Anatomy of skin

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Skin anatomy and physiology

Healthy skin is made of three layers that form a protective barrier between your body and the environment:

  • The epidermis – the skin’s protective outer layer
  • The dermis – which contains connective tissue, hair follicles and sweat glands
  • The subcutaneous layer – the deepest layer made from fat and connective tissue

The outermost layer of the epidermis, the stratum corneum, helps prevent excessive loss of moisture, which is critical to protecting skin’s natural barrier. 1

At the simplest level, the stratum corneum can be compared to a brick wall. The bricks are corneocytes, protein-rich cells comprised mostly of keratin microfibrillar matrix, while the mortar is composed of lipids. 1

Image adapted from Hoffman, 2014 (2)

Image adapted from Hoffman, 2014 (2)

Intercellular lipids

The intercellular lipids in the stratum corneum are primarily made up of ceramides, free fatty acids and cholesterol. These lipids are organised to provide a tight and effective barrier to transepidermal water loss (TEWL). 3

In dry skin conditions such as eczema, the stratum corneum has lower levels of intercellular lipids. 4  Increasing age also correlates to changes in the stratum corneum’s lipid content. The overall decrease in total stratum corneum lipids in elderly skin is approximately 30%. 3

Corneocytes contain natural moisturising factor (NMF)

Corneocytes (the bricks) comprise the major part of the stratum corneum, they are regularly replaced through desquamation and renewal from lower epidermal layers. They contain natural moisturising factor (NMF), a complex mixture of free amino acids, amino acid derivatives and salts. 4 NMFs are humectants, as their physiological action is to absorb water from the atmosphere, to maintain hydration of the stratum corneum. This water retention helps maintain the turgidity of the corneocytes, to preserve barrier function and help prevent cracking of the skin. 4

Low levels of NMF are associated with severe cases of xerosis. 4 Urea is one component of NMF, and has an important role to play in keeping the stratum corneum hydrated, for example there is a urea deficit in the stratum corneum of people with atopic dermatitis. 3

Adapted from Harding, 2004 (1)

Adapted from Harding, 2004 (1)

Skin pH

The acidic pH of the skin (~5), commonly referred to as the acid mantle, also has a significant role in establishing the epidermal permeability barrier, as well as the epidermal antimicrobial barrier. Among the important roles of the acid mantle, the acidic pH enables the optimal function of epidermal enzymes, such as β-glucocerebrosidase, acid sphingomyelinase and secreted phospholipase A2 (sPLA2), which are vital for the production of ceramide molecules and free fatty acids (the mortar) required for epidermal barrier homeostasis and appropriate control on stratum corneum integrity and cohesion. 5

Elevations in the skin surface pH is associated with disruption in skin barrier function as well as abnormal desquamation of the skin. 5 Disrupted skin barrier function has been associated with skin conditions such as xerosis, eczema, atopic dermatitis and psoriasis.

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How dry and itchy skin develops


Dry skin is the visible manifestation of excessive TEWL which can lead to more serious conditions developing in a gradual progression of distinct stages: 6

1) Exogenous damage to the stratum corneum

  • Damage to the major lipids and proteins, and loss of NMF causes disruption of the permeability barrier
  • The result is decreased hydration and enzymatic function, and, though non-visible, the beginning of dry, flaky, skin
  • This increase in TEWL can be partially countered by self-repair mechanisms of the stratum corneum, however;

2) If disruption of the permeability barrier continues

  • This can lead to an overstressed stratum corneum and the start of visible changes to the skin, in the form of microfissuring
  • Without appropriate therapy, continued exogenous damage can lead to reduced elasticity, larger scales, linear fissuring, and finally;

3) Asteatotic Eczema

  • Visible inflammation, persistent scaling and flaking, itchy, dry skin

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Useful information for patients

Take a look at our treatment planner for more information on how skin works to help patients after their consultation.

References

  1. Harding CR. (2004). Dermatol Ther. 17(suppl.1): 6-15
  2. Hoffman M. (2014). WebMD. Picture of the Skin. [accessed: April 2017] via http://www.webmd.com/skin-problems-and-treatments/picture-of-the-skin#1
  3. Verdier-Sévrain S, Bonté F. (2007). J Cosmet Dermatol. 6(2): 75-82
  4. Voegeli D. (2007). Nurs Stand. 22(7): 62-68
  5. Seung Hun Lee, Se Kyoo Jeong, Sung Ku Ahn. (2006). Yonsei Med J. 47(3): 293-306
  6. High W, Del Rosso JQ, Levin J. (2011). J Clin Aesthet Dermatol. 4(9): 22-42

UK/E45-NHS/0317/0004a