Tissue engineering became one of the hot topics of medicine just after the review article by Langer and Vacanti in 1993 with title “Tissue Engineering,” and today tissue engineering is known to be a multidisciplinary field that applies the principles of engineering and life sciences toward the development of biological substitutes that restore, maintain, or improve tissue function or a whole organ [1]. The strategies of tissue engineering or in other words “regenerative medicine” require both relation and communication of cells with tissue by signaling pathways [1,2,3].

To clarify such confusion, technical terminology including “tissue engineering,” “regenerative medicine,” “cellular therapy,” and “cell transplantation” should be classified briefly. They are classified depending on the scaffold use. So, according to this, regenerative medicine involves two concepts: “cell therapy” without any use of scaffold and “tissue engineering” that needs scaffold as a support [4].

1 Native Extracellular Matrix as a Host

For almost all kind of “tissue engineering” applications, the “one and only” desire is to have the scaffold material to degrade and to be broken down in time and as this degradation process is going on, the biomaterial should be replaced with cells or tissue and newly synthesized extracellular matrix ECM. In addition, these degradation end products must be biologically inert so that they can be rapidly cleared away from the body without any biological reaction.

Polymers are the well-known materials for tissue engineering, and there are two mechanisms for their degradation: chemical in which the polymer chain is broken down by chemical reactions like hydrolysis and enzymatic in which enzymes secreted by cells in culture recognize and cleave specific sites within biological polymers [5].

Cells in human tissues are in an environment (in a solid matrix) called extracellular matrix (ECM). According to the function of the tissue, there are different types of ECM in human tissues. The functions of ECM can be divided into five different categories (Table 5.1).

Table 5.1 Functions of extracellular matrix (ECM) in native tissue and tissue engineering products (scaffolds)
Full size table

Firstly, ECM provides structural support and physical environment for the cells of that tissue to grow and respond to signals. Secondly, ECM gives the tissue its structural and also mechanical character, like elasticity or rigidity basically depending on the function of that tissue. Thirdly, ECM actively offers bioactive signals to the cells inside the matrix for cellular activities. Fourthly, ECM is also a reservoir of growth factors. Fifthly, ECM creates an optimum physical environment during degradation process during tissue dynamic processes like morphogenesis, homeostasis, and wound healing [6, 7].

So theoretically the best scaffold for “tissue engineering” should have the ECM of the target tissue. But the complex composition and the dynamic nature of ECM make it almost impossible to mimic the same. Therefore, the main strategy of scaffolding in tissue engineering is to mimic the functions of native ECM [8].

  • Scaffolds should have enough empty space for new tissue formation and remodeling after implantation. On the other hand, this scaffold consisting of porous structure for transport of nutrients and metabolites should be mechanically stable. And also, after implantation the speed of degradation of this biomaterial should match with the new matrix production of new tissue. The pore structures affect the cell responses and their further organization in the tissue. Hydrogel type scaffolds have water-filled channels and polymer networks, whereas there are space open channels for the general scaffolds. The porous structure relies basically on the fabrication and manufacturing. These methods include fiber bonding, solvent casting/particulate leaching, gas foaming, and phase separation [8, 9].

  • Scaffolds should be made of biocompatible biomaterial so that the cellular components of the engineered tissue and the cells of host tissue suit properly. Natural and synthetic polymers have excellent flexibility to adapt their shape to wanted forms via different manufacturing techniques [8].

  • The bioactivity level of the scaffold should be high enough to carry out and regulate the activity of native tissue cells and the cells inside the engineered ECM. The scaffold, at the same time, is a delivery vehicle for exogenous growth-stimulating signals such as growth factors. Hydrogels can be filled up with proteins, and the release of these proteins can be triggered via swelling of the scaffold [10].

  • Scaffolds, on the first hand, must provide mechanical and shape stability to the defective area of the native tissue defect. Therefore, mechanical properties of the scaffold should match that of the host tissue.

Regardless of the approach used for each clinical application, the objective is the same, specifically, the restoration of structure and function. Similarly, regardless of which strategy is selected, the response of the host to the implanted construct will dictate the success or failure of the eventual outcome.

2 Humoral Response to a Scaffold

It is a fact that all kind of materials implanted into a human recipient are subject to response by the host’s immune system. The host response to implanted materials is unavoidable. This response occurs immediately, and there are factors affecting the level of this response. These are directly related to the character of the material especially the constituents of the implant and also anatomical location of the implant.

We have a broad knowledge about the foreign body reaction for the classical biomaterials which are composed of nondegradable synthetic and metallic components and used for long-term implantation like hip and knee prosthesis. On the other hand, tissue engineering end products, scaffolds, trigger a different response than those preferred for long-term treatment like materials used for arthroplasty without degradation. There should be a balance between tissue reaction against the biomaterial implanted and the structural and mechanical properties of that biomaterial in vivo.

The foreign body response is known to have negative effects for material lifetime in the body and local tissue as well.

Immediately after implantation of biomaterial, a cascade of events regarding the host response starts. This starts from protein biofilm formation on the biomaterial then continues with acute inflammation then to chronic inflammation and granulation tissue formation and ends with fibrosis and capsule formation around the biomaterial [11, 12].

2.1 Blood Material Interaction and Biofilm Formation

Release of blood into the wound site during the surgical procedure results in degranulation of platelets and start of inflammatory process. With the blood, contact proteins (components of coagulation system or plasma-derived proteins) attach to the surface of the biomaterial within seconds of implantation. These proteins on the surface of biomaterial (or scaffold) serve as an anchor for the inflammatory cells migrating to the region. And finally a biofilm is formed around the scaffold [12, 13].

2.2 Acute Inflammation

Arrival of inflammatory cells, especially neutrophils, will start the process with the release of chemoattractant factors. However, as soon as neutrophils arrive, they interact with the proteins on the biomaterial surface. This will lead to phagocytosis by neutrophils and/or macrophages or destruction of the pathogen via the complement pathway. In both processes there will be an erosion of implanted material which is not favored in short term after implantation [12, 14].

2.3 Chronic Inflammation

The chronic inflammation is typically characterized by the presence of activated macrophages. This process may last from weeks to months depending on the nature of the implanted material and the anatomical location. Different from acute inflammation, angiogenic component is prominent in chronic inflammation, and there is new ECM formation in and around the scaffold. And also, unlike acute inflammation, the foreign body giant cells will replace macrophages [12].

2.4 Granulation Tissue Formation, Foreign Body Reaction, and Tissue Encapsulation

Chronic inflammation can progress to a granulation tissue phase, in which the deposition of new ECM and the growth of vasculature into the implantation eventually form a dense layer of connective tissue. This will end up with “foreign body reaction” and eventually encapsulation of the biomaterial within a dense layer of collagenous connective tissue will be seen [12, 15].

The foreign body response with an outcome of tissue encapsulation is considered an undesirable outcome for tissue engineering and regenerative medicine strategies which seek to promote functional recovery [12].

This inflammatory process or host immune response will dictate the success or outcome of the scaffold. So that, immediately after surgical placement of the scaffold, the properties of the scaffold will start changing. These properties can be listed as mechanical (strength and porosity), chemical (biodegradability), and biological (biocompatibility).

There are a number of factors which dictate the host response to tissue engineering and regenerative medicine constructs, and these factors are:

  • The choice of biomaterial, and we can also subclassify the factors affecting the biomaterial choice.

    • Chemistry of the material (hydrophobic or hydrophilic)

    • Its ability to degrade

    • Surface structure

    • Processing methods

  • The types of cells included within the scaffold.

  • The bioactive factors or pharmacological agents used with the implanted scaffold.

The scaffolds are used typically either as a delivery vehicle for the cells or to provide mechanical and functional support immediately just after implantation. To minimize the host immune response, the cells used in tissue engineering and regenerative medicine are preferably autologous in type. When allograft and xenograft cells are preferred, the host immune response easily detect them [8]. Recently, induced pluripotent stem cells (iPSC) have inspired great interest as a potential source of autologous cells for tissue engineering and regenerative medicine. Yamanaka et al. discovered induced pluripotent stem cells which can differentiate into any type of cell as a result of the action of four reprogramming factors: c-Myc, Klf4, Oct3/4, and Sox2 [16]. But there is still a long way to go since the effects of their reprogramming upon the host response to these cells following placement is currently unknown, and there is still a potential for these cells for carcinogenesis [17]. Host immune response can also be triggered by the cellular debris due to cell death around the scaffold. Therefore, it is also important to carefully consider the fate of the cells and the potential mechanisms of cell death.

Xenogeneic and allogeneic cellular antigens are recognized by the host immune response immediately, and this will activate the immune system and cause the production of pro-inflammatory mediators and cause transplant tissue rejection. This is known as Th1-type immune response and is commonly associated with rejection. Th2-type immune response is characterized by production of a different set of chemokines and are more commonly associated with the process of transplant acceptance [12, 18].

Autologous cells are known to be the only way to avoid any kind of immune response but collecting and expanding autologous cells is time consuming and technically demanding.

Alternatively, engineered tissues could incorporate progenitor cells (like mesenchymal stem cells) that may suppress host immune responses directly or indirectly through decreased expression of MHC, and these progenitor cells have a broad differentiation capacity so that they are capable of differentiating into multiple cell types [19, 20].

3 Signaling

Intracellular cell signaling is the whole process starting from an extracellular mechanical and/or chemical stimuli and translation of these stimuli into a cellular response. An extracellular signal, which can be a cytokine, growth factor, or hormone, is transmitted through the cellular membrane into the cytoplasm. Once it is inside the cell, it may either continue to the nucleus via second messengers or interact with other cell components, and finally the result can be a change in gene expression, phenotype, or metabolism.

Signaling can be autocrine type, which occurs when signaling molecules released from a cell bind to receptors located on the same cell, paracrine type, which signaling refers to signaling molecules that bind to receptors located on neighboring cells, or endocrine type signaling when systemically circulating signaling molecules (e.g., hormones) bind to receptors located in cells external to their place of production.

Cytokines, growth factors, and hormones are some of the extracellular signaling molecules that initiate signaling pathways. Cytokines (e.g., interleukins, interferons) are primarily used for maintaining cell homeostasis and the body’s defensive pathways. Growth factors, closely related to cytokines, are primarily used in the regulation of cell growth and proliferation such as TGF-β superfamily and insulin-like growth factor (IGF). Hormones (e.g., PTH, growth hormone [GH]) interact with cells through endocrine signaling.

Signaling pathways occur through the attachment of an extracellular signal, a ligand, to a cell receptor protein either spanning or extending from the plasma membrane of the cell. Receptor proteins are most commonly transmembrane, structurally consisting of three segments: extracellular, intracellular, and a hydrophobic segment located within the plasma membrane [21].

ECM molecules are capable of interacting with different type of receptors. Integrins which are known to be an important receptor participate in both “outside-in” and “inside-out” signaling [22].

Outside-in signaling occurs when an extracellular ligand binds the receptor and initiates intracellular signaling. And in inside-out signaling, intracellular signaling increases the affinity of the receptor for its ligand. Binding of the receptor to the ligand, in turn, initiates outside-in signaling.

In matrix-induced signaling, there are three categories of cell-ECM interactions; the first type of interactions are involved in adhesion and migration, the second type of interactions are involved in adhesion and migration, and the third type of interactions are involved in apoptosis and epithelial-mesenchymal transitions [22, 23].

Ideal scaffold is the one which is composed of an ideal extracellular matrix same or close to native ECM seeded with autologous cells in conjunction with immunosuppressant drugs and also with growth and differentiation factors added to the matrix favoring cell-ECM interactions.

Although ECM molecules can be used successfully in tissue engineering, the use of natural ECM molecules has several disadvantages, especially risk of generating an immune response, possible contamination, and ease of degradation. Additionally, artificial biocompatible materials have limitations that, unlike natural ECM, they are generally incapable of transmitting growth and differentiation factors to cells.

In the last few years, “semi-synthetic biomaterials” are the new design of scaffolds in which functional regions of ECM molecules are incorporated into artificial biomaterials to create additional functionality.