Skin has three layers

• The epidermis, the outermost layer of skin, provides a waterproof barrier and creates our skin tone.
• The dermis, beneath the epidermis, contains tough connective tissue, hair follicles, and sweat glands.
• The deeper subcutaneous tissue (hypodermis) is made of fat and connective tissue.

The skin’s color is created by special cells called melanocytes, which produce the pigment melanin. Melanocytes are located in the epidermis.

Skin Conditions

• Rash: Nearly any change in the skin’s appearance can be called a rash. Most rashes are from simple skin irritation; others result from medical conditions.
• Dermatitis: A general term for inflammation of the skin. Atopic dermatitis (a type of eczema) is the most common form.
• Eczema: Skin inflammation (dermatitis) causing an itchy rash. Most often, it’s due to an overactive immune system.
• Psoriasis: An autoimmune condition that can cause a variety of skin rashes. Silver, scaly plaques on the skin are the most common form.
• Dandruff: A scaly condition of the scalp may be caused by seborrheic dermatitis, psoriasis, or eczema.
• Acne: The most common skin condition, acne affects over 85% of people at some time in life.
• Cellulitis: Inflammation of the dermis and subcutaneous tissues, usually due to an infection. A red, warm, often painful skin rash generally results.
• Skin abscess (boil or furuncle): A localized skin infection creates a collection of pus under the skin. Some abscesses must be opened and drained by a doctor in order to be cured.
• Rosacea: A chronic skin condition causing a red rash on the face. Rosacea may look like acne, and is poorly understood.
• Warts: A virus infects the skin and causes the skin to grow excessively, creating a wart. Warts may be treated at home with chemicals, duct tape, or freezing, or removed by a physician.
• Melanoma: The most dangerous type of skin cancer, melanoma results from sun damage and other causes. A skin biopsy can identify melanoma.
• Basal cell carcinoma: The most common type of skin cancer. Basal cell carcinoma is less dangerous than melanoma because it grows and spreads more slowly.
• Seborrheic keratosis: A benign, often itchy growth that appears like a “stuck-on” wart. Seborrheic keratoses may be removed by a physician, if bothersome.
• Actinic keratosis: A crusty or scaly bump that forms on sun-exposed skin. Actinic keratoses can sometimes progress to cancer.
• Squamous cell carcinoma: A common form of skin cancer, squamous cell carcinoma may begin as an ulcer that won’t heal, or an abnormal growth. It usually develops in sun-exposed areas.
• Herpes: The herpes viruses HSV-1 and HSV-2 can cause periodic blisters or skin irritation around the lips or the genitals.
• Hives: Raised, red, itchy patches on the skin that arise suddenly. Hives usually result from an allergic reaction.
• Tinea versicolor: A benign fungal skin infection creates pale areas of low pigmentation on the skin.
• Viral exantham: Many viral infections can cause a red rash affecting large areas of the skin. This is especially common in children.
• Shingles (herpes zoster): Caused by the chickenpox virus, shingles is a painful rash on one side of the body. A new adult vaccine can prevent shingles in most people.
• Scabies: Tiny mites that burrow into the skin cause scabies. An intensely itchy rash in the webs of fingers, wrists, elbows, and buttocks is typical of scabies.
• Ringworm: A fungal skin infection (also called tinea). The characteristic rings it creates are not due to worms.

Epidermis

The epidermis is the outer layer of the skin. It is a mosaic of cells glued together and its thickness depends on the location on the body. On the palms and soles the epidermis is thick, flexible and resists mechanical injury. On the eyelids it is very thin and allows maximum movement. The epidermis prevents loss of water and body fluids, resists mechanical and chemical injury and protects against bacteria, viruses and parasite infections. The pigment in the epidermis plays an important role in protecting the skin from ultraviolet radiation.

The hair follicles, sweat glands, sebaceous (oil) glands and apocrine glands develop from the epidermal cells, but their deeper parts extend into the dermis. The glands open onto the surface of the skin via small ducts.

Hair grows from the hair follicle, which is found in all skin except the palms and soles.

Nails are specialised plates of hard keratin that develop from the epidermis overlying the small bones at the ends of the fingers and toes.

There are 3 main groups of cells in the epidermis:

1. Keratinocytes (skin cells)
2. Melanocytes (pigment cells)
3. Langerhans cells (immune cells).

The main cell in the epidermis is the keratinocyte, which develops from the bottom or basal layer and then migrates upwards over a period of about four weeks to the outer surface (stratum corneum) where it is shed.

Langerhans cells are specialised immune cells that are an important part of the body’s immune response to foreign materials and infections.

The melanocytes produce pigment. All humans have the same number of melanocytes. The difference in skin colour occurs because in darker skin melanocytes produce more pigment. The melanin pigment protects the cells of the epidermis and the tissues in the dermis from sun damage. Lighter skinned people are more susceptible to developing sun damaged skin because their melanocytes produce less melanin (skin pigment).

Dermo-epidermal junction

This is a complex region where the dermis and epidermis are attached to each other via specialised cells and molecules. It contains the basement membrane.

Dermis

The dermis lies beneath the epidermis and is 20 to 30 times thicker than the epidermis. It is composed of a dense network of specialised proteins (collagen and elastin) organised into fibres of differing sizes and properties.  A complex gel of different proteins surrounds these fibres. All together this is known as the extracellular matrix.

Within the extracellular matrix are blood and lymphatic vessels, nerves, the bottom part of the hair follicles and sweat glands.

Subcutis (subcutaneous layer)

This is a specialised area under the dermis, which contains a network of collagen fibres and fat cells (adipocytes). It protects the body from external trauma and insulates from cold. It acts as a main storage site for fat and therefore energy. There are many blood and lymphatic vessels and nerves passing through the subcutis.