Cells need to do more than just proliferate in the laboratory—they also have to organize exactly the way they would in the human body, and to do that they need the right mix of nutrients, growth factors, and a scaffold. The nutrient solution with all of its ingredients—the cell culture medium, in other words—is first heated to 37 degrees Celsius, because cells feel most comfortable at body temperature. The scientists then dispense the liquid into plastic dishes coated with a gel-like scaffold material, onto which they pipette the cells, one type of cell per dish. The keratinocytes become the epidermis, or top protective layer of the skin, while the other cell type—the fibroblasts—forms the dermis below that. Once they are in their respective dishes, the cells are finally placed in the incubator to mature and multiply. As soon as enough cells have formed, the researchers combine the cell types into a two-layer model while adding growth factors. After that, it’s back to the incubator for two to three weeks until the cells have joined to form a firm, transparent skin the size of a one-cent coin.