Inside out – outside in the skin: different experimental approaches to evaluate the epidermal response to different exogenous stimuli

Elena Bianca Donetti
Laboratorio di Morfologia Strutturale e Ultrastrutturale
Dipartimento Scienze Biomediche per la Salute
UniMi


From an anatomical point of view, the borders between the human body and the environment are the skin, the respiratory system, and the intestine. The skin preserves homeostasis thanks to an inside-out barrier avoiding loss of water and nutrients. The outside-in barrier in the skin prevents damages induced by xenobiotics, radiations or microbes and plays a major role in skin absorption. Mammalian skin is composed by three compartments: the epidermis, the dermis, and the subcutis. Epidermis, the most superficial skin layer, is a cornified stratified squamous epithelium composed by keratinocytes, melanocytes, Langerhans cells, and Merkel cells. The physical and immunological epidermal barrier against a wide range of exogenous stimuli is formed thanks to a strong cooperation between keratinocytes and Langerhans cells. Many experimental models were developed to investigate the early epidermal response to environment, comprising in vitro keratinocyte monolayer, full thickness 3-D human skin model, commercially available reconstructed human epidermis. To overcome the intrinsic limitations of these approaches, in our lab we standardized a simple but effective experimental model using three-dimensional organotypic cultures of normal human skin obtained from aesthetic surgery maintained at the air-liquid interface in a Transwell system mimicking the physiological condition. The effects of gamma rays, UVA and UVB rays, and, more recently, of different proinflammatory psoriatic cytokines were tested on keratinocyte proliferation, differentiation, adhesion, and fine structure by transmission electron microscopy. Altogether, these studies prove that this experimental approach offers the advantage of understanding the intrinsic, specific, and early effects of different exogenous stimuli on differentiated keratinocytes.