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Skin 1: the Structure and Functions of the Skin

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Skin 1: the Structure and Functions of the Skin Copyright EMAP Publishing 2019 This article is not for distribution except for journal club use Clinical Practice Keywords Skin/Skin function/Skin assessment/Epidermis/Dermis Systems of life This article has been Skin double-blind peer reviewed In this article... l How the skin is structured l Functions of the skin l Specialised cells and structures Skin 1: the structure and functions of the skin Key points Author Sandra Lawton, Queen’s Nurse and nurse consultant and clinical lead The skin is the dermatology, The Rotherham NHS Foundation Trust. largest organ in the human body Abstract Skin diseases affect 20-33% of the population at any one time, and around 54% of the UK population will experience a skin condition in a given year. Nurses Approximately half observe the skin of their patients daily and it is important they understand the skin so of the UK population they can recognise problems when they arise. This article, the first in a two-part series will experience a on the skin, looks at its structure and function. skin condition in any given year Citation Lawton S (2019) Skin 1: the structure and functions of the skin. Nursing Times [online]; 115, 12, 30-33. Nurses observe patients’ skin daily, so need to be able kin diseases affect 20-33% of the consists largely of fat. These structures are to identify problems UK population at any one time described below. when they arise (All Parliamentary Group on Skin, S1997) and surveys suggest around Epidermis Key functions of 54% of the UK population will experience a The epidermis is the outer layer of the skin, the skin include skin condition in a given year (Schofield et defined as a stratified squamous epithe- protection, al, 2009). Nurses will observe the skin daily lium, primarily comprising keratinocytes regulation of body while caring for patients and it is impor- in progressive stages of differentiation temperature, and tant they understand it so they can recog- (Amirlak and Shahabi, 2017). Keratinocytes sensation nise problems when they arise. produce the protein keratin and are the The skin and its appendages (nails, hair major building blocks (cells) of the epi- How others respond and certain glands) form the largest organ dermis. As the epidermis is avascular (con- to people who have in the human body, with a surface area of tains no blood vessels), it is entirely skin conditions is 2m2 (Hughes, 2001). The skin comprises dependent on the underlying dermis for an important 15% of the total adult body weight; its nutrient delivery and waste disposal consideration thickness ranges from <0.1mm at its thin- through the basement membrane. for nurses nest part (eyelids) to 1.5mm at its thickest The prime function of the epidermis is part (palms of the hands and soles of the to act as a physical and biological barrier to feet) (Kolarsick et al, 2011). This article the external environment, preventing reviews its structure and functions. penetration by irritants and allergens. At the same time, it prevents the loss of water Structure of the skin and maintains internal homeostasis The skin is divided into several layers, as (Gawkrodger, 2007; Cork, 1997). The epi- shown in Fig 1. The epidermis is composed dermis is composed of layers; most body mainly of keratinocytes. Beneath the epi- parts have four layers, but those with the dermis is the basement membrane (also thickest skin have five. The layers are: known as the dermo-epidermal junction); l Stratum corneum (horny layer); this narrow, multilayered structure l Stratum lucidum (only found in thick anchors the epidermis to the dermis. The skin – that is, the palms of the hands, layer below the dermis, the hypodermis, the soles of the feet and the digits); Nursing Times [online] December 2019 / Vol 115 Issue 12 30 www.nursingtimes.net Copyright EMAP Publishing 2019 This article is not for distribution except for journal club use Clinical Practice Systems of life Fig 1. Cross-section through the skin molecules are arranged in a highly organised fashion, fusing with each other and the cor- neocytes to form the skin’s lipid barrier against water loss and penetration by aller- Sweat gland Melanocytes Hair gens and irritants (Holden et al, 2002). Sweat The stratum corneum can be visualised Capillaries Oil as a brick wall, with the corneocytes Stratum corneum forming the bricks and lamellar lipids (keratin) forming the mortar. As corneocytes con- Epidermis tain a water-retaining substance – a nat- ural moisturising factor – they attract and Basement membrane hold water. The high water content of the corneocytes causes them to swell, keeping Sebaceous gland the stratum corneum pliable and elastic, Dermis and preventing the formation of fissures and cracks (Holden et al, 2002; Cork, 1997). This is an important consideration when Hair follicle applying topical medications to the skin. These are absorbed through the epidermal Fat layer Blood vessels barrier into the underlying tissues and structures (percutaneous absorption) and Nerve transferred to the systemic circulation. The stratum corneum regulates the amount and rate of percutaneous absorp- l Stratum granulosum (granular layer); keratin, microfilaments and microtubules tion (Rudy and Parham-Vetter, 2003). One l Stratum spinosum (prickle cell layer); (keratinisation). The outer layer of the epi- of the most important factors affecting l Stratum basale (germinative layer). dermis, the stratum corneum, is com- this is skin hydration and environmental The epidermis also contains other cell posed of layers of flattened dead cells (cor- humidity. In healthy skin with normal structures. Keratinocytes make up around neocytes) that have lost their nucleus. hydration, medication can only penetrate 95% of the epidermal cell population – the These cells are then shed from the skin the stratum corneum by passing through others being melanocytes, Langerhans cells (desquamation); this complete process the tight, relatively dry, lipid barrier and Merkel cells (White and Butcher, 2005). takes approximately 28 days (Fig 3). between cells. When skin hydration is Between these corneocytes there is a com- increased or the normal skin barrier is Keratinocytes. Keratinocytes are formed by plex mixture of lipids and proteins (Cork, impaired as a result of skin disease, division in the stratum basale. As they 1997); these intercellular lipids are broken excoriations, erosions, fissuring or prema- move up through the stratum spinosum down by enzymes from keratinocytes to pro- turity, percutaneous absorption will be and stratum granulosum, they differen- duce a lipid mixture of ceramides (phospho- increased (Rudy and Parham-Vetter, 2003). tiate to form a rigid internal structure of lipids), fatty acids and cholesterol. These Melanocytes. Melanocytes are found in the Fig 2. Layers of the skin stratum basale and are scattered among the keratinocytes along the basement mem- brane at a ratio of one melanocyte to 10 Stratum basal cells. They produce the pigment mel- corneum anin, manufactured from tyrosine, which Stratum is an amino acid, packaged into cellular lucidum vesicles called melanosomes, and trans- ported and delivered into the cytoplasm of Stratum the keratinocytes (Graham-Brown and granulosum Epidermis Bourke, 2006). The main function of mel- Stratum anin is to absorb ultraviolet (UV) radiation spinosum to protect us from its harmful effects. Stratum Skin colour is determined not by the basale number of melanocytes, but by the Basement number and size of the melanosomes membrane (Gawkrodger, 2007). It is influenced by sev- Dermis eral pigments, including melanin, caro- tene and haemoglobin. Melanin is trans- ferred into the keratinocytes via a melanosome; the colour of the skin there- fore depends of the amount of melanin produced by melanocytes in the stratum FRANCESCA CORRA FRANCESCA basale and taken up by keratinocytes. Nursing Times [online] December 2019 / Vol 115 Issue 12 31 www.nursingtimes.net Copyright EMAP Publishing 2019 This article is not for distribution except for journal club use Clinical Practice Systems of life Melanin occurs in two primary forms: Fig 3. Desquamation process closely interlaced elastic fibres and thicker l Eumelanin – exists as black and brown; bundles of collagen (White and Butcher, l Pheomelanin – provides a red colour. 2005). It also contains fibroblasts, mast Skin colour is also influenced by expo- Desquamation cells, nerve endings, lymphatics and epi- sure to UV radiation, genetic factors and dermal appendages. Surrounding these hormonal influences (Biga et al, 2019). structures is a viscous gel that: Stratum 14 days l Allows nutrients, hormones and waste Langerhans cells. These are antigen (micro- corneum products to pass through the dermis; organisms and foreign proteins)- l Provides lubrication between the presenting cells found in the stratum spi- collagen and elastic fibre networks; nosum. They are part of the body’s l Gives bulk, allowing the dermis to act immune system and are constantly on the as a shock absorber (Hunter et al, 2003). lookout for antigens in their surround- ings so they can trap them and present Specialised dermal cells and structures. The them to T-helper lymphocytes, thereby fibroblast is the major cell type of the activating an immune response (Graham- 14 days dermis and its main function is to synthe- Brown and Bourke, 2006; White and sise collagen, elastin and the viscous gel Butcher, 2005). within the dermis. Collagen – which gives the skin its toughness and strength – Merkel cells. These cells are only present in makes up 70% of the dermis and is con- very small numbers in the stratum basale. tinually broken down and replaced; They are closely associated with terminal fila- elastin fibres give the skin its elasticity ments of cutaneous nerves and seem to have a (Gawkrodger, 2007). However both are role in sensation, especially in areas of the affected by increasing age and exposure to body such as palms, soles and genitalia l Cushioning the deeper structures from UV radiation, which results in sagging (Gawkrodger, 2007; White and Butcher, 2005).
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