To revise the anatomy and physiology of normal skin to better understand disease processes
Important characteristics of the structure of the skin include:
The skin of an adult occupies an area of 1.5 – 2 m2 and weighs about 20kg. It varies in thickness from 0.5 to several centimetres in thickness. The thinnest sites are the eyelids and scrotum. The thickest are the soles and palms.
Skin is made up of:
These layers are modified according to the needs of the specific area of the body. For example, the scalp is covered with thick hair, the palms have particularly thick epidermis and the face contains large numbers of sebaceous glands.
The epidermis is a dynamic structure acting as a semi-permeable barrier with a layer of flat “dead” (anuclear) cells at the surface (stratum corneum). The epidermis regenerates in orderly fashion by cell division of keratinocytes in the basal layer, with maturing daughter cells becoming increasingly keratinised as they move to the skin surface. Keratinocytes adhere because of desmosome complexes attached to the cell membranes.
Langerhans cells are monocytes within the epidermis that recognise and process small molecules penetrating the skin surface. These antigen-presenting cells are identified in tissue sections by characteristic surface markers.
Pigment cells in the basal layer (melanocytes) protect the skin from ultraviolet radiation.
The basement membrane zone is the communication channel between epidermis and dermis. Adherence is via hemidesmosomes attached to the cell membrane opposite similar complex on adjacent cell. Desmosomes make and break as keratinocytes move from basal layer to surface.
The dermis supports the epidermis by providing it with nutrients and toughness. It is mainly made up of collagen, with elastic fibres, ground substance, nerves, blood vessels and cellular infiltrations.
The papillary dermis is the upper portion beneath the epidermis, characterised by thin haphazardly arranged collagen fibres, thin elastic fibres and ground substance. The lower portion is the reticular dermis, composed of coarse elastic fibres and thick collagen bundles parallel to the skin surface.
Subcutaneous tissue is made up of lobules of adipose cells, which contain fat. These are surrounded by connective tissue, larger blood vessels and nerves.
Eccrine sweat glands produce a hypotonic solution of water, sodium chloride, urea, ammonia and uric acid. They are most dense on palms, soles, axillae and forehead and are absent on the lips, labia minora, glans penis and prepuce. The glands are dermal structures under hypothalamic control. Sweat is produced in response to exercise, high temperature (internal or environmental), spicy food, emotional stress and sometimes spontaneously. It reaches the skin surface via a coiled duct.
Pilosebaceous structures produce terminal hair on the scalp and vellus hair on the body surface (short, thin, light coloured). They are absent on the palms and soles. Smooth muscles (arrector pili) cause erection of the hairs on exposure to cold or fear (goose bumps).
The hair cycle includes a growth phase (anagen) that lasts several years; a short involutional phase (catagen); and a resting phase that lasts for several months (telogen). Anagen hairs have a pointed tip and telogen hairs have a clubbed or bulbous tip.
Hair colour depends on the amount of melanin within the cortex during anagen: dark hair has more eumelanin melanosomes; fair hair fewer with more lamellated phaeomelanin; red hair has erythromelanin; grey/white hair very few melanocytes.
Sebaceous glands produce sebum. They are most concentrated on scalp and face where circulating androgens induce increased secretion at puberty. Sebum is secreted into the hair follicle except it in the labia, prepuce, nipple and areola, where it is secreted directly onto the skin surface.
Apocrine glands are found in axillae and perianal regions and become active after puberty. They produce a thick secretion that rapidly becomes colonised by bacteria resulting in characteristic body odour. The ducts open into the pilosebaceous follicle
The nail plate is a specialised form of stratum corneum growing from keratinocytes in the matrix.
Skin has an average pH value of 5.5, creating the acid mantle. This is the result of acidic substances such as amino acids, lactic acid and fatty acids in perspiration, sebum and the hormones. There are resident protective microflora (bacteria and yeasts) but the acid mantle repels pathogenic microorganisms and reduces body odour.
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.
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 (granulomata), violaceous (lichen planus) or scarlet (psoriasis).
The pigment melanin is formed by the action of tyrosinase on dopamine metabolites in melanosomes. Melanosomes are specialised pigment granules in the cytoplasm of melanocytes. Most skins produce eumelanin predominantly, but the melanin in red-headed celts is phaeomelanin.
The amount of melanin depends on:
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).
Melanin pigmentation is promoted by:
It may be reduced by
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%. Skin permits a stable internal environment, not just holding the body together but also protecting internal organs from environmental dangers. Its protective properties are described below.
Skin is a physical barrier to mechanical and chemical injury and is mainly impermeable to water. It is strong, flexible and shock-resistant. It is a poor conductor of heat, thus a barrier to heat injury, tolerating temperatures of 40°C though higher temperatures may cause marked tissue destruction. Severe burns are rare on palms and soles because of their particularly thick epithelial cover. Superficial layers of epidermis protect underlying tissues from excessive UV radiation mainly due to the thickness of the keratinocyte layer, melanin and appendages. Sunburn is rare on palms and soles because of thick epithelial cover and rare in black skin because of melanin throughout epidermis.The physical barrier of the keratinocyte “brick wall”, the acid mantle and tolerance of commensal bacteria and yeasts repel pathogens.
The immunological function of the skin is to protect against infection and allergy. Besides mechanical factors to protect the body from invasion by bacteria, fungi and viruses there is a complex innate and acquired humoral and cellular immunological system.
Skin also has metabolic functions as it synthesises vitamin D, proteins, lipids and other structures; stores electrolytes, water, vitamins, carbohydrates and protein; and secretes fibronectin and the other compounds required to heal a wound.
Adherent keratinocytes and sebum prevent skin from drying out or conversely from absorbing excessive water. Sweat moistens skin surface and permits excretion of waste products and salt. In hot weather the body is cooled by evaporation of sweat and cutaneous blood vessels dilate to give off heat. In cold weather cutaneous blood vessels constrict to conserve heat and subcutaneous tissue is insulating as well as a source of fuel. Contraction of arrector pili (goose bumps) produces heat.
Skin also has a communication role. Sensory nerves warn of heat, cold, contact, mechanical injury and infestation by relaying information about temperature, pain, touch, vibration and pruritus (itch). One’s emotional state can be recognised by appearance, colour, sweating,, goose bumps, odour and facial expressions.