Lipids and the Permeability and Antimicrobial Barriers of the Skin

Abstract

The primary purpose of the epidermis of terrestrial vertebrates is to produce the stratum corneum, which serves as the interface between the organism and the environment. As such, the stratum corneum provides a permeability barrier which both limits water loss through the skin and provides a relatively tough permeability barrier. This provides for a degree of resistance to mechanical trauma and prevents or limits penetration of potentially harmful substances from the environment. The stratum corneum consists of an array of keratinized cells embedded in a lipid matrix. It is this intercellular lipid that determines the permeability of the stratum corneum. The main lipids here are ceramides, cholesterol, and fatty acids. In addition, the skin surface of mammals, including humans, is coated by a lipid film produced by sebaceous glands in the dermis and secreted through the follicles. Human sebum consists mainly of squalene, wax monoesters, and triglycerides with small proportions of cholesterol and cholesterol esters. As sebum passes through the follicles, some of the triglycerides are hydrolyzed by bacteria to liberate free fatty acids. Likewise, near the skin surface, where water becomes available, some of the ceramides are acted upon by an epithelial ceramidase to liberate sphingosine, dihydrosphingosine, and 6-hydroxysphingosine. Some of the free fatty acids, specifically lauric acid and sapienic acid, have been shown to have antibacterial, antifungal, and antiviral activity. Also, the long-chain bases have broad spectrum antibacterial activity.

Seborrheic dermatitis—Looking beyond Malassezia

“…Sebum is a key component of a functional epidermal barrier . Although it is not known whether there are significant changes in the lipid composition of secreted sebum in SD and dandruff patients, it has been well established that Malassezia metabolism of sebum lipids does lead to changes in skin surface lipid composition .…”

Collagen Hydrolysates for Skin Protection: Oral Administration and Topical Formulation

“…This function is regulated by the hypothalamus. Lipids and ceramides of the external layers of the skin provide a barrier against the excessive loss of water [24]. Dryness of the skin could appear if these emerged lipids are removed by the use of detergents.…”

“…Melanin is the natural pigment in the skin that absorbs ultraviolet (UV) light [25]. The skin protects against chemical and microbiological attacks [24]. Lamellar granules release intercellular lipids in the underlying stratum granulosum.…”

“…TEWL provokes a gradient of humidity concentration, resulting in a water content higher at the dermis level and decreasing at the surface of the skin [58]. The SC is a very effective barrier against loss or gain of moisture [24]. The water content at the SC level is not uniformly distributed through its thickness, the more superficial layers are in equilibrium with the ambient water content, while the deeper layers exchange their free water with the epidermis [59].…”

A Quantitative MALDI-MSI Study of the Movement of Molecules in Biological Systems

“…The stratum corneum (SC) represents the principal skin barrier and this function is essentially due to the lipid composition and organisation within it (Grubauer et al, 1989;Bouwstra et al, 1999;Wertz, 2018). In the SC each corneocyte is surrounded by an envelope of cross-linked proteins with which a layer of lipids (lipid envelope) are covalently bound, forming the cornified envelope structure (Abraham and Downing, 1990;Nemes and Steinert, 1999;Candi, Schmidt and Melino, 2005).…”