Since the first utilization in 1957 (Neumann, 1957), the use of tissue expansions have become widespread in maxillary and craniofacial surgery (Kobus, 2007), burn scar excision (Hafezi et al., 2009), breast reconstruction following mastectomy (Lohsiriwat et al., 2013), ophthalmology (Hou et al., 2012), management of omphalocele (Clifton et al., 2011), nasal reconstruction (Kheradmand et al., 2011), scalp alopecia (Guzey et al., 2015) and other deformities in plastic reconstructive surgery (Motamed et al., 2008; Laurence et al., 2012; Santiago et al., 2012). Tissue expander generates new tissues, by exploiting the viscoelastic properties of the skin and adjusted histological changes which follows the principle of the controlled mechanical skin overstretch (Argenta, 1984; Pamplona et al., 2014). It involves the insertion of a biomimetic and bioinspired material (i.e., hydrogel tissue expander) adjacent to a wound or defect that needs to be resurfaced (Motamed et al., 2008; Swan et al., 2012). The expanded tissue can then be used to resurface a defect or incorporate permanent prostheses (Kasper et al., 2012; Swan et al., 2012).

Nevertheless, tissue expansion for the reconstructive surgery are also associated with a variety of complications (Adler et al., 2009; Huang et al., 2011). Swan et al. (2012) observed mucoperiosteal ulceration while using uncoated self-inflating anisotropic hydrogel tissue expander in the porcine hard palate. Minor side effects on skin histology and circulation resulted in skin stretching with staples or hypodermic needles, thus proving the Pavletic device to be non-feasible in primary wound closure (Tsioli et al., 2015). Incidence of infection, being the most common complication (Huang et al., 2011), has witnessed a total of 16 cases out of 215 children who underwent reconstruction with tissue expanders (Adler et al., 2009). However, the pivotal concern is to ensure normal tissue patterning and prevent tumor or scar formation (Huang and Ingber, 1999; Aarabi et al., 2007).

Recent studies revealed that rapid changes in extension, alignment, and collagen adapt to mechanical expansion (i.e., stretch or strain). Both elastin and collagen realign in a parallel fashion in response to stretch and/or expansion (Verhaegen et al., 2012; Tsioli et al., 2015), and the elongation occurs to the direction of stretching (Figure 1). Mechanical stretch on tissue is related to several physiological phenomena such as cellular growth enhancement and/or expansion with a significantly higher vascularity of expanded tissue (Yano et al., 2004). Strain beyond physiological limit may lead to alteration of cell function such as tumor formation, necrosis and/or apoptosis (Chen et al., 1997; Huang and Ingber, 1999; Wernig et al., 2003; Knies et al., 2006). Hence lies the clinical implications of tissue expansion (Swenson, 2014; Kwon et al., 2016).

In physiological condition, tissue development and remodeling are highly regulated. A number of studies have focused on the cellular and molecular mechanisms (such as integrated network of cascades, implicating growth factors, cytoskeleton, protein kinase family, synthesis of DNA, expression of gene) leading to the increase of skin surface area (Plenz et al., 1998; Takei et al., 1998; Skutek et al., 2003; Knies et al., 2006; Jaalouk and Lammerding, 2009; Wong et al., 2011; Wu et al., 2015). Under mechanical stress, the cell phenotype and the nature of the physical stimuli determine which signal transduction pathways are activated during tissue expansion (Hsieh and Nguyen, 2005). This review, will focus the reports of molecular events of skin-derived cells in response to mechanical strain. The response of cells to mechanical stretch, the roles of growth factors, effects on extracellular matrix, cell membrane, and stretch induced ion channels, roles of second messengers, and cellular interactions will be organized from the extracellular to intracellular pathways with future perspectives in the conclusion.

The viscoelastic properties of skin to increase surface area in response to forces are the basic biology of tissue expansion (Bascom and Wax, 2002). The external forces are transmitted through the multi-layered skin which consists of epidermis connected to the dermis and the underlying subcutaneous tissues (Schwartz and DeSimone, 2008). The morphological and physiological consequences of tissue expansion on various layers of skin and other cellular and muscular components are summarized in Table 1.

Numerous researchers have linked the mechanisms that lead to an increased length with skin's elasticity (Kenedi et al., 1975; Bader and Bowker, 1983; Larrabee Jr and Sutton, 1986). Gibson et al. (1965) associated the increase in skin length with the interstitial displacement of fluids and skin's creep behavior. Austad et al. (1982) reported that the increased length was as a result of cellular proliferation. Siegert et al. (1993) simplified these findings relating the strain, time and mechanism of skin expansion as shown in Figure 2. Because of its elasticity, the skin expands practically without temporal delay after expansion pressure is exerted. Interstitial displacement of fluids can be seen (in oedema) after skin expansion. Larrabee Jr et al. (1986), Gibson et al. (1965) and Wilhelmi et al. (1998) suggested that the biological creep (i.e., the generation of new tissue) is due to the chronic stretching forces. It is also most likely that similar events such as interstitial fluid displacement and elasticity beyond the tolerance limit of the tissue might induce necrosis and/or apoptosis of the tissue (Linder-Ganz and Gefen, 2004).

Cell stretching, in some contexts causes apoptosis, and in others promotes cell proliferation (Takei et al., 1998; Skutek et al., 2003). Similarly, apoptosis and proliferation pathways share many common elements, and they converge and influence each other at different levels (Wernig et al., 2003). Application of mechanical stretch (stimulus) activates mechanosensitive ion channels, G-protein coupled receptors, protein kinases, integrin-matrix interactions and other membrane-associated signal-transduction molecules to convert physical cues to biologic responses (Schwartz and DeSimone, 2008; Jaalouk and Lammerding, 2009) (Figure 3).

The cellular growth, tissue integrity and eventually the reestablishment of the barrier function of the skin is executed and regulated by the coordinated efforts of several cell types (keratinocytes, fibroblasts, macrophages, platelets etc.) and numerous growth factors (biologically active polypeptides) (Werner et al., 2007; Gurtner et al., 2008). The epidermal growth factor (EGF) family, transforming growth factor beta (TGF-β) family, fibroblast growth factor (FGF) family, vascular endothelial growth factor (VEGF), platelet derived growth factor (PDGF), connective tissue growth factor (CTGF), interleukin (IL) family are all important in stress (either mechanical or physiological) induced cell growth (Werner et al., 1994; Shimo et al., 1999; Steiling and Werner, 2003; Shirakata et al., 2005; Secker et al., 2008). The functions of growth factors depend on source and binding with specific receptors and can act by paracrine, autocrine, juxtacrine, and endocrine mechanisms (Barrientos et al., 2008). Earlier studies showed that, EGF, FGF-2, TGF-β, PDGF, and VEGF levels are increased in early after injury and decreased at chronic states and IL-1 and 6, and TNF-α levels increased both in early and chronic states (Brown et al., 1986; Frank et al., 1995). The functions of various growth factors are summarized in Table 2.

Among the growth factors families, the EGF family and the TGF-β family are thought to play central roles (Hashimoto, 2000) and they provide dual-mode regulation of keratinocyte growth via the proliferation-stimulating effect of EGF and the proliferation-inhibiting effect of TGF-β (Amendt et al., 2002; Secker et al., 2008). Although, these growth factors appear to share several downstream pathways of cell membrane molecules, the direct effects of mechanical stress on TGF and EGF are yet to be investigated (Takei et al., 1998). Although, human epidermal keratinocytes express ErbB1, ErbB2, and ErbB3, they do not express ErbB4 (Hashimoto, 2000). Similarly, signals originating from ErbB1 play crucial roles in mediating the pro-survival and proliferative programs of keratinocytes (Shirakata et al., 2010). The expression of cadherins, integrins, and various other ECM components that contribute to the maturation of new blood vessels are regulated by FGF2 (Cross and Claesson-Welsh, 2001). HB-EGF shows a starring role in the reepithelialisation and granulation tissue formation (Marikovsky et al., 1996). The strongest autocrine stimulation to cell growth is provided by amphiregulin (Piepkorn et al., 1994).

Mechanical stress to the cell surface activates the mechanosensitive ion channels along with other membrane-associated signal-transduction molecules (De Filippo and Atala, 2002; Wang et al., 2009). The precise mechanism of activation and modulation of ion channels by mechanical forces that results in biologically meaningful signals are subjects of intensive research (Martinac, 2014). Sachs (1991) reported that, in order to make conformational changes of a channel, external forces must do work on the channel and be dominated by the distance the force move. Howard and Hudspeth (1988) estimated that the stress activated channels change their dimensions by 4 nm between the closed and open states. These stretch-induced ion channels are mainly cation (Ca²⁺, K⁺, and Na⁺) channels and a few anion (Cl⁻) channels (Jackson, 2000; Nilius and Droogmans, 2001).

The vast majority of channels open because of the changes in lipid bilayer, membrane fluidity or tension and are regulated by voltage, extracellular ligands, phosphorylation, influx of Ca²⁺ and direct (physical interactions between G-protein subunits and the channel protein) or indirect (via second messengers and protein kinases) interaction with activated G proteins (Christensen, 1987; Maroto et al., 2005; Lumpkin and Caterina, 2007; Hahn and Schwartz, 2009). The mechanosensitive activities of ion channels are cell dependent and vary from cell to cell (Hsieh and Nguyen, 2005). The elevated intracellular Ca²⁺ levels are cytotoxic and provide the apoptotic stimulus in multiple cell types. The studies of past decades indicated the involvement of different ions in stretch induced response and cytoskeleton are also associated (Jackson, 2000; Wang et al., 2001). However, the precise ion channels related mechanisms for tissue expansion are yet to be studied.

The exact role of second messengers system in response to tissue expansion (i.e., epithelial cell proliferation) is not clearly elucidated (De Filippo and Atala, 2002). Several investigations in last decades of the past century reported that, cyclic adenosine monophosphate (cAMP) plays an important role to influence cell growth, differentiation, proliferation and protein synthesis depending on the source of cells and experimental conditions (Bang et al., 1992; Florin-Christensen et al., 1993; Zhang et al., 2016). Takei et al. (1997) found significant increase of protein production in keratinocytes subjected to cyclic strain. Moreover, net collagen amount decreases when the levels of cAMP in skin fibroblasts is increased. Study of Acute and chronic cyclic strain reduces adenylate cyclase activity in cultured coronary vascular smooth muscle cells that could promote strain-induced cell contraction (Wiersbitzky et al., 1994). The findings of previous researches on second messengers are listed in the Table 3.

Inositol phosphate (IP), c-fos, and phospholipids (PL) are thought to mediate extracellular signals to the nucleus but the precise mechanisms need further reaserch (Takei et al., 1998). Moreover, Molinari (2015), proposed hydrogen ion (H⁺) as a second messenger to mediate Ca²⁺ mobilization especially in IP3/Ca²⁺ signaling pathway. At the beginning of 21st century, Buscà et al. (2000) reported that the BRAF gene (which mediates growth signaling at a level just below RAS) can be activated by cAMP in melanocytes. Extracellular signals (growth factors) that activate G-protein-couples receptor can result in the activation of adenylate cyclase to upregulate cAMP leading to the activation of RAS and further activation of BRAF and the downstream cascades (Simonds, 1999; Davies et al., 2002; Pollock and Meltzer, 2002). Likewise the studies on second messengers have been done on different cell lines, this study was also performed with cultured cell lines derived from human tumors, so further investigations are needed to be executed with expanded tissue and acutely stretched skins to determine the precise roles of the ubiquitous and archetypal intracellular second messengers.

In this article, recent advances in tissue expansion in the field of plastic and reconstructive surgery were described with a special focus on the biological response and the activated pathways leading to either proliferation or apoptosis. Emphasis was given on the roles of membrane bound molecules such as integrins, G-protein, growth factors, stretch-activated ion channels, and secondary messengers. Although, studies of past decades demonstrated that, mechanical stimulation is capable to activate highly integrated signaling cascades resulting in the new skin production, questions remain on how different types of stimulation works on, different cells following different signal transduction pathways. For example, studies on the cells from the kidney differ significantly compared to the cells of skin which are subjected to constant expansion or mechanical forces. Moreover, studies using cultured cells rather than intact tissue (skin) cannot clarify the exact effects of tissue expansion. Similarly, stimulus such as shearing, heat, and shock cannot provide natural microenvironment to better understand how cell adapt to changes during tissue expansion. Furthermore, the signaling pathways activated by different biochemical factors were investigated in linear methods such as single pathway analysis, which is insufficient to describe multiple signaling pathways involved in cell proliferation and/or apoptosis. Therefore, in depth comparative proteomic and genomic analysis with expanded tissue or acutely stretched skin would reveal the pathways and molecules responsible for cell proliferation and/or apoptosis ultimately skin regeneration.

Concept development: MTR. Writing the manuscript: MAR, MTR, MSH, ZR, NY, and JC. Literature review for data collection: MAR, MTR, MSH, and ZR. Figure and Tables: MAR, MTR, MSH.