Skin Structure And Function
How The Skin Works: What It Does And What Can Go Wrong With It

Skin Structure And Function

Contents

• What Is The Skin?
• Structure Of The Skin
• What Is The Function Of The Skin?
• Disorders Of The Skin

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The skin is an outer protective layer of the body, also known as an integument. It is the largest organ of the body and is water-resistant. It has many functions including protecting us from weather and infections and shaping the body.

Facts

• Skin is composed of two layers, the epidermis and the dermis.
• It has 8 functions: secretion, heat regulation, absorption, protection, elimination, sensation, Vitamin D and melanin production.
• The skin is affected by 7 different types of disease: congenital, bacterial, viral, fungal, pigmentation disorders, skin cancers and other general conditions.
• The skin is the largest organ in the human body. If you stretched it out flat it would measure between 11 and 18 square meters and account for 12 percent of your total body weight.
• Skin is 6mm thick on the soles of your feet and only 0.5mm thick on your eyelids.
• Although the skin may feel smooth, its surface is made of tiny flakes of dead cells that are far too small to see unless magnified.

What Does It Mean When Skin Sheds?

If you look around any room you will see dust. Dust, amongst other things, contains millions of shed, or desquamated skin cells that have fallen off the body. Whereas dead blood cells are destroyed inside the body, dead skin cells are destroyed outside the body - that is on the outer layer of the skin. Once they are dead, they exfoliate - peel off. This is known as desquamation. Try rubbing your skin when it is very dry and you will see small particles coming away from the surface. These are dead skin cells.

Did You Know?

The color of your hair is affected by the amount of melanin (the pigment made by the skin that gives your skin and hair color) in your body. For example, gray hair is caused by a decrease in melanin production. Instead of the pigment providing color, there are air bubbles in the hairs and to the naked eye the hair now looks gray.

Structure Of The Skin

What Is The Epidermis?

The epidermis is the layer of skin that we can see. It varies in thickness, depending on the part of the body - it is thickest on the soles of the feet and palms of the hand and thinnest on eyelids and nipples. The cells on the surface are constantly falling off (shedding): this is called desquamation. Every minute of the day we lose about 30,000 to 40,000 skin cells from the epidermis. These cells being constantly replaced from below as cells in the basal layer of the epidermis multiply and are pushed up to the surface. It takes about a month for new cells to form and push their way up to the surface, ready for shedding. The further they rise, the thinner and flatter they become, gradually dying in the process. Cell turnover tends to take a little longer in areas that receive the most friction - that is the palms of the hands and the soles of the feet. There is no blood supply to the epidermis, hardly any nerve supply and it receives nutrients and fluids from the lymphatic vessels in the dermis. In total there are 5 layers in the epidermis. These are:

Structure of Epidermis

1. Stratum Corneum: Surface, outside of the skin.
• Hardened, flattened dead cells that overlap and create a tough, waterproof protection.
• Cells are constantly being shed - desquamation.
• Cell membrane is not visible.

2. Stratum Lucidum - Clear Layer
• Denucleated cells but not completely hard.
• Most easily visible under a microscope (only on palms and soles).
• Cell membranes becoming less visible.

3. Stratum Granulosum - Granular Layer
• Cells have a distinct nucleus but cell membranes are dying.
• Contain granules which are visible in healing tissue after trauma.

4. Stratum Spinosum - Prickle Cell Layer
• Cells are living and membranes are intact; they have fibrils which interlock.
• Capable of mitosis (cell division/reproduction) under friction or pressure i.e. on soles of feet or palms of hands.

5. Stratum Germinativum - Basal Layer
• The primary site of cell division/ reproduction (mitosis) in the skin.
• Cells are living. It is in this layer that cells are made. They take about 28-30 days to move up from here through the five layers of the epidermis before being shed.
• This layer also contains a pigment known as melanin that gives skin its natural color, whether red, yellow or black. Melanin is produced by cells called melanocytes. 95 percent of cells in the basal layer divide and reproduce, the other 5 percent produce melanin.

What Is The Dermis?

The dermis is commonly known as the true skin. Unlike the epidermis, this layer is connected to the blood and lymph supply as well as the nerves. Running through the dermis are sweat and sebaceous glands, hair follicles and muscles cells.

Structure of Dermis

The dermis is made of connective tissue, mainly areolar tissue (connective tissue that holds organs in place) which is tough and elastic. It contains white collagen fibers and yellow elastic tissue known as elastin. Collagen plumps the skin and elastin keeps it supple and elastic. Both diminish with age. It Contains:

1. Specialized Cells
• Fibroblasts: responsible for the production of areolar tissue, collagen and elastin. Fibroblasts can be damaged by ultraviolet light (sunlight. This is why excess sun reduces the skin’s elasticity and plumpness).
• Mast cells: produce histamine as an allergic response and heparin, a natural anticoagulant (blood thinner).
• Histiocytes: also produce histamine.
• Leucocytes: white blood cells which help to fight infection and disease. See, what is blood?

2. Nerve Endings
The dermis contains nerve endings that alert the brain and thus the body to heat, cold, pressure and pain. These are part of the defense system of the body.

3. Sweat Glands
The sweat glands stretch from deep within the dermis to the outer layer of the epidermis; there are two kinds:
• Eccrine: excrete watery sweat and control body temperature. Found all over the body especially under the arms, the forehead, palms of the hands and the soles of the feet.
• Apocrine: excrete milky fluid. Body odor is produced when the sweat from these glands mixes with bacteria. The milky fluid is odorless until it mixes with bacteria on the surface of the skin. Found in larger quantities in the groin and axillae (armpits). Both excess heat and emotional stress can trigger the apocrine glands to secrete more perspiration. To control sweating and odor most people use deodorants. Antiperspirants reduce sweating while deodorants mask the odor. Most products combine both an antiperspirant and deodorant.

4. Hair Follicles
Travel through the epidermis and the dermis. Tiny muscles, called erector pili, are attached to each hair and help with temperature control of the body by pulling the hair upright and trapping a layer of air (causing goose pimples).

5. Sebaceous Glands
The sebaceous glands are connected to the hair follicles. They produce sebum, a fatty acid which keeps the skin moist and which lubricates the hair shaft and they are therefore found in hairy areas, not on the palms of the hands or soles of the feet. Sweat and sebum combine on the surface of the skin to form the acid mantle, a protective shield which helps to control bacteria levels and prevents infections and disease and also acts as a natural moisturizer. The pH balance of the skin is 4.5-5.6 and this acid environment helps to prevent bacterial growth. Excess sebum production can lead to oily skin and acne.
Related:
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6. Blood Supply
A system of blood vessels including microscopic capillaries which are one cell thick.

7. Lymphatic Capillary
Part of the lymphatic system. Works in conjunction with the blood supply (circulatory system).

8. Papilla
Small conical projections at the base of the hair containing blood vessels and nerves which supply the hair with nutrients.

Below The Dermis: Layer Of Fat

Below the dermis is a layer of fat that helps to keep the body insulated against temperature extremes and also gives shape to the body. Muscle fibers, nerves and blood vessels also run through this subcutaneous fat, as do the roots of the oil and sweat glands.

The skin has eight main functions:

• Secretion: secretes sebum.
• Heat regulation: cools and warms body.
• Absorption: of drugs or essential oils.
• Protection: keeps out bacteria and creates a barrier against rays of the sun.
• Elimination: of waste products.
• Sensation: skin is the organ of touch.
• Vitamin D production.
• Melanin production.

1. Secretion

The skin secretes sebum from the sebaceous glands. This fatty substance lubricates the hair shafts and when combined with perspiration on the surface of the skin, it creates a natural moisturizer which acts as a protective barrier against bacteria. If secretion levels are high, it leads to oily skin. If it low, it leads to dry skin.
Related Questions
What is dry skin?
What is the best treatment for dry skin?

2. Heat Regulation

Body temperature is maintained in healthy humans at 36.8°C. Organs involved in heat production are the muscles, liver and digestive organs. Heat is absorbed and maintained in the subcutaneous layer of adipose tissue. Heat regulation is controlled in the following ways:

Cooling: Vasodilation is when the smooth muscle in the blood vessel walls relaxes so that the vessel becomes wider. When the body becomes hot, the capillaries nearer the surface of the body vasodilate allowing more blood to reach the surface of the skin. The pores dilate allowing the heat to be lost from the body. This causes the skin to flush - known as hyperemia. Sweating will occur simultaneously and the evaporation of perspiration from the surface has a cooling effect on the body.

Warming: Vasoconstriction is when the smooth muscle in the blood vessel walls contracts so that the vessel becomes narrower. When the body becomes cold, the capillaries nearer the surface of the body vasoconstrict so that blood is moved away from the extremities to the major organs, thus ensuring they are kept warm. As a result the skin appears pale and heat loss is inhibited. The erector pili muscles contract causing body hair to stand on end, trapping air against the surface of the skin, which is then warmed by body heat. Shivering occurs, caused by rapid and repeated muscle contractions which work to raise body temperature.

3. Absorption

The skin is a waterproof covering but some chemical substances, such as drugs and essential oils, can penetrate the skin through the layers, the hair follicles and sweat glands. The amount of penetration is affected by the health and condition of the skin. Splits, cuts, tears and irregularities caused by disease or disorder increase the risk of infection.

4. Protection

The skin acts as a barrier to the body's invasion by micro-organisms like bacteria. The naturally acid pH of the skin's surface inhibits bacterial production. Melanin, the pigment produced by the melanocytes in the basal layer of the epidermis, has a protective function. It helps to protect against ultraviolet light damage to tissues. Sensory nerve endings found at differing levels in the dermis warn of possible trauma and, by reflex action, prevent greater damage to the body.

5. Elimination

Some toxins are eliminated from the body through the skin via the sweat glands. The toxins normally take the form of waste salts and water.

6. Sensation

Specialized nerve endings found in the dermis make the body aware of its surroundings. They warn of pain, cold, heat, pressure and touch. Different receptors lie at different levels in the skin. Pain and touch receptors are closer to the surface. All receptors warn of and prevent trauma to the skin and underlying structures.

7. Vitamin D Production

Vitamin D is essential for the formation and maintenance of bone. Vitamin D production is stimulated by ultraviolet light which converts a fatty substance in the skin, 7-dehydrocholesterol, into vitamin D. This circulates in the blood and any excess is stored in the liver. Lack of vitamin D results in rickets in children.

8. Melanin Production

In the sun, the body makes melanocyte-stimulating hormone (MSH) which causes the melanocytes in the basal layer of the epidermis to produce melanin, a substance which produces a darkening of the skin to protect the underlying structures. The pigment protects the body from harmful effects of the sun's rays since dark colors absorb radiation.

Diseases and disorders of the skin can be classified under seven categories:

1. Congenital: Exists at birth, may be inherited.
2. Bacterial: Bacteria are single-celled organisms found both inside and outside the body. Some are harmful and others are good for us. They can invade normal skin areas or wounds.
3. Viral: Viruses are sub-microscopic organisms that can only multiply once inside cells they have invaded.
4. Fungal: An organism that can live on skin, hair and nails and thrives in warm, moist conditions.
5. Pigmentation Disorders: A number of pigments contribute to skin color, for example melanin. This can accumulate more or less intensively in some areas.
6. General: Other features visible on the skin surface which may be considered a disorder.
7. Skin Cancer: There are 3 main types of skin cancer. The leading cause of all types is excessive exposure to sunlight.

Congenital

Eczema: Eczema can be found all over the body but most often on the inside of the knee (in the popliteal space) and elbow joints, on the face, hands and scalp. The skin becomes extremely dry and itchy causing great discomfort. Skin has scaly dry patches with bleeding at points. Not contagious.
Psoriasis: Chronic inflammatory skin disease characterized by red patches covered with silvery scales that are constantly shed. Size of scales vary from minute spots to quite large sheets of skin. Points of bleeding may occur beneath scales. Affects whole body or specific areas, like face and scalp. Not infectious.

Bacterial

Rosacea: Gives a flushed, reddened appearance. Occurs on the face, this condition can be aggravated by anything causing vasodilation - heat, sunshine, spicy food, alcohol, cold. Affects both men and women especially menopausal women. Not related to acne vulgaris. Not contagious.
Acne vulgaris: Normally caused by hormonal imbalances which increase sebum production leading to blocked glands and infection. The skin has a shiny, sallow appearance with papules, pustules and comedones. It is prone to open pores. Where pustules have cleared there is often pitting and scarring. The main sites for infection are the face, back, chest and shoulders. Not contagious.
Spots: What is a spot?: A bacterial infection of the skin, causing inflammation around a hair follicle.
Carbuncles: A skin infection that often involves a group of hair follicles. The infected material forms a lump which occurs deep in the skin.
Folliculitis: Bacterial infection of the pilo-sebaceous duct (sebaceous gland and hair follicle) causing inflammation. Common in adolescence. Possible link with acne vulgaris.
Impetigo: A bacterial infection causing thin-roofed blisters which weep and leave a thick, yellow crust. Highly contagious.
Syphilis: Syphilis is a bacterial STD which is caught through sexual contact. An open sore (called a chancre) on the genital area is one symptom of the disease.
Chancroid: Raised sore in the vulva or vaginal area, caused by chancroid, a bacterial STD.

Viral

Warts: A small horny tumor found on the skin, often on fingers and thumbs. Caused by viral infection. Genital warts are warts which are transferred through sexual contact. Highly contagious.
Verrucas: Warts found on the feet. Highly contagious.
Herpes simplex: A viral infection commonly known as cold sores; not confined to the mouth, can spread over the face and other parts of the body. Genital herpes is an STD which manifests as sores in the genital area. Appears as small blisters which if left alone will dry up leaving a crust which falls off. Highly contagious when active. See, how to treat cold sores.
Herpes zoster: A viral infection commonly known as shingles. Adult form of chicken pox. Usually affects spinal nerves and one side of the thorax. Highly contagious.

Fungal

Tinea corporis, pedis: Infections which attach themselves to keratinised structures like the skin. Tinea corporis is commonly known as ringworm and can be found anywhere on the body. Tinea pedis is commonly known as athlete's foot. Highly infectious. See, how to treat athlete's foot.

Parasitical Infestation

Pediculosis: The infestation with lice resulting in severe itching. This can occur on the head (capitis), body (corporis) and pubic (pubis) areas. Pubic lice, also known as crabs, is an infestation of lice in the hairs of the pubic region.
Scabies: Scabies is a contagious skin infection caused by the itch mite; characterized by persistent itching and skin irritation.

Pigmentation Disorders

Dermatosis Papulosa Nigra: Is a condition of many small, benign skin lesions, characterized by dark-brown papular lesions on the face and upper body, mainly found on a black skin.
Papilloma: Benign epithelial tumor forming a rounded mass.
Vitiligo: Complete loss of color in well-defined areas of the face and limbs. A form of leucoderma (an abnormal whiteness of the skin due to absence of pigmentation); begins in patches but may converge to form fairly large areas; most obvious in darker skins.
Albinism: Complete lack of melanocytes resulting in lack of pigmentation in skin, hair and eyes. Sufferers have poor eyesight and extreme ultraviolet sensitivity. This is an inherited condition.
Chloasma: Butterfly mask often caused by pregnancy and the contraceptive pill; a hyper pigmentation condition involving the upper cheeks, nose and occasionally forehead. Discoloration usually disappears spontaneously at the end of pregnancy.
Ephelides/Freckles: Small pigmented areas of skin which become more evident on exposure to sunlight and are found in greatest abundance on the face, arms and legs; fair-skinned individuals are more prone to freckles.
Lentigo: Also known as liver spots; dark patches of pigmentation which appear more distinct than freckles and have a slightly raised appearance and more scattered distribution.
Moles (papilloma): Common occurrence on the face and body and present in several different forms, varying in size, color and vascular appearance. Flat moles are called sessile whilst those raised above the surface, or attached by a stalk are pedunculated. See, what are skin moles?
Naevae: Birth mark; if pigmented may occur on any part of the body and are often found on the neck and face, being sometimes associated with strong hair growth. Vary in size from pinhead to several centimeters and in rare cases may be extremely large. Pigmentation varies from light brown to black. Strawberry naevae (pink or red birth marks) often affect babies, eventually disappearing after a few years.
Port wine stain: A large area of dilated capillaries causing a pink to dark red skin color which makes it contrast vividly with the surrounding skin. The stain is commonly found on the face.

General Disorders

Blisters: An elevation of the skin filled with serous fluid. Did you know? The surface of a blister is made of raised epidermis and the fluid inside it is serum. How to treat blisters.
Cyst: A harmless swelling filled with air, fluids or semi-solid material. The cyst is enclosed by a layer of epidermis which can be removed painlessly because it contains no nerves or blood vessels.
Keloid scars: A scar that forms at the site of a healed wound. Keloids are rubbery, firm raised areas of skin that can vary in color.
Striae: Another name for stretch marks. .
Skin tags: Medically termed verrucae filliformis, skin tags are small flaps of excess skin typically found in the neck, groin and armpits. They are harmless and may eventually fall off.
Xanthomas: Raised bumps in the skin that contain yellow fatty deposits, linked to high levels of cholesterol.
Burns: Skin injury caused by exposure to flame, heat or friction.
Cellulitis: Inflammation of tissues beneath the skin. It is characterized by fever, redness, swelling and pain.
Staph Infection: Methicillin-resistant Staphylococcus aureus (MRSA): A dangerous, potentially life-threatening infection caused by Staphylococcus aureus bacteria (commonly called a Staph infection) that is resistant to the broad spectrum antibiotics commonly used to treat it.
Pressure sores/bed sores: Ulcers (open sore) that appear on areas of the skin which are under pressure from lying in bed, wearing a cast, sitting in wheelchairs, or being immobile for long periods of time.
Broken capillaries: Appear as small thin threads under the skin, typically on the face. Sometimes referred to as spider veins. It happens when the tiny blood vessels (capillaries) near the surface of the skin dilate. This may be hereditary or caused by other factors such as spicy foods or drinking too much alcohol.
Crow's feet: Fine lines (wrinkles) around the eyes caused by habitual expressions (squinting or laughing) and daily movement. The premature appearance of crow's feet may be due to eye strain and is often associated with edema (swelling) around and under the eyes. See, What are wrinkles?
UV/Sun damage: UV rays stimulate rapid production of the basal cells. This in turn causes the stratum corneum to thicken and dry out. Over exposure to UVA can lead to premature ageing and over-exposure to UVB is linked to skin cancer. See, how to treat sunburn.
Urticaria - hives, nettle rash: Usually an allergic reaction to a product, food or plant. Urticaria is characterized by welts which are pinkish in color. Very itchy. Can lead to secondary infection by bacteria through scratching.
Allergic reaction: When the skin or body is irritated, it produces histamine (part of the defense mechanism) in the skin. This can cause red, blotchy patches on the skin, stinging, watery, eyes, swellings and a runny nose. Reaction can be slight or intense.
Comedones/blackheads: Blackheads are caused by a build-up of oil secretions which have become trapped in the hair follicles and have subsequently dried out and hardened. The dark color comes from oxidation (contact with the air). More common in puberty when oil production is naturally higher.
Dermatitis: Allergic inflammation of the skin. Main symptom is erythema - skin redness, itching and skin lesions. Also known as contact dermatitis, there are many causes including clothing, cosmetics, chemicals, plants and drugs. Not contagious.
Milia/whiteheads: Tiny white bumps on the skin. Whiteheads form when sebum (oil) becomes trapped in a blind duct with no surface opening. The condition is most common on dry skin and milia appear on the eyelids and between the eyebrows. Milia can form after injury, e.g. sunburn on the face or shoulders, and are sometimes widespread.

Gland Disorders

Bromidrosis/osmidrosis: Fetid or foul-smelling perspiration which is caused by decomposition of the sweat and cellular debris by the action of bacteria and yeasts.
Anhidrosis: The reduced ability or inability to sweat.
Hyperhidrosis: Is the condition characterized by abnormally increased perspiration.

Skin Cancer

Basal cell carcinoma: Occurs on exposed parts of the skin, especially face, nose, eyelid, cheek.
Squamous cell carcinoma: Squamous cells are those found on the surface of the body, on the top layer of the skin. Squamous cell carcinoma is said to be caused by sunlight, chemicals or physical irritants. It starts very small but grows rapidly, becoming raised.
Malignant melanoma: A malignant tumor of melanocytes. It usually develops in a previously benign mole. The mole has become larger and darker, ulcerated and the tumor eventually spreads.

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