Understanding experimental biology of skin equivalent: from laboratory to clinical use in patients with burns and chronic wounds.

A major breakthrough in burn wound care was the early excision of the burn and its immediate coverage with a skin autograft. A search for a skin-graft substitute began to reduce the autografting-related trauma at the donor site. One entry was skin equivalence, which contains 3 components: (1) living fibroblasts, suspended in (2) a native collagen matrix, the surface of which is covered with (3) viable keratinocytes. The tissue-cultured dermal fibroblasts are derived from human foreskin. The fibroblasts are grown in cell culture dishes as a monolayer and are retrieved by limited trypsin digestion. The fibroblast suspension is mixed with serum-supplemented culture medium and native acid-soluble collagen. The entire mixture, called a dermal equivalent, is placed in a bacteriological Petri dish before transfer to a 37 degrees C incubator. The collagen rapidly polymerizes, trapping cells in the dermal equivalent. During the initial 4 hours, fibroblasts elongate and spread, causing a decrease in the thickness of the dermal equivalent. After 6 hours, the dermal equivalent undergoes a decrease in diameter as a consequence of the reorganization of the collagen. A freshly isolated suspension of human skin-derived keratinocytes is seeded on the surface of a several-day-old floating dermal equivalent. The keratinocytes proliferate, covering the surface of the dermal equivalent. The keratinocytes deposit basement membranes beneath them and undergo epidermal cell differentiation, leading to the formation of a basal layer beneath differentiated cell layers. Both cell populations retain viability and release cell factors that have a positive effect on wound closure. The placement of skin equivalence within a chronic wound may share structural attributes with a skin graft, but its function is to accelerate closure.