Structure and function of the epidermis related to barrier properties
Introduction
The skin is a continuously self-renewing organ that covers the surface of the body and separates it from the outside world with which it connects in a dynamic way. It provides protection against external agents such as mechanical and chemical insults, heat, infections, water, and electromagnetic radiation. The skin is divided into two main structural compartments: the epidermis or epithelial component coating on the surface, and the in-depth dermis or connective component of nutrition. Both skin compartments cooperate in the formation of a highly specialized matrix structure, the basement membrane, which physically separates the two compartments, providing a stabilizing as well as dynamic interface.1 Although the whole skin structure actively participates in the host defense, epidermis is important in preventing loss of water and other components of the body to the environment (inside–outside barrier) and in protecting the body from a variety of environmental insults (outside–inside barrier).
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Epidermis and the first-line epidermal barrier
Epidermis is a continually renewing epithelium, usually subdivided into several layers or strata, starting with the basal layer (or stratum basale) just above the dermis and proceeding upward through the spinous and granular layers to the top layer, the stratum corneum (Figure 1). Its main function is to protect the skin from potentially hazardous environmental threats, providing physical, chemical, biochemical (antimicrobial, innate immunity), and adaptive immunologic barriers.2 The physical
Second-line epidermal barrier
In addition to the stratum corneum, the entire skin, as a whole, serves a protective function. The lower epidermal layers are also significant in barrier function, so that after the stratum corneum, they represent a second-line epidermal barrier (Figure 1). In fact, a low to moderate increase in TEWL occurs after removal of the stratum corneum by tape stripping, whereas loss of the entire epidermis through suction blisters leads to a severe disturbance in barrier function. Loss of the stratum
Dermis and hypodermis
The dermis, a layer of connective tissue that includes collagen and elastic fibers, is highly vascularized and provided with a good network of lymphatic vessels. It hosts sweat and sebaceous glands as well as hair follicles, and provides a good mechanical, compact, supple support.29, 30 The dermis contains numerous cells, including fibroblasts and macrophages. Fibroblasts are responsible for the synthesis and the renewal of extracellular matrix, whereas macrophages contribute to eliminate
Skin immune defence
The human skin represents the first line of defence against potentially hazardous environmental threats, such as infection by microbes (viruses, bacteria. and fungi), and is also the largest organ of the body, functioning as an interface between the human host and the environment.31 Although for many years the skin was believed to be an inert barrier, it is a complex organ that has the ability to rapidly discriminate between self and not-self, starting a sufficient innate nonspecific immune
Conclusions
The skin acts as a two-way barrier to prevent the inward or outward passage of water and electrolytes. The main epidermal barrier is localized to the stratum corneum; however, a second-line epidermal barrier is represented by the sealing effect that the tight junctions of the granular layer exert on neighboring keratinocytes.
The immune defense of the skin has several facets, including immediate, nonspecific mechanisms (innate immunity), and delayed, stimulus-specific responses (adaptive
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Adone Baroni and Elisabetta Buommino equally contributed to this work.