The Skin Structure

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Skin provides a physical and chemical barrier between the outside environment and the inside tissues of the body (5, 6). This ‘barrier function’ is critical to protect underlying tissues from pathogens, chemicals, and environmental exposures (5, 7). Structurally, skin is comprised of two main layers, the epidermis and the dermis (Figure 1) (6, 7). The epidermis, the upper layer, is responsible for many of the barrier functions of the skin. The dermis is the structural and nutritive support network underneath the epidermis. Since each layer has its own unique structure and function, the nutritive requirements of each must be considered separately. Unlike many organs, skin nutrition may be enhanced directly through topical applications. 

Epidermis: the outer barrier

Human epidermis comprises the outermost layers of skin cells, ranging from 0.1 mm to 0.6 mm thick, depending on the location on the body (7)Keratinocytes compose the vast majority (90-95%) of cells within the epidermis. These cells are found in layers of varying size and thickness (6). The bottom or basal layer of the epidermis consists of a layer of round, undifferentiated keratinocytes that is supported by contact to the underlying dermis. Cells in this layer are constantly dividing in order to produce new cells that will make up the remainder of the epidermal layers (6, 7). Thus, it is in this fashion that skin is replenished, with new cells from the basal layer replacing the outer layer of skin cells that is shed over time.

Once a keratinocyte leaves the basal layer, it begins the process of cell differentiation known as keratinization (6). During keratinization, keratinocytes undergo several morphological changes that result in the synthesis of unique structural proteins (especially keratin) and the secretion of a variety of specialized lipids that will comprise key components of the epidermal barrier.

The outermost layer of skin, the stratum corneum, interacts with the outside environment. Lipids secreted by cells during the keratinization process are now assembled with extracellular proteins into a protective layer. This barrier is often likened to a ‘brick and mortar’ system: large, flattened, metabolically inactive corneocytes (the protein-rich ‘bricks’) are sealed together with a variety of extracellular lipids and proteins (the ‘mortar’) (5, 6). The chemical properties and structural design of this layer slow absorption and limit penetration of the skin, as well as limit the loss of vital nutrients and water from the underlying tissue. As new layers of cells are produced, the outer cells of the stratum corneum are enzymatically detached from this layer and shed in a process called desquamation (5).

Further, other cells contribute to the function of the epidermis. Melanocytes are cells that produce melanin, a compound involved in skin pigmentation produced in response to ultraviolet (UV) light exposure (6). Melanin can absorb energy from UV light to shield underlying tissues from damage. 

The dermis is the inner layer of skin situated between the epidermis and other tissues of the body, such as connective tissue, subcutaneous fat, muscle, and bone. The dermis can vary in depth from 0.3 mm to 4 mm depending on body location and is generally at least ten times thicker than the epidermis (7, 8). However, nearly 75% of the weight of the dermis is a matrix of collagen, an extracellular protein that allows for both structural support and elasticity of the skin. Thus, the primary role of the dermis is a mechanical support network for the epidermis, providing integrity and flexibility to skin. Blood vessels that supply nutrients for all skin layers are found in the dermis (6).

Healthy skin has the ability to respond to challenges that would otherwise undermine its structure and function. Balanced nutrition complements the host of endogenous factors that preserve skin health. Moreover, skin that functions properly has aesthetically pleasing properties, giving skin a healthful appearance and feel. 

Although the lower layers of the epidermis are moist, there is a sharp decline in extracellular water content as the cells migrate outward toward the skin surface (20, 21). This is partially by design: the hydrophobic environment found in the stratum corneum slows the passage of water from the body out into the atmosphere, a phenomenon known as Transepidermal Water Loss (TEWL). Since water loss is directly related to the skin’s ability to maintain its barrier functions, measurements of TEWL are often used to determine skin dysfunction. However, small amounts of water are needed for the stratum corneum to maintain its structure. A mixture of stratum corneum components form a water-binding barrier, together known as natural moisturizing factor (NMF), to retain moisture content even in dry environments (20).

Dry skin can be caused by many factors, but it is usually accompanied by changes in the epidermal barrier and increased TEWL (more water lost to the environment). Intrinsic changes in the lipid barrier or NMF of the stratum corneum can disrupt the barrier and cause water loss. This can stem from simple chemical exposures, such as washing with detergents (20) or from more complex nutritional deficiencies, such as a lack of essential fatty acids (22). However, dry skin can also be an effect of atmospheric conditions or exposures. Changes in temperature, air flow, and humidity can pull water away from the skin and reduce barrier integrity. If left untreated, dry skin is often predisposed to insults from other sources, leading to cycles of cell damage and inflammation that perpetuate the condition.

An important strategy to treat dry skin is to maintain the lipid barrier and NMF components of the stratum corneum (20). This can be achieved either through nutritional support of the underlying epidermis or by the use of a variety of topical applications (23).

Skin laxity and wrinkles

Little is known about the origin of wrinkles (24). However, skin laxity is associated with poor support of the epidermis by the underlying dermis. Loss of collagen, damage to collagen and elastin fibers in the dermis, or structural changes in the junction between the dermis and epidermis are thought to contribute to wrinkle formation (25-27).

Skin laxity may be accentuated by loss of blood vessels in the dermis (28). Additionally, smoking and photodamage increase skin wrinkling (27, 29). Thus, avoiding these sources of damage would benefit skin tone. Nutritional factors are thought to influence wrinkling (1), but their roles are not clear.

Skin aging

Skin is often the first visible manifestation of the aging process. However, the effects of age on skin appearance are often similar to the effects associated with photodamage and environmental exposures. This makes the changes induced by chronological age on skin — often referred to as intrinsic aging — difficult to distinguish from other effects (27).

Instrinsic skin aging is characterized by decreasing support from the dermis to the epidermis. Ridges on the interface between the two layers are diminished, preventing the dermis from providing adequate mechanical support to the epidermis (27, 33)Collagen levels are lower and extracellular proteins in the dermis are more disorganized in skin of older individuals compared to younger adults (27, 33, 34). These changes result in increased skin fragility and laxity, as well as decreased size of the dermis and reduced vascularization, which reduces nutrient availability to the skin (27, 29). Aged skin keratinocytes are relatively slow to differentiate and shed, which alters their ability to maintain the stratum corneum. These changes may lead to an overall dull skin appearance and loss of protective ability of skin (27).

Antioxidant protection is often thought to diminish in the skin of older individuals.

Specialized lipids required for the development of the stratum corneum, such as sterols and ceramides, are synthesized in the epidermis from amino acids, carbohydrates, and phospholipids. However, differentiating keratinocytes also utilize fatty acids from circulating stores or dermal fat layers for energy. The extruded fatty acids that make up the mortar of the stratum corneum can absorb lipid-soluble materials placed on the external surface of this outermost skin layer. This is especially the case for sebum, a waxy substance secreted from the sebaceous glands that are attached to hair follicles, but it is also true for topically applied materials.

The epidermal layers of the skin do not contain blood vessels that supply the cells with nutrients; blood vessels are found only in the dermis. Additionally, as the epidermis develops, its unique protein and lipid structure (the aforementioned “bricks and mortar” model) prevents the circulation of extracellular fluids (7). Therefore, the outer layers of the epidermis are provided with less nutritional support than the underlying cells. 

he stratum corneum prevents the passage of many different types of molecules, but some compounds pass through to the underlying layers. In general, uncharged or lipid-soluble molecules pass through the epidermis and may also penetrate the dermis. Concentrations of nutrients in the skin may be comparable to that achieved through oral ingestion. Yet, topical application may be a more efficient, targeted method for supplying nutrients to the skin, especially to the epidermis (12, 39).

References:

1.Freinkel RK, Woodley D. The biology of the skin. New York: Parthenon Pub. Group; 2001.

2.https://www.researchgate.net/publication/43561145_Phytochemical_Constituents_and_Pharmacological_Activities_of_Calendula_officinalis_Linn_Asteraceae_A_Review