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Home , Apocrine sweat gland, Arrector pili muscle, Cortex (hair), Cuticle (hair), Eccrine sweat gland, Epidermis

Copyright EMAP Publishing 2020 This article is not for distribution except for journal club use

Clinical Practice Keywords //Nails/Sweat /Sebaceous glands Systems of This article has been Skin double-blind peer reviewed In this article... ● The four main accessory structures of the skin ● Structure and function of hair and nails ● The role of sweat and sebaceous glands

Skin 2: accessory structures of the skin and their functions

Key points Author Sandra Lawton is Queen’s Nurse, nurse consultant and clinical lead Accessory structures dermatology, The Rotherham NHS Foundation Trust. of the skin include the hair, nails, Abstract Understanding the skin requires knowledge of its accessory structures. sweat and These originate embryologically from the and include hair, nails, sweat sebaceous glands glands and sebaceous glands. All are important in the skin’s key functions, including protection, and its sensory roles. This article, the second in a Hair’s primary two-part series, looks at the structure and function of the main accessory structures functions are of the skin. protection, warmth and sensory Citation Lawton S (2020) Skin 2: accessory structures of the skin and their functions. reception Nursing Times [online]; 116; 1, 44-46.

Nails protect the tips of the ccessory structures of the skin l Distribution of sweat- products; and toes include the hair, nails, sweat l Psychosocial – hair plays an glands and sebaceous glands. important role in determining self The two main types AThese structures embryologi- image and social perceptions of – cally originate from the epidermis and are (Bit.ly/RUAccessoryStructures; eccrine and often termed “appendages”; they can extend Kolarsick et al, 2011; Graham-Brown and – are responsible down through the into the hypo- Bourke, 2006) . for regulating body dermis (Bit.ly/RUAccessoryStructures). The hair mainly comprises dead kerati- temperature nised cells. Strands of hair originate from Hair the , which is an epidermal Sebaceous glands The hair is a structure growing out penetration of the dermis. Hair follicles lubricate and of the epidermis. It is found on all areas of are set at an angle into the dermis, with the waterproof the skin the body except the palms of the , bulb (germinal matrix) sitting deep down, and hair, and keep soles of the feet and the , and has sev- just above or in the hypodermis (Graham- them pliable eral functions: Brown and Bourke, 2006). The structure of l Protection – hair on the head protects the dermal papilla (bulb), germinal matrix the skull from the sun, while that in the and root. nose and ears and around the eyes The visible hair shaft is the only part () traps and excludes dust that is not anchored to the hair follicle and particles, which may contain allergens the shape plays a role in determining hair and microbes. prevent sweat texture. The rest of the hair (hair root) is and other particles from dripping into anchored in the follicle and lies below the the eyes; surface of the skin (Fig 1). The hair root l Sensory reception – the hair is far more ends deep in the dermis at the hair bulb, sensitive than the skin surface to air and includes a layer of mitotically active movement or other disturbances in the basal cells called the . environment, for example, head-lice Cells of the hair matrix divide and dif- infestation; ferentiate to form the three layers of the l Thermoregulation; hair:

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Fig 1. of hair Each cycle of hair growth is predeter- mined and goes through three phases: l Anagen or growth phase – cells divide rapidly at the hair root, pushing the Hair shaft hair shaft up and out. Hair typically grows at the rate of 0.3mm per day in this phase, which usually lasts from two to seven years; l Catagen or resting phase – lasting only Epidermis Hair two or three weeks, this marks a transition from the hair follicle’s active growth; l Telogen or shedding phase – strands of hair are released, no new growth occurs and the follicle is at rest. This lasts for around two to four months before Sebaceous glands another anagen phase begins. On Dermis average Outer root l Fifty are lost and replaced per day Hair sheath (Gawkrodger and Ardern-Jones, 2016; follicle Inner root Bit.ly/RUAccessoryStructures; sheath Graham-Brown and Bourke, 2006). Hair colour Dermal papilla Hair is similar to the skin in that it gets its colour from the pigment , which is produced by melanocytes in the hair papilla. Hair colour is genetically deter- mined but, as we age, melanin production decreases and the hair loses its colour, “Hair colour is genetically before, or at, birth (Gawkrodger and becoming grey and/or white. determined but, as we age, Ardern-Jones, 2016; Graham-Brown and Bourke, 2006). Sebaceous glands melanin production The pilosebaceous unit comprises the hair decreases and the hair Hair growth follicle, hair shaft, arrector pili and seba- loses its colour” Hair growth is determined by genetic and ceous glands. The sebaceous glands are hormonal factors, and begins with the pro- found along the hair follicles and secrete a l Medulla – the central core of the hair; duction of by the basal cells liquid substance called sebum (a mix of l Cortex – this surrounds the medulla; in the matrix of the hair bulb. Keratinisa- ) in response to hormonal stimula- l Cuticle – an outer layer of very hard, tion is complete when these cells are tion. The glands vary in size and number, keratinised cells. pushed through the follicle to the skin sur- but are mostly on the , face, upper Hair texture is determined by the shape face, forming the visible shaft of hair; this torso and anogenital areas. They are rela- and structure of the cortex and by the shaft is dead and composed entirely of ker- tively inactive during childhood, but shape of the hair follicle: atin (Bit.ly/RUAccessoryStructures). become very active during . l Round hair follicle – hair will be straight and coarse; Fig 2. Eccrine and apocrine sweat glands l Oval hair follicle – hair will be wavy; l Flat hair follicle – hair will be curly. The hair bulb surrounds the hair papilla, Sweat pore which is made of connective and con- tains and endings from the dermis (Bit.ly/RUAccessoryStruc- tures). Also attached to each follicle is a small bundle of , the arrector pili, which contracts with cold, and emotion Hair follicle to erect the hairs and produce goosebumps (Gawkrodger and Ardern-Jones, 2016). There are three types of hair in : l Terminal – scalp, eyebrows/lashes, , axillae, pubic areas; l Vellus – finer downy hair covering the body;

FRANCESCA CORRA FRANCESCA l  – present in utero and shed

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Fig 3. Structure of the divide, mature, keratinise and move forward to the nail plate; l Nail plate – this hard and translucent Edge of nail portion is composed of keratin, and varies in thickness from 0.3mm to Nail matrix Nail plate Root of nail 0.5mm ; l Cuticle () – the cuticle is Cuticle the fold of skin at the proximal end of the nail; l Paronychium – the lateral fold of skin bone on the sides of the nail; Dermis l Nail bed – this is adherent connective Epidermis tissue underlying the nail; it is rich in blood vessels, making it appear pink, except at the base, where a thick layer of over the nail matrix Sebum helps lubricate and waterproof “Apocrine sweat includes forms a crescent-shaped region called the skin and hair, keeping them pliable; the lunula (“little moon”); the fatty acids of sebum also have antibac- organic compounds that l Hyponychium – the area beneath terial properties and prevent water loss make the sweat thicker the free edge of the nail, farthest from the skin in low- environ- from the cuticle; it consists of a ments (Bit.ly/RUAccessoryStructures). than eccrine sweat and thickened layer of stratum corneum subject to bacterial (Bit.ly/RUAccessoryStructures; Sweat glands decomposition and Gawkrodger and Ardern-Jones, 2016). Also known as sudoriferous glands (from the Latin sudor, meaning sweat), these pro- subsequent smell” Nail growth duce sweat to cool the body when it The rate of nail growth varies from person becomes warm. Sweat glands develop from glands, lie deeper in the dermis and are to person: fingernails average between epidermal projections into the dermis and sometimes found in the hypodermis. Less 0.5mm and 1.2mm per week, but toenails are classified as eccrine glands, as the numerous than eccrine glands, they are grow more slowly (Weller et al, 2013). The are excreted by found in the axillae as well as in more- growth of nails is a complex process and, through a without affecting the cells localised sites – namely the , peri- in humans, is genetically controlled to of the gland. There are two types of sweat neum and scalp. ensure the nail is flat; this is unlike the pro- glands – eccrine and apocrine – each of Apocrine sweat, as well as containing cess in other animals and primates that which secretes slightly different products water and salts, includes organic com- have claws. Nail growth can also be nega- (Bit.ly/RUAccessoryStructures). pounds that make it thicker than eccrine tively affected by trauma and inflamma- sweat and subject to bacterial decomposi- tion (Graham-Brown and Bourke, 2006). Eccrine sweat glands tion and subsequent smell. The release of Eccrine sweat glands are part of the body’s this sweat is under nervous and hormonal Conclusion thermoregulatory system and help to control. Apocrine sweat plays a role in the The accessory structures of the skin, maintain . There are more poorly understood including the hair, nails, sweat glands and than two million of these and, although response and regulating body temperature sebaceous glands, play an important role they are found over the whole body, they (Bit.ly/RUAccessoryStructures; Graham- in the skin’s key functions. NT are more numerous in the forehead, Brown and Bourke, 2006). axillae, palms of the hands and soles of the References Gawkrodger DJ, Ardern-Jones M (2016) feet. These coiled structures are usually at Nails Dermatology: An Illustrated Colour Text. London: the junction between the dermis and the Nails consist of hardened and densely Elsevier. subcutaneous layer, with a duct leading packed keratin, and protect the extremi- Graham-Brown R, Bourke J (2006) Mosby’s Colour Atlas and Text of Dermatology. London: through the dermis and epidermis to a ties of our fingers and toes from mechan- Mosby Elsevier. pore on the skin surface, where the sweat is ical damage. Offering protection for the Kolarsick PAJ et al (2011) Anatomy and physiology released (Graham-Brown and Bourke, fingertips, they facilitate grasping and tac- of the skin. Journal of the Dermatology Nurses’ 2006; Fig 2). tile sensitivity in the finger. The fingertip Association; 3: 4, 203-213. Weller RB et al (2013) Clinical Dermatology. The sweat, released by exocytosis, is has many nerve endings and receives Oxford: Blackwell Science. hypotonic and mostly water, with some information about objects we touch; the salt, antibodies, traces of metabolic waste nail acts as a counterforce, providing even

and , an antimicrobial peptide. more sensory input. CLINICAL The components making up the struc- SERIES Skin series Apocrine sweat glands ture of the nail are shown in Fig 3 and Part 1: Structure and functions of Apocrine sweat glands become active described below: the skin Bit.ly/NTTheSkin1 Dec 2019 l  during puberty, and are also coiled struc- Nail matrix – the growing part of the Part 2: Accessory structures of the tures; their ducts open into hair follicles nail, still under the skin at the nail’s skin and their functions Jan 2020

FRANCESCA CORRA FRANCESCA (Fig 2). They are larger than eccrine sweat proximal end, containing cells that

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