Functions of the skin

Learning objectives
Introduction
Protective properties
Skin colour
Activity

Learning objectives

• Describe the principal functions of the skin and its components

Introduction

An intact skin is essential for life, illustrated by the serious nature of extensive thermal burns - the mortality risk of a 40% total body surface area burn in a 70-year-old patient is 94%.

Secretions
The surface of the scalp, face and upper trunk of adults is a hydrolipid film made up of sebum, water, salts and metabolic products. Sebum is produced by sebaceous glands within the hair follicular apparatus. The lipids protect against irritants, allergens and certain toxins and prevent water loss.

Eccrine sweat glands produce a salty solution. They are found over most of the body, but are often profuse over the scalp and forehead, axillae, palms and soles. This arises in response to exercise and high temperature (internal or environmental).

Apocrine glands are found in axillary, pubic and perianal regions resulting in a sticky secretion prone to malodour. The smell is due to colonization by bacteria.

Protective properties

Skin permits a stable internal environment, not just holding the body together but also protecting internal organs from environmental dangers.

Barrier function
Physical	Protects body from mechanical damage i.e. friction and impact. Mainly impermeable to water. Adherent flat cells of stratum corneum, supporting keratinocytes & collagen provide tensile strength. Sebum aids pliability. Elastin allows skin to recover shape after deformation. Subcutaneous tissue absorbs shock.
Thermal	Poor conductor of heat, thus a barrier to heat injury, tolerating <40 deg C. Higher temperatures may cause marked tissue destruction. Severe burns rare on palms and soles because of thick epithelial cover. Deep epidermal appendages (scalp) are also protective.
Antimicrobial	Protects from bacteria, fungi and viruses. Keratinocytes provide physical barrier & produce cytokines. Sebum & acid mantle repel pathogenic organisms. Commensal bacteria/yeasts protect against pathogens. Skin is a major immunological organ.
Chemical	Keratinocytes protect from chemicals. Excessive exposure causes irritant dermatitis and chemical burns.
Radiation	Superficial layers of epidermis protect dividing DNA in basal layer and below from excessive UV radiation Due to thickness of keratinocyte layer, melanin, urocanic acid & appendages. Sunburn rare on palms and soles because of thick epithelial cover. Sunburn rare in black skin because of melanin throughout epidermis. Sunburn greater after bathing because of loss of water-soluble urocanic acid. Hair provides a physical barrier.
Maintain fluid balance
Evaporation	Adherent keratinocytes & sebum prevent skin from drying out. Sweat moistens skin surface.
Permeation	Adherent keratinocytes & sebum prevent absorption of excess water.
Excretion	Waste products & salt disposed of in sweat.
Regulation of body temperature
Cooling	Evaporation of sweat. In hot weather cutaneous blood vessels dilate to give off heat.
Warming	Insulating subcutaneous tissue. In cold weather cutaneous blood vessels constrict to conserve heat. Hair protects scalp. Arrector pili contraction (goose bumps) produces heat.
Immunological function
Infection	Protect against microbial attack.
Allergy	Protect against allergic reaction.
Metabolic function
Synthesis	Vitamin D (subcutaneous tissue) Structural proteins, glycans, lipids, signalling molecules
Storage	Fuel in subcutaneous adipose cells Electrolytes, water, vitamins, carbohydrates, protein
Wound healing	Secrete fibronectin & other compounds required for restoration of skin integrity.
Communication
Sensory nerves	Warn of heat, cold, contact, mechanical injury, infestation. Temperature, pain, touch, vibration, pruritus.
Physical state	Aesthetic appearance. Odour (sexual attraction): apocrine gland secretion. Deodorant (low level of bacterial decomposition).
Emotional state	Facial expressions. Skin colour (erythema, pallor). Sweating, goose bumps.

Skin colour

Skin colour varies from white (absence of pigmentation) to black (densely melanized). The colour depends on the quantity and depth of melanin and other chromophores.

Oxidised haemoglobin	Red
Deoxidised haemoglobin	Blue
Haemosiderin	Red-brown
Carotene	Orange-yellow
Bilirubin bound to elastin	Green-yellow
Dermal exogenous pigment	Tattoos, makeup, drugs, metallic compounds

The thickness and quality of keratinocytes and dermal components may also affect skin colour. A thick layer of horny cells can appear white (psoriasis), yellow (seborrhoeic dermatitis), dirty-brown (ichthyosis) or black (eschar). Inflammatory infiltrates may result in plaques that are yellow-brown (granulomas), violaceous (lichen planus) or scarlet (psoriasis).

The pigment melanin is formed by the action of tyrosinase on dopamine metabolites in melanosomes (specialised pigment granules) in the cytoplasm of melanocytes. Skin produces predominantly brown to black eumelanin and a little phaeomelanin; the hair of red-headed celts contains mainly phaeomelanin.

The amount of melanin depends on:

• Genetic factors: ethnicity/phototype.
• Active melanogenesis.

There are melanocytes in the normal skin of all races. However, dark-skinned people produce more melanin, and it is distributed to keratinocytes throughout the epidermis. The more superficial the pigment, the more effective the protection it provides against damage caused by ultraviolet radiation. Albinos have inactive melanocytes.

Melanin is found in the dermis in some melanocytic naevi (moles) and as a result of inflammation affecting the level of the basement membrane (postinflammatory pigmentation).

Skin colour

Cutaneous colours

Variation in melanisation

Melanin pigmentation is promoted by:

• Ultraviolet radiation
• Hormones (MSH, ACTH, androgens, oestrogens, progesterones)
• Inflammation
• Friction

It may be reduced by:

• Melatonin
• Corticosteroids

Activity

Compare phaeomelanin and eumelanin. What is the explanation for variations in skin colour between different individuals and different races?

 

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