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Home , Skin, Stratum basale, Stratum lucidum

Copyright EMAP Publishing 2019 This article is not for distribution except for journal club use

Clinical Practice Keywords /Skin function/Skin assessment// Systems of life This article has been Skin double-blind peer reviewed In this article... ● How the skin is structured ● Functions of the skin ● 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 in the body Abstract Skin diseases affect 20-33% of the population at any one time, and around 54% of the UK population will experience a 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 . 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 regulation of body while caring for patients and it is impor- in progressive stages of differentiation , and tant they understand it so they can recog- (Amirlak and Shahabi, 2017). Keratinocytes sensation nise problems when they arise. produce the and are the The skin and its appendages (nails, major building blocks (cells) of the epi- How others respond and certain ) form the largest organ dermis. As the epidermis is avascular (con- to people who have in the , with a surface area of tains no 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 delivery and waste disposal consideration thickness ranges from <0.1mm at its thin- through the basement membrane. for nurses nest part () to 1.5mm at its thickest The prime function of the epidermis is part (palms of the 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 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 (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);

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Fig 1. Cross-section through the skin are arranged in a highly organised fashion, fusing with each other and the cor- neocytes to form the skin’s barrier against water loss and penetration by aller- Sweat Hair gens and irritants (Holden et al, 2002). Sweat The can be visualised Oil as a brick wall, with the Stratum corneum forming the bricks and lamellar (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 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 applying topical medications to the skin. These are absorbed through the epidermal Fat layer Blood vessels barrier into the underlying tissues and structures (percutaneous ) and 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 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 (); 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 (Cork, impaired as a result of skin disease, division in the . As they 1997); these intercellular lipids are broken excoriations, erosions, fissuring or prema- move up through the down by enzymes from keratinocytes to pro- turity, percutaneous absorption will be and , 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 to 10 Stratum basal cells. They produce the 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 , including , 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.

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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 (White and Butcher, l Pheomelanin – provides a red colour. 2005). It also contains , 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 , hormones and waste Langerhans cells. These are antigen (micro- corneum products to pass through the dermis; 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 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 is the major 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, 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 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). mechanical ; and stretching of the skin as the person l Providing nourishment to the gets older and/or is exposed to greater Basement membrane zone epidermis; amounts of UV radiation (White and (dermo-epidermal junction) l Playing an important role in Butcher, 2005). This is a narrow, undulating, multi-layered healing. Mast cells contain granules of vasoactive structure lying between the epidermis and The network of interlacing connective chemicals (the main one being histamine). dermis, which supplies cohesion between , which is its major component, is They are involved in moderating immune the two layers (Amirlak and Shahabi, 2017; made up of collagen, in the main, with and inflammatory responses in the skin Graham-Brown and Bourke, 2006). It is some elastin. Scattered within the dermis (Graham-Brown and Bourke, 2006). composed of two layers: are several specialised cells (mast cells and Blood vessels in the dermis form a com- l Lamina lucida; fibroblasts) and structures (blood vessels, plex network and play an important part in l Lamina densa. lymphatics, sweat glands and nerves). thermoregulation. These vessels can be The is the thinner layer The epidermal appendages also lie divided into two distinct networks: and lies directly beneath the stratum within the dermis or subcutaneous layers, l Superficial plexus – made up of basale. The thicker is in but connect with the surface of the skin interconnecting arterioles and venules direct contact with the underlying dermis. (Graham-Brown and Bourke, 2006). lying close to the epidermal border, and It undulates between the dermis and epi- wrapping around the structures of the dermis and is connected via rete ridges dermis, the superficial plexus supplies called dermal papillas, which contain cap- “About 1 million nerve and nutrients to the cells; illary loops supplying the epidermis with fibres serve the skin – l Deep plexus – found deeper at the nutrients and oxygen. sensory perception border with the subcutaneous layer, its This highly irregular junction greatly vessels are more substantial than those increases the surface area over which the serves a critically in the superficial plexus and connect exchange of oxygen, nutrients and waste important protective and vertically to the superficial plexus products occurs between the dermis and social/sexual function” (White and Butcher, 2005). the epidermis (Amirlak and Shahabi, 2017). The lymphatic drainage of the skin is important, the main function being to Dermis Layers of dermis. The dermis is made up of conserve plasma proteins and scavenge The dermis forms the inner layer of the two layers: foreign material, antigenic substances and skin and is much thicker than the epi- l The more superficial papillary dermis; (Amirlak and Shahabi, 2017). dermis (1-5mm) (White and Butcher, 2005). l The deeper reticular dermis. About 1 million nerve fibres serve the Situated between the basement membrane The papillary dermis is the thinner skin – sensory perception serves a criti- zone and the subcutaneous layer, the pri- layer, consisting of loose cally important protective and social/ mary role of the dermis is to sustain and containing capillaries, elastic fibres and sexual function. Free sensory nerve end- support the epidermis. The main func- some collagen. The reticular dermis con- ings are found in the dermis as well as the tions of the dermis are: sists of a thicker layer of dense connective epidermis (Merkel cells) and detect pain,

FRANCESCA CORRA FRANCESCA l Protection; tissue containing larger blood vessels, and temperature. There are also

Nursing Times [online] December 2019 / Vol 115 Issue 12 32 www.nursingtimes.net Copyright EMAP Publishing 2019 This article is not for distribution except for journal club use Clinical Practice For more articles on dermatology, go to Systems of life nursingtimes.net/dermatology specialised receptors – Pacinian corpus- This is achieved by alterations to the blood Social and sexual function cles – that detect and vibration; flow through the cutaneous vascular . How an individual is perceived by others is and Meissner’s corpuscles, which are During warm periods, the vessels dilate, important. People make judgements based touch-sensitive. the skin reddens and beads of sweat form on what they see and may form their first The autonomic nerves supply the blood on the surface (vasodilatation = more blood impression of someone based on how that vessels and sweat glands and arrector pili flow = greater direct heat loss). In cold person looks. Throughout history, people muscles (attached to the hair) (Gawk- periods, the blood vessels constrict, pre- have been judged because of their skin, for rodger, 2007). venting heat from escaping (vasoconstric- example, due to its colour or the presence tion = less blood flow = reduced heat loss). of a skin condition or scarring. Skin condi- 15%The skin of The secretion and evaporation of sweat tions are visible – in this skin-, beauty- and QUICK an adult human from the surface of the skin also helps to image-conscious society, the way patients FACT as a percentage cool the body. are accepted by other people is an impor- of total body weight tant consideration for nurses. Sensation Hypodermis Skin is the ‘-of-touch’ organ that trig- Summary The hypodermis is the subcutaneous gers a response if we touch or feel some- This article gives an overview of the struc- layer lying below the dermis; it consists thing, including things that may cause ture and functions of the skin. Part 2 will largely of fat. It provides the main struc- pain. This is important for patients with a provide an overview of the accessory struc- tural support for the skin, as well as insu- skin condition, as pain and itching can be tures of the skin and their functions. NT lating the body from cold and aiding extreme for many and cause great distress. shock absorption. It is interlaced with Also touch is important for many patients References All Parliamentary Group on Skin (1997) An blood vessels and nerves. who feel isolated by their skin as a result of Investigation into the Adequacy of Service colour, disease or the perceptions of others Provision and Treatments for Patients with Skin Functions of the skin as many experience the fact that they are Diseases in the UK. Bit.ly/APGSkin Amirlak B, Shahabi L (2017) Skin . The skin has three main functions: seen as dirty or contagious and should not Bit.ly/MedscapeSkinAnatomy l Protection; be touched. Biga LM et al (2019) Anatomy and . l Thermoregulation; The 5.1: layers of the l Sensation. skin. Bit.ly/OregonSkinLayers “In this skin-, beauty- and Cork MJ (1997) The importance of skin barrier Within this, it performs several impor- function. Journal of Dermatological Treatment; tant and vital physiological functions, as image-conscious society, 8: Suppl 1, S7-S13. outlined below (Graham-Brown and how patients with skin Gawkrodger DJ (2007) Dermatology: An Bourke, 2006). Illustrated Colour Text. Edinburgh: Churchill conditions are accepted Livingstone. Graham-Brown R, Bourke J (2006) Mosby’s Protection by other people is an Color Atlas and Text of Dermatology. London: The skin acts as a protective barrier from: Mosby. important consideration Holden C et al (2002) Advised best practice l Mechanical, thermal and other physical for nurses” for the use of emollients in eczema and other injury; dry skin conditions. Journal of Dermatological l Harmful agents; Immunological surveillance Treatment; 13: 3, 103-106. Hughes E (2001) Skin: its structure, function l  Excessive loss of moisture and protein; The skin is an important immunological and related pathology. In: Hughes E, Van l Harmful effects of UV radiation. organ, made up of key structures and cells. Onselen J (eds) Dermatology Nursing: A Depending on the immunological Practical Guide. Edinburgh: Churchill response, a variety of cells and chemical Livingstone. Thermoregulation Hunter J et al (2003) Clinical Dermatology. One of the skin’s important functions is to messengers () are involved. These Oxford: Blackwell Science. protect the body from cold or heat, and specialised cells and their functions will be Kolarsick PAJ et al (2011) Anatomy and maintain a constant core temperature. covered later. physiology of the skin. Journal of Dermatology Nurses’ Association; 3: 4, 203-213. Rudy SJ, Parham-Vetter PC (2003) Biochemical functions Percutaneous absorption of topically applied Nursing Times The skin is involved in several biochemical medication. Dermatology Nursing; 15: 2, Self-assessment 145-152. processes. In the presence of sunlight, a Schofield J et al (2009) Skin Conditions in online form of D called cholecalciferol is the UK: A Health Care Needs Assessment. synthesised from a derivative of the steroid Bit.ly/SkinConditionsUK Test your knowledge with Nursing cholesterol in the skin. The liver converts White R, Butcher M (2005) The structure and functions of the skin. In: White R (ed) Skin Care Times Self-assessment after reading cholecalciferol to calcidiol, which is then in Wound Management: Assessment, Prevention this article. If you score 80% or more, converted to calcitriol (the active chemical and Treatment. Aberdeen: UK. you will receive a personalised form of the vitamin) in the kidneys.

certificate that you can download and is essential for the normal CLINICAL store in your NT Portfolio as CPD or absorption of calcium and phosphorous, SERIES Skin series revalidation evidence. which are required for healthy (Biga Part 1: Structure and functions of et al, 2019). The skin also contains recep- the skin Dec 2019 To take the test, go to: tors for other steroid hormones (oestro- Part 2: Accessory structures of the nursingtimes.net/NTSASkin gens, progestogens and ) skin and their functions Jan 2020 and for vitamin A.

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