Keratinocytes that are grown in the laboratory so that a small biopsy sample from uninjured skin may grow as a sheet and expand to have a surface area 1,000 to 10,000 times the area of the sample. The sheet can be used to cover wounds such as burns. The culture technique requires 2 to 3 weeks, and the regeneration of tissue below the sheet may not be complete for 5 to 6 months.