A negative pressure instrument (Electronic Diversities, Finksburg, MD, USA) constructed to produce standard suction blisters upon application of negative pressure, was used on healthy skin (ex vivo: abdominal skin; in vivo: lower forearm). Subcutaneous fat was partially removed from ex vivo skin using a scissor. Subsequently, skin (10 × 10 cm2) was placed (not fixed, not kept in medium) on a styrofoam lid that was covered with aluminium foil to provide (at least partial) backpressure. Suction chambers with 5 openings (Ø = 5 mm) on the orifice plate were attached to skin, topped with a styrofoam lid and pressed with 1 kg weight in order to avoid movement of the plate. A pressure of 200–250 millimeter (mm) mercury (Hg) (ex vivo) or 150–200 mm Hg (in vivo) caused the skin to be drawn through the openings creating typical suction blisters of different size within 6–8 h (ex vivo) and 1–2 h (in vivo). Suction blister fluid (~110 µl/5 blisters) was collected using a syringe with a needle. Cells within the fluid were counted and placed on adhesion slides for staining and analysis. Blister roof epidermis was cut with a scissor, fixed with ice-cold acetone (10 minutes) and used for staining. For comparison and control, epidermal sheets were prepared from unwounded skin biopsy punches (Ø = 6 mm; 3.8% ammonium thiocyanate (Carl Roth GmbH + Co. KG, Germany) in PBS (Gibco, Thermo Fisher, Waltham, MA, USA), 1 h, 37 °C). Removal of the blister roof created a wound area. Biopsies (Ø = 6 mm) from wounded and unwounded areas were cultivated for 12 days in either duplicates or triplicates in 12 well culture plates and Dulbecco’s modified Eagle’s medium (DMEM) (Gibco) supplemented with 10% fetal bovine serum (FBS) (Gibco) and 1% penicillin-streptomycin (Gibco) and were cultured at the air-liquid interphase. Medium was changed every second day.
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