Protective Barriers Provided by the Epidermis

Abstract

The skin is the largest organ of the body and consists of an epidermis, dermis and subcutaneous adipose tissue. The skin surface area is often stated to be about 1.8 to 2 m² and represents our interface with the environment; however, when one considers that microorganisms live in the hair follicles and can enter sweat ducts, the area that interacts with this aspect of the environment becomes about 25-30 m². Although all layers of the skin, including the adipose tissue, participate in antimicrobial defense, this review will focus mainly on the role of the antimicrobial factors in the epidermis and at the skin surface. The outermost layer of the epidermis, the stratum corneum, is physically tough and chemically inert which protects against numerous environmental stresses. It provides a permeability barrier which is attributable to lipids in the intercellular spaces between the corneocytes. In addition to the permeability barrier, there is an innate antimicrobial barrier at the skin surface which involves antimicrobial lipids, peptides and proteins. The skin surface has a low surface pH and is poor in certain nutrients, which limits the range of microorganisms that can survive there. Melanin and trans-urocanic acid provide protection from UV radiation, and Langerhans cells in the epidermis are poised to monitor the local environment and to trigger an immune response as needed. Each of these protective barriers will be discussed.

Keywords: Langerhans cells; antimicrobial lipids; antimicrobial peptides; barrier function; epidermis; melanocytes.

Figure 1

Transmission electron micrograph of an intercellular space in the stratum corneum [5]. The human skin specimen was fixed with ruthenium tetroxide. Bar = 50 nm.

Figure 2

Ceramide structures from porcine and human stratum corneum [5]. The numbers (1–6) on the right indicate ceramides from porcine stratum corneum, while human ceramides are indicated on the left in the nomenclature of Motta et al. [12]. The porcine ceramide designated as 4/5 separates into two separate bands. The more mobile fraction contains mostly 20- through 28-carbon α-hydroxyacids amide-linked to sphingosine and dihydrosphingosine bases. The less mobile fraction is mostly α-hydroxy palmitic acid amide-linked to the same bases. In the human CER AS, there is an α-hydroxyacid-containing component with a more uniform chain length distribution.

Figure 3

The linoleate-rich acylglucosylceramide (center) is the precursor of the acylceramide (bottom) and the covalently bound ω-hydroxyceramide [5]. This lipid is attached to the outer surface of the cornified envelope and has been referred to as the corneocyte lipid envelope, or CLE.