ADSCs (Lonza Group AG, Basel, Switzerland) were co-cultured with normal human dermal fibroblasts (NHDFs) as previously reported28 (link)30 (link). Briefly, NHDFs were seeded in six-well plates (IWAKI, Shizuoka, Japan) and cultured in Dulbecco's modified Eagle's Medium (DMEM) (Gibco, Life Technologies, Carlsbad, CA, USA) containing 10% fetal bovine serum (Sigma-Aldrich, St. Louis, MO, USA) for 24 hours, while ADSCs were seeded on 0.4-µm Millicell® hanging cell culture inserts (Merck Millipore, Darmstadt, Germany) coated with type IV collagen (Nitta Gelatin, Osaka, Japan) and placed onto the plates. All-trans retinoic acid (Sigma-Aldrich) was added at 1 µM to the upper chamber. After culturing for 3 days, 25 ng/ml of bone morphogenetic protein 4 (R&D Systems, Minneapolis, MN, USA) was also added to the upper chamber. After 4 days, the media were replaced with keratinocyte serum-free medium (KSFM) (Thermo Fisher Scientific, Waltham, MA, USA). After 7 days of culture in KSFM, ADSCs were removed from the co-culture system and cultured on a dish coated with type IV collagen in KSFM for an additional 14 days.