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17.3 SKIN MECHANOBIOLOGY 351

Mechanosensing is then followed by the process of mechanotransduction that occurs through alterations at the sub-
cellular and molecular levels through intracellular proteins (e.g., vinculin and talin), kinases and phosphatases (e.g.,
focal adhesion kinase (FAK)), cytoskeletal components (i.e., actin filament and microtubule), or intermediate filaments
(e.g., keratin, dermin, plectin, lamin, and vimentin) [93]. Thus it is clear why the mechanosensing force is mechano-
transduced into a biological response related with the activated signaling pathway that can be, for instance, adhesion,
survival, apoptosis, proliferation, migration, or differentiation [93, 96, 97].
The effect of the stiffness and topography over skin cells has been largely reviewed, but the result of mechanical
forces on skin cells has only been considered recently and will be the focus of this section. In the skin, different cells
are able to mechanosense mechanical forces. The most distinctive mechanoreceptors are neurons that are able to sense
touch, itch, pain, and temperature stimuli. Merkel cells, the Ruffini corpuscle, and the Meissner’s corpuscle both at the
epidermis and dermis of the skin are also able to mechanosense external forces and transmit the signal to sensory neu-
rons called low-threshold mechanoreceptors (LTMRs) that in turn send the signal to the central nervous system [98,
99]. Other cells existing in the skin, including endothelial cells [100], adipocytes [101], stem cells [102], Langerhans cells
[103], and melanocytes [104, 105], also sense mechanical stimuli, but the effect of mechanical loading over skin cells has
only been explored for the main skin resident cells—fibroblasts and keratinocytes. This is probably because these cells,
in addition to the external stimuli, are continuously exposed to internal passive tension forces resulting from cell-cell
(keratinocytes and fibroblasts) and cell (fibroblast)-ECM interactions. External forces exerted at air-epidermis interface
enhance the active tension in the skin and on both cell types due to the stretching of cell junctions and collagen fibrils.
The mechanical stimuli are transmitted along the air-epidermis interface throughout the epidermis due to hemides-
mosomes existing in the epidermal-dermal junction that maintain the mechanical continuity along the two skin layers
[106]. Animal models have been used to understand the effect of forces over the different skin cells [25–27]. However,
these studies are mainly performed using rodent skin that is both anatomically and mechanically different from
human skin [107]. Moreover, human skin is a complex organ, and the continuous exposition to different stimuli,
including other than mechanical ones such as chemical, radiation, electric, etc., renders difficult the understanding
of isolated effects. Hence, the effect of external forces over keratinocytes and fibroblasts has been mainly explored
in vitro by the application of external forces onto cells.

17.3.2 Effect of Forces Over Fibroblasts and Keratinocytes
The first study of the effect of forces over skin cells was in 1990. G€ ormar et al. showed that cyclic compression forces
of pestle-shaped weights over keratinocytes induced their differentiation [108]. Similar studies using compression
forces conducted in dermal skin fibroblasts showed increased production of metalloproteinases (MMPs) in response
to the stimulus [109]. To our knowledge, these were the only studies that explored the effect of compression forces over
skin cells. This may be due to the limited significance of compression forces, as keratinocytes and fibroblasts in the skin
are only subjected to tension forces. Thus skin mechanobiology studies have focused on the effect of tension forces of
different orientations, magnitudes, frequencies, and periods over keratinocytes (Table 17.2) and fibroblasts (Tables 17.3
and 17.4).
In the late 90’s, researchers started developing in-house mechanical loading units to study the effect of tension on
cells cultured onto silicone substrates [110, 111]. Keratinocyte mechanobiology was empowered with the arising of
mechanical stretching units for sell [16, 112–115]. Tension forces showed to affect keratinocytes by activating the
epithelial-mesenchymal transition [115], promoting proliferation [112, 113, 115–118] through the activation of ERK
signaling pathway [111, 116, 119], and by activating antiapoptotic mechanisms [120, 121] mediated by the activation
of the Akt signaling pathway [117, 120, 121]. Moreover, tension forces induced keratinocytes migration [122, 123],
basement membrane ECM production [16, 118], expression of basal layer-associated keratins [16, 116], and decreased
expression of upper layer-associated ones [116]. These responses seem to be mediated by integrin β1 [111, 119, 123].
These studies revealed that tension mechanical loading has a prominent effect on keratinocytes and that the observed
responses are independent of force orientation, magnitude, frequency, and period of the loading (Fig. 17.8).
The first observations regarding the mechanobiology of fibroblasts refer to 1983 when Allen and Schor detected the
contraction of free-floating collagen gels loaded with dermal fibroblasts [127, 128]. In the absence of gel anchorage (no
mechanical loading), the contractile tension existing between dermal fibroblasts and collagen gels becomes unbal-
anced prevailing fibroblasts traction forces that lead to gel contraction. This did not occur if gels were anchored
because the mechanical loading due to anchoring maintained a balanced between the cell traction and the cell-
ECM forces [127] (Fig. 17.9). The dermal fibroblasts in the anchored gels showed an elongated morphology and pro-
duced higher amounts of ECM in contrast with the round morphology and the reduced synthesis of the mediators of
ECM synthesis observed with fibroblasts in free-floating gels [127, 129–131] (Fig. 17.9, Table 17.3). These responses

II. MECHANOBIOLOGY AND TISSUE REGENERATION