Five Functional Aspects of the Epidermal Barrier

Abstract

The epidermis is a living, multilayered barrier with five functional levels, including a physical, a chemical, a microbial, a neuronal, and an immune level. Altogether, this complex organ contributes to protect the host from external aggression and to preserve its integrity. In this review, we focused on the different functional aspects.

Keywords: ceramides; desmosomes; epidermis; immunity; keratinization; microbiome; sensory nerves; skin barrier; stratum corneum; tight junctions.

Figure 1

Modification of intercellular junction along keratinocyte differentiation. Kc: keratinocytes Ds: desmosome; CDs: corneodesmosome; TJ: tight junction; AJ: adherens junction; K: keratin; Tg: transglutaminase; SPR: small proline-rich proteins.

Figure 2

Changes in desmosome composition during keratinocyte differentiation.

Figure 3

Human epidermis after solar irradiation (magnification × 120, HES stain). UVB light stimulates melanin secretion which acts as a built-in sunscreen. Epidermal melanin is a primary absorber of ultraviolet radiation, thus protecting the underlying epidermal and dermal elements.

Figure 4

Crosstalk between microbiological, immunological, and sensory nerve barriers of the epidermis. Bacteria and skin cells are organized in a symbiosis in which nutrients are shared and protection from environmental hazards (e.g., pathogens) is provided by the different actors. Bacteria produce their own AMPs and stimulate immune cells and keratinocytes to induce immune vigilance and modulate the epidermal structure. In return, the epidermis produces nutrients and ligands essential for the establishment of commensal flora and develops an anti-pathogen defense via the production of AMPs, the action of cells such as neutrophils and macrophages, the production of cytokines to potentiate the action of these cells and chemokines to recruit them from the bloodstream. Immune cells and keratinocytes also interact with nerve endings (“neuroinflammation”) to potentiate the response to pathogens and participate in sensory awareness. Finally, the skin barrier informs the body by sending nerve impulses and circulating skin cells to lymphoid organs.