Date Published: March 15, 2019
Author(s): Seyedeh Atefeh Mobasseri, Sebastiaan Zijl, Vasiliki Salameti, Gernot Walko, Andrew Stannard, Sergi Garcia-Manyes, Fiona M. Watt.
In human skin the junction between epidermis and dermis undulates, the width and depth of the undulations varying with age and disease. When primary human epidermal keratinocytes are seeded on collagen-coated polydimethylsiloxane (PDMS) elastomer substrates that mimic the epidermal-dermal interface, the stem cells become patterned by 24 h, resembling their organisation in living skin. We found that cell density and nuclear height were higher at the base than the tips of the PDMS features. Cells on the tips not only expressed higher levels of the stem cell marker β1 integrin but also had elevated E-cadherin, Desmoglein 3 and F-actin than cells at the base. In contrast, levels of the transcriptional cofactor MAL were higher at the base. AFM measurements established that the Young’s modulus of cells on the tips was lower than on the base or cells on flat substrates. The differences in cell stiffness were dependent on Rho kinase activity and intercellular adhesion. On flat substrates the Young’s modulus of calcium-dependent intercellular junctions was higher than that of the cell body, again dependent on Rho kinase. Cell patterning was influenced by the angle of the slope on undulating substrates. Our observations are consistent with the concept that epidermal stem cell patterning is dependent on mechanical forces exerted at intercellular junctions in response to undulations in the epidermal-dermal interface.
In human skin the epidermal-dermal junction undulates and epidermal stem cells are patterned according to their position. We previously created collagen-coated polydimethylsiloxane (PDMS) elastomer substrates that mimic the undulations and provide sufficient topographical information for stem cells to cluster on the tips. Here we show that the stiffness of cells on the tips is lower than cells on the base. The differences in cell stiffness depend on Rho kinase activity and intercellular adhesion. We propose that epidermal stem cell patterning is determined by mechanical forces exerted at intercellular junctions in response to the slope of the undulations.
Mammalian skin is built from two histologically and physiologically distinct tissue compartments: an epithelial layer called the epidermis and an underlying connective tissue layer called the dermis. In humans, the interface between the epidermis and dermis is not flat but undulates . The interfollicular epidermis (IFE) comprises multiple cell layers, with the stem cell compartment attached to an underlying basement membrane  and cells undergo terminal differentiation as they move through the suprabasal layers . Extrinsic signals such as interactions with neighboring cells, extracellular matrix (ECM) adhesion, tissue stiffness and secreted factors are known to regulate the behavior of stem cells . Physical forces such as cell shape, shear forces and substrate stiffness all affect the balance between stem cell proliferation and differentiation . Internal and external mechanical loading affects the biology of both epidermis and dermis and is mediated through mechanochemical transduction processes that involve both cell-cell and cell-ECM adhesion .
When primary human keratinocytes are seeded on a collagen-coated undulating polydimethylsiloxane (PDMS) elastomer substrate that mimics the topography of the epidermal-dermal junction in healthy, young skin, the cells become patterned such that the β1 integrin bright stem cells localise to the tips of the features . Here we show that the cells at the tips of the features are softer than those at the base, correlating with enhanced accumulation of E-cadherin, Desmoglein 3 and F-actin at cell-cell borders. The differential stiffness of the cells was dependent of Rho kinase activity. Our observations lead us to propose that epidermal stem cell patterning is achieved through forces exerted on stem cells by cells on the slopes, mediated by intercellular adhesion. This would be consistent with earlier work showing that keratinocytes are stiffer than other cell types because of their keratin intermediate filament network , and that mutations or antibody treatments that impair desmosomes alter the viscoelastic properties of keratinocytes , , .