Download 060808Anatomy and function of the skin part 2

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