Table 1.

Relationship Between Anatomic Structure and Viscoelastic Function of the Skin

Skin Layer	Structure	Role in Viscoelasticity
Stratum corneum—outermost layer of epidermis	Structure of up to 25–30 rows of corneocytes; includes fibrous keratin; “brick and mortar” arrangement, in conjunction with other stratified layers in the epidermis, increases tensile strength (resistance to longitudinal stress), and resistance to damage (Micali, Lacarrubba, Bongu, & West, 2001) Water content is 15–30% (Johnsen, Haugsnes, Martinsen, & Grimnes, 2010)	Supports pliability (ease in change of shape from baseline) Promotes strength, elastic behavior, and resistance to loss of skin integrity with movement, stretching, and application of force
Basement membrane zone (BMZ)	Collection of three cell layers between the epidermis and the dermis (lamina lucida, lamina densa, and lamina propria; Bruckner-Tuderman & Stanley, 2007; Chan, 1997); comprised of proteins (primarily laminins, proteoglycans, and types IV and VII collagens; Chan, 1997) Desmosomes (cells responsible for adhesion) serve as binding cells between basal layer of skin and upper lamina lucida (Bruckner-Tuderman & Stanley, 2007; Chan, 1997) Anchoring fibrils and a matrix of fibers at varying stages of maturity connect thicker lamina densa layer to upper layer of dermis (Barland, Zettersten, Brown, Ye, Elias, & Ghadially, 2004; Chan, 1997) BMZ semipermeable to water; limits water passage to maintain skin hydration and support viscoelasticity	Lamina layers extremely flexible due to construction of multiple-microfibrillar subdensa and protein-based supra-lamina desmosomes (Bruckner-Tuderman & Stanley, 2007; Chan, 1997) Supports epidermis and provides strong adhesion between the epidermal and dermal layers to protect against shearing forces (Chu, 2007); when force applied on parallel plane to skin, it has a viscoelastic response of expanding and then contracting fiber matrix and associated fluids Serves as an anchor to surrounding layers; disruption of BMZ leads to amorphous structure within epidermis and dermis causing skin structure breakage and reduced viscoelastic response
Dermis—layer between the epidermis and subcutaneous tissues	Within papillary region (uppermost layer of dermis), a networking of thin elastin protein fibers (oxytalan fibers and the elaunin fibers cross-linked via desmosomes) is in loose matrix with procollagen (a precursor to collagen that originates within ground substance) and ground substance (Chu, 2007; Haake et al., 2001; Schafer, Pandy, Ferguson, & Davis, 1985; Uitto, Chu, Gallo, & Eisen, 2007) Reticular region (below papillary region and above hypodermis) is comprised of ground substance and a thicker mesh of collagen fibers wound among thicker elastic fibers assembled from elastin and microfibrils (Haake et al., 2001; Schafer et al., 1985; Uitto et al., 2007)	With force, elastin molecules stretch in linear pattern, cross links maintain structure; quick elastic reaction provides immediate response to force, followed by slower viscous response and then full return to baseline Elastic fibers are thinner in papillary region and used for quick response but break more easily; elastic fibers in reticular region thicker, more bundled with collagen, and provide slower, viscoelastic behavior and greater tensile strength (Uitto et al., 2007; Wysocki, 1999)
Hypodermis—innermost and thickest layer of skin; connects dermis to bone or connective tissue	Adipose tissue is present in the hypodermis, but thickness of this layer may vary (Agache, 2006; Agache & Diridollou, 2006; Tortora & Grabowski, 1993)	Thickness of adipose deposits maintains shape of skin, protects it from underlying (bony) structures, and is positively correlated with skin strength and elasticity (Agache & Varchon, 2004; Smalls et al., 2006); positive and protective effects may negated in obesity (Yosipovitch, DeVore, and Dawn (2007) Problems with obesity include impaired skin barrier repair, decreased lymphatic flow, decreased strength of collagen structures, impaired circulation, decreased wound healing, and skin disorders that change the structure and impair the function of the skin (Yosipovitch, DeVore, & Dawn, 2007)