Skin is the largest organ in the human body and serves numerous functions including protection, the absorption of substances, and thermoregulation. Skin has a complex stratified structure consisting of three main layers:
1) the tightly packed cells of the epidermis form a barrier against intruders and water loss, and includes an outer layer called the stratum corneum, consisting of overlapping, nonviable corneocytes that protect against threats such as UV radiation and pathogens;
2) the dermis, which supports the epidermis and contains nerve endings, sweat glands, sebaceous glands, hair follicles, and blood vessels. The basal layer of the dermis is where keratinocytes constantly undergo mitosis to replenish cells; and
3) the subcutaneous layer, or hypodermis, which contains mostly adipocytes and acts to protect underlying tissues, as an energy reserve, and to provide some thermoregulation via insulation.
Historically, animal models have been used to test the toxicity and efficacy of cosmetics1 and transdermal drug absorption.2 Due to ethical concerns, however, these animal models have gradually been replaced by “cruelty-free” in vitro organotypic skin models that use primary human cells and cell culture inserts to recapitulate the stratified epidermal architecture.3
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