993). Presumably, the adhesion-dependent synthesis of PI(4,5) P2 is largely caused by Rac and Rho, two plasma membrane molecules regulating cell–ECM interaction.

In addition to the examples discussed above, several alternative models for cooperation between ECM receptors and other receptors have emerged. Another clear situation whereby ECM can collaborate with soluble factors is where interactions of an ECM molecule with an adhesion receptor directly activates a growth factor receptor without the need for that receptor's soluble ligand. This seems to be true when β1 integrins directly activate a PDGFβ receptor (Howe et al., 1998). A second model entails the ligand-mediated activation of growth factor receptors that have been recruited to ECM-dependent adhesion sites. The increased concentration of receptor tyrosine kinases increases the efficiency of receptor activation, as reported in the case of epithelial growth factor receptor, PDGF receptor, and insulin receptor (Vuori and Rouslahti, 1994).

A third model suggests that the adhesion structures act as a molecular scaffold for the downstream component of signaling cascades, thus allowing more efficient propagation of the signal. For example, within focal adhesions, integrins physically bridge the ECM to the network of cytoplasmic actin microfilaments, a situation that seems particularly well suited for providing an appropriate molecular scaffold for signaling components. This type of event has been reported in fibroblasts for activation of the MAPK cascade (Fincham et al., 2000).

Aplin et al. (2002) studies reveal that in fibroblasts, not only is growth factor activation of the ERK cascade enhanced when cells are adherent, but cell adhesion differentially regulates the nucleocytoplasmic distribution of MAP kinase members; ERK accumulation in the nucleus occurs more efficiently in adherent cells, whereas nuclear accumulation of active p38 and active JNK are unaffected by changes in adhesion.

Another highly controversial question about ECM–cell interactions is that the ECM molecule composition is heterogeneous in different tissues, and in the same tissue at different stages of development. The functional significance of this apparent redundancy of the ECM component variation is that, at a given time end place, the ECM has the potential to provide specific environmental information to cells. For example, in the case of mammary epithelial cells, for a correct signaling induced by prolactin and insulin, the adhesion of cells to the basement membrane components is necessary, while their interaction with ECM proteins, such as collagen I, inhibits the intracellular signal cascades induced by these hormones (Lee and Streuli, 1999).

Post-translational modifications also affect the ways in which ECM components interact with each other and with cells. The degree of glycosylation of fibronectin and laminin and the amount of calcium bound by thrombospondin have all been shown to modulate cell adhesion.

Self-aggregation of laminin-nidogen complexes is dependent on calcium ions (Paulsson et al., 1988) while fibronectin becomes incorporated in ECM through transglutaminase-catalyzed cross-linking (Barry and Mosher, 1988). Self-assembly or cross-linking to other matrix components might affect cell adhesive activity by increasing the local concentration of cell-binding sites, or conversely, obscuring the sites. Non-covalent interactions among matrix molecules can affect the activity of adhesive glycoproteins such as fibronectin: these observations have led to the categorization of thrombospondin, tenacin, osteonectin, and, in some circumstances, laminin, as “anti-adhesive” matrix glycoproteins (Sage and Bornstein, 1991). In addition, soluble proteoglycans can inhibit cell adhesion to collagen and fibronectin (Ruoslahti, 1989).

GROWTH FACTORS–ECM INTERACTIONS

  1. Top of page
  2. Abstract
  3. GENERAL OVERVIEW
  4. ECM COMPOSITION
  5. ECM RECEPTORS
  6. CELL–ECM INTERACTIONS
  7. GROWTH FACTORS–ECM INTERACTIONS
  8. TISSUE ENGINEERING
  9. CONCLUSIONS AND FUTURE PERSPECTIVES
  10. LITERATURE CITED

The second way by which the ECM may affect cell function is through harboring growth factors or growth factor-binding protein. In this case ECM plays an active role in the mobilization of growth molecules rather than passively sequestering such factors. The interactions between growth factors and ECM regulate cell behavior in many ways. For instance, the direct binding of growth factors to the ECM can affect the local concentration as well as the biological activity of growth factors (Li et al., 2003). On the other hand, the transcription, translation, and post-translational modification of ECM macromolecules have been shown to be regulated by various growth factors (Rachfal and Brigstock, 2003).

The binding of growth factors to the ECM is regulated by the GAG side chains. The presence of basic aminoacid clusters within regions of α-helical structure of growth factors seems to mediate the binding to the negatively charged heparin sulfate side chains of proteoglycans (Huhtala et al., 1999). The biological consequences of the matrix-bound growth factors are related to their controlled release. The ECM is able to limit the diffusion of soluble factors, and thus provides a local store of biologically active molecules that persists after growth factor production has ceased.

In the case of fibroblast growth factor (FGF), its binding to ECM induces a decrease in its degradation compared to free FGF (Moscatelli, 1988). On the contrary, TGF-b and PDGF bound to ECM are inactive and can be released by the proteolysis of the matrix (Dallas et al., 1995).

The regulation of specific gene expression is again regulated by growth factor–ECM interaction, as in the case of tumor necrosis factor (TNF) production that is increased by neutrophil adhesion to fibronectin (Ortiz et al., 1995). All of these observations support the idea that growth factors and ECM proteins collaborate in creating distinct cellular environments or “niches” that regulate proliferation and differentiation, a concept originally formulated for stem cells in self-renewing tissues.

Further evidence has come from a number of experimental models of differentiation. In bone marrow, differentiation of progenitor stem cells along separate lineages is directed by growth factors, several of which are presented in a functionally active form by matrix components secreted by fibroblasts of the bone marrow stroma (Gordon, 1988).

TISSUE ENGINEERING

  1. Top of page
  2. Abstract
  3. GENERAL OVERVIEW
  4. ECM COMPOSITION
  5. ECM RECEPTORS
  6. CELL–ECM INTERACTIONS
  7. GROWTH FACTORS–ECM INTERACTIONS
  8. TISSUE ENGINEERING
  9. CONCLUSIONS AND FUTURE PERSPECTIVES
  10. LITERATURE CITED

Tissue-engineering approaches typically employ exogenous three-dimensional ECMs to engineer new natural tissues from natural cells (Abatangelo et al., 2001). The exogenous ECMs are designed to bring the desired cell types into contact in an appropriate three-dimensional environment, and also to provide mechanical support until the newly formed tissues are structurally stabilized and specific signals occur to guide the gene expression of cells forming the tissue (Putnam and Mooney, 1996). One approach to designing exogenous ECMs for tissue engineering is to mimic the functions of the ECM molecules naturally found in tissues. These native ECMs act as a scaffolding to bring cells together in a tissue, to control the tissue structure, and to regulate the cell phenotype (e.g., tissue-specific gene expression).

All synthetic ECMs used to engineer tissues have three primary roles. First, the synthetic ECMs facilitate the localization and delivery of cells to specific sites in the body. Second, they define and maintain a three-dimensional space for the formation of a new tissues with appropriate structure. Third, they guide the development of new tissues with their appropriate functions (Bouhadir and Mooney, 1998).

The synthetic ECM should provide temporary mechanical support sufficient to withstand in vivo forces and maintain a potential space for tissue development. This mechanical support by the synthetic ECM should be maintained until the engineered tissue has sufficient mechanical integrity to support itself, as in the case of the Kim and Mooney (1998) studies that showed the ability of synthetic ECMs based on non-woven mesh of polyglycolic acid bonded at their fiber crosspoints with poly-l-lactic acid to resist cellular contractile forces and maintain their predefined structure during the process of smooth muscle tissue development in vitro.

The cells composing the engineered tissue must express the appropriate genes to maintain the tissue-specific function of the engineered tissue. The function of seeded cells is strongly dependent on the specific cell-surface receptor (e.g., integrins) used by the cell to interact with the material, on interactions with surrounding cells, and on the presence of soluble growth factors (Deuel, 1997). These factors can be controlled by incorporating or integrating a variety of signals, such as cell-adhesion peptides and growth factors (Park and Bae, 2002), into the synthetic ECM or by subjecting it to mechanical stimuli (Tian et al., 2002).

CONCLUSIONS AND FUTURE PERSPECTIVES

  1. Top of page
  2. Abstract
  3. GENERAL OVERVIEW
  4. ECM COMPOSITION
  5. ECM RECEPTORS
  6. CELL–ECM INTERACTIONS
  7. GROWTH FACTORS–ECM INTERACTIONS
  8. TISSUE ENGINEERING
  9. CONCLUSIONS AND FUTURE PERSPECTIVES
  10. LITERATURE CITED

Tissue engineering is an emerging, interdisciplinary field in biomedical engineering and aims at regenerating new biological tissues for replacing diseased or damaged tissues/organs using cells seeded and cultured on “intelligent scaffolds.” At the present, some devices are commercially available for tissue engineering of skin and cartilage, and many will be available in the near future for the engineering of other tissues (i.e., bone, fat tissue, nervous tissue).

We hope that engineering tissues may provide an alternative to current therapies used to treat the loss or failure of tissue function. The development of synthetic ECMs is a critical component of this field and research in this area is based on the understanding of native ECM molecules and their interactions with living cells. The matrices developed for tissue engineering may in turn provide a novel experimental system to elucidate the mechanisms by which native ECMs regulate tissue development.

LITERATURE CITED

  1. Top of page
  2. Abstract
  3. GENERAL OVERVIEW
  4. ECM COMPOSITION
  5. ECM RECEPTORS
  6. CELL–ECM INTERACTIONS
  7. GROWTH FACTORS–ECM INTERACTIONS
  8. TISSUE ENGINEERING
  9. CONCLUSIONS AND FUTURE PERSPECTIVES
  10. LITERATURE CITED