The skin is composed of three layers, the epidermis (50–100 μm), dermis (1–2 mm) and hypodermis (1–2 mm) (). The barrier to percutaneous absorption lies within the stratum corneum, the most superficial layer of the epidermis. The function of the stratum corneum is to reduce water loss, provide protection against abrasive action and microorganisms, and generally act as a permeability barrier to the environment.
The anatomical layers of the cutaneous tissue.
The stratum corneum is a 10–20 μm thick, multilayer stratum of flat, polyhedral-shaped, 2 to 3 μm thick, non-nucleated cells named corneocytes. Corneocytes are composed primarily of insoluble bundled keratins surrounded by a cell envelope stabilized by cross-linked proteins and covalently bound lipids. Corneodesmosomes are membrane junctions interconnecting corneocytes and contributing to stratum corneum cohesion. The intercellular space between corneocytes is composed of lipids primarily generated from the exocytosis of lamellar bodies during the terminal differentiation of the keratinocytes. These lipids are required for a competent skin barrier function.
The epidermis is composed of 10–20 layers of cells. This pluristratified epithelium also contains melanocytes involved in skin pigmentation, and Langerhans’ cells, involved in antigen presentation and immune responses. The epidermis, as for any epithelium, obtains its nutrients from the dermal vascular network.
The epidermis is a dynamic structure and the renewal of the stratum corneum is controlled by complex regulatory systems of cellular differentiation. Current knowledge of the function of the stratum corneum has come from studies of the epidermal responses to perturbation of the skin barrier such as: (i) extraction of skin lipids with apolar solvents; (ii) physical stripping of the stratum corneum using adhesive tape; and (iii) chemically-induced irritation. All such experimental manipulations lead to a transient decrease of the skin barrier efficacy as determined by transepidermal water loss. These alterations of the stratum corneum generate an increase of keratinocyte proliferation and differentiation in response to this “aggression” in order to restore the skin barrier. This increase in the keratinocyte proliferation rate could directly influence the integrity of the skin barrier by perturbing: (i) the uptake of nutrients, such as essential fatty acids; (ii) the synthesis of proteins and lipids; or (iii) the processing of precursor molecules required for skin barrier function.
