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Anatomy and Function of the Skin Part 2 – the Epidermis

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Anatomy and Function of the Skin Part 2 – the Epidermis CLINICAL systems of life Anatomy and function of the skin Part 2 – the epidermis stratum corneum stratum lucidum stratum granulosum stratum spinosum stratum basale Basement membrane Dermis fiG 1. the layeRs of the ePiDeRmis Author sandra lawton, msc, RGN, oND, eNB the underlying dermis for nutrient delivery and waste 393, RN (Child), is nurse consultant in dermatology, disposal through the dermo-epidermal junction. The Queen’s Medical Centre, Nottingham University epidermis is defined as a stratified squamous Hospitals NHS Trust. epithelium, consisting primarily of keratinocytes in progressive stages of differentiation (Revis and The epidermis is the outer layer of the skin varying in Seagle, 2006). Its prime function is to act as a thickness from 0.1mm to 1.4mm (Gawkrodger, 1992) physical and biological barrier to the environment, (Fig 1). It consists of a number of layers: preventing penetration by irritants and allergens and l Stratum corneum; loss of water while maintaining internal homeostasis l Stratum lucidum; (Gawkrodger, 1992; Cork, 1997). l Stratum granulosum; l Stratum spinosum; Layers of epidermis l Stratum basale; The epidermis is composed primarily of layers of l Basement membrane. densely packed keratinocytes that produce the protein It contains no blood vessels and is dependent on keratin and are the major cells of the epidermis. 28 NT 8 August 2006 Vol 102 No 32 www.nursingtimes.net keywoRDs n Skin n Epidermis n Anatomy and physiology Keratinocytes are formed by division in the fiG 2. the PRoCess of DesquamatioN RefeRences stratum basale (basal or germinative layer). As the keratinocytes move up through the stratum Butcher, M., White, r (2005) The spinosum (prickle cell layer) and stratum granulosum Structure and Functions of the Skin. (granular layer), they differentiate to form a rigid In: White, R. (Eds). Skin Care in Desquamation t Wound Management: Assessment, internal structure of keratin, microfilaments and 1,000 cells/cm2/hour microtubules (keratinisation). prevention and treatment. Aberdeen: Wounds UK. The outer layer of the epidermis, the stratum two corneum (horny layer), is composed of layers of weeks Cork, M. (1997) The importance of flattened dead cells (corneocytes) that have lost their stratum corneum skin barrier function. Journal of nucleus. These cells are then shed from the skin 15–20 cell layers t Dermatological Treatment; 8: Suppl (desquamation). The complete process from t 1, 7–13. formation to desquamation takes approximately 28 two days (Hill, 1994) (Fig 2). gawkrodger, D.J. (1992) An weeks Between these cells (corneocytes) there is a Dermatology: An Illustrated Colour complex mixture of lipid and proteins (Cork,1997). Text. Edinburgh: Churchill These intercellular lipids are broken down by Livingstone. enzymes from keratinocytes to produce a lipid mixture of ceramides (phospholipids), fatty acids graham-Brown, r., Bourke, J.F. and cholesterol. These molecules are arranged in a (1998) Mosby’s Color Atlas and highly organised fashion, fusing with each other and t Text of Dermatology. London: Mosby. the corneocytes to form the skin’s lipid barrier against water loss and penetration by allergens and Hill, M.J. (1994) Skin Disorders. irritants (Holden et al, 2002). Other epidermal cells St Louis: Mosby. The stratum corneum can be visualised as a brick Keratinocytes make up about 95% of the epidermal wall, with the corneocytes forming the bricks and the cells, the others being melanocytes, Langerhan cells Holden, C. et al (2002) Advised lamellar lipids forming the mortar. Corneocytes and Merkel Cells (Butcher and White, 2005). best practice for the use of contain a water-retaining substance – natural MeLANoCyTeS – are found in the stratum basale emollients in eczema and other dry moisturising factor – and so attract and hold water. (basal or germinative layer) and are interspersed skin conditions. Journal of The high water content of the corneocytes causes among the keratinocytes along the basement Dermatological Treatment; 13: 3, these cells to swell, thereby keeping the stratum membrane (dermo-epidermal junction) at a ratio of 103–106. corneum pliable and elastic, and preventing the one melanocyte to 10 basal cells. They produce the revis, D.r., Seagle, M.B. (2006) formation of fissures and cracks (Cork, 1997; Holden pigment melanin, which is manufactured from the Skin Anatomy emedicine. et al, 2002). This is an important consideration when amino acid tyrosine, packaged into melanosomes, Available: www.emedicine.com/ applying topical medications to the skin. transported and delivered into the cytoplasm of the plastics/topic389.htm. keratinocytes (Graham-Brown and Bourke, 1998). The Percutaneous absorption key function of melanin is protection, absorbing rudy, S., Parham-Vetter, P. (2003) Percutaneous absorption refers to the absorption of ultraviolet radiation and protecting us from its Percutaneous absorption of topically topical medications and other substances through the harmful effects. Skin colour is determined not by the applied medication. Dermatology epidermal barrier into the underlying tissues and numbers of melanocytes but by the number and size Nursing; 15: 2, 145–152. structures, which then transfer to the systemic of the melanosomes (Gawkrodger ,1992). Skin colour circulation. Three elements play a role in percutaneous is also influenced by ultraviolet light, genetic factors absorption – the features of normal skin, changes in and hormonal influences (Hill, 1994). skin barrier function and vascular changes. LANgerHAN’S CeLLS – are antigen-presenting cells in The stratum corneum (horny layer) regulates the the stratum spinosum (prickle cell layer). They amount and rate of percutaneous absorption (Rudy represent one part of the body’s immune system and and Parham-Vetter, 2003). Two of the most important are constantly on the lookout for antigens (micro- factors affecting percutaneous absorption are skin organisms and foreign proteins) in order to trap them hydration and environmental humidity. and present them to T helper lymphocytes, thus In healthy skin with normal hydration, activating an immune response (Graham-Brown and medicaments can only penetrate the stratum Bourke 1998, Butcher and White, 2005). corneum (horny layer) by passing through the tight, MerkeL CeLLS – are only present in very small relatively dry, lipid barrier between cells. When skin numbers and are found in the stratum basale (basal hydration is increased or the normal skin barrier is or germinative layer). They are closely associated This article has been double-blind impaired, as a result of skin disease, excoriations, with terminal filaments of cutaneous nerves and peer-reviewed. erosions, fissuring or prematurity, percutaneous seem to have a role in sensation, especially in areas For related articles on this subject absorption will be enhanced (Rudy and Parham- such as palms, soles and genitalia (Gawkrodger, and links to relevant websites see Vetter, 2003). 1992; Butcher and White, 2005). n www.nursingtimes.net NT 8 August 2006 Vol 102 No 32 www.nursingtimes.net 29.
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