The most superficial layer of the skin, the epidermis plays a role of protection against the aggressions to which the skin is subjected. Infection of the epidermis with parasites can lead to inflammatory reactions. And strong sun exposure causes skin lesions that can develop into cancer.  

The epidermis is the outermost layer of the skin. It is formed by the juxtaposition of cells on four layers:

• The basal layer rests on the underlying dermis
• The Malpighi layer
• The granular layer
• The stratum corneum, the most superficial

The cells that make up the epidermis are of three kinds:

• Keratinocytes represent 90 to 95% of epidermal cells
• Melanocytes (which produce melanin) in the basal layer
• Langerhans cells

In addition, the epidermis is crossed by several structures that come from the underlying dermis:

• The sweat gland ducts that excrete sweat
• Pilosebaceous follicles, anatomical sac-like formations in which hairs form and which serve as excretory ducts for the sebum produced in the sebaceous glands

Finally, the epidermis is made up of skin appendages such as hair and nails.

At the interface between the organism and the external environment, the epidermis forms a vital barrier which plays a role of protection against the attacks to which the skin is subjected.

Renewal of the stratum corneum

The primary function of the epidermis is to ensure the permanent renewal of the stratum corneum which forms a semi-permeable protective barrier. This outermost layer is made up of dead keratinocytes without a nucleus (called corneocytes) but rich in keratin. This fibrous, water-insoluble protein ensures impermeability and protection of the skin against external agents.

UV protection

Within the epidermis, several mechanisms allow the skin to protect itself against the skin damage that can be caused by prolonged exposure to the sun.

Indeed, exposure to UV causes:

• An increase in the production of keratinocytes (thickening of the epidermis) to limit the penetration of radiation
• Stimulation of the production of melanin, a pigment that filters UV rays and absorbs free radicals generated in cells by UV rays to prevent them from causing DNA damage

Antimicrobial barrier

Keratinocytes and Langerhans cells play a role in the body’s immune response.

In addition, the very structure of the epidermis helps fight against pathogenic attacks:

• The stratum corneum is an almost impassable barrier
• Cell renewal eliminates dead cells on the surface and installed microorganisms
• The acidic pH and low humidity of the stratum corneum are unfavorable for microbial growth
• The epidermis contains antimicrobial peptides and lipids

Sun damage

Skin damage caused by prolonged UV exposure is known as photoaging (fine deep wrinkles, irregular pigmentation, yellowish skin, etc.) Prolonged exposure to the sun can also lead to precancerous growths called actinic keratoses.

Skin cancers

• Basal cell carcinoma is characterized by a shiny papule that appears on the skin and slowly grows in size
• Squamous cell carcinoma forms a red, scaly, crusted area that may become raised and harden
• Melanoma develops on healthy skin or on an existing mole

Diseases of the pilosebaceous follicle

• Acne is a condition that clogs the sebaceous glands that secrete sebum.
• Epidermal cysts can develop when the ducts of the sebaceous glands become blocked. The secretions then accumulate under the skin causing the cyst to grow, which can become infected.

Pathological desquamation

The removal of cells from the horny layer of the skin (peeling) is superficial and invisible. But it can be pathological when the epidermis peels off in shreds as in Stevens-Johnson syndrome and Lyell syndrome, two forms of the same potentially fatal skin condition. These syndromes are caused by drug intolerance (antibiotics, anticonvulsants) or by bacterial infections, vaccinations or transplant rejection.

Parasitic skin infections

Parasitic skin infections can be caused by:

• Yeasts like Candida ou Malassezia furfur
• Dermatophytes like Epidermophyton, Microsporum and Trichophyton
• The scabies mite parasite
• A papillomavirus whose infection is manifested by the appearance of warts

Atopic dermatitis

It is linked in part to an alteration of the barrier function of the epidermis. The skin allows too much water to evaporate (dry skin), and it more easily lets in allergens that can trigger an inflammatory reaction (eczema flare-ups).

Psoriasis

It is a chronic inflammation of the skin which causes a runaway of the cells of the epidermis which renew themselves in just 3 days (instead of 28 days).

Most epidermal pathologies are taken care of by a dermatologist.

Sun damage

Different treatments can improve superficial changes in the skin:

• 5-fluoro-uracil, an anticancer drug
• Topical α-hydroxy acids, moisturizers at low concentrations, they can cause lesions on the skin at high concentrations
• Imiquimod, an immune response modifier that behaves like an anti-tumor agent
• Dynamic photo therapy

Some effects of photoaging can also be reduced with a chemical peel. As for actinic keratoses, if they are few and large, they can be removed by cryotherapy or electrosurgical curettage.

Skin cancers

The therapies considered for basal cell and squamous cell carcinomas are identical: curettage, electro-drying, surgical excision, cryotherapy, photodynamic therapy or sometimes radiotherapy. The treatment of melanoma is mainly based on surgical excision.

Diseases of the pilosebaceous follicle

The treatment of acne involves a healthy lifestyle (avoiding the sun, stress, tobacco) associated with local dermatological treatment (cleansing lotion, anti-acne treatment and moisturizer) or combined (local treatment + antibiotics or oral hormones).

Epidermal cysts can be removed if they cause discomfort.

Pathological desquamation

Stevens-Johnson and Lyell syndromes require hospitalization, if possible in a burn center or intensive care unit to avoid infection. Any suspicious medication should be stopped.

Parasitic skin infections

• Mycotic infections are treated with antimycotic drugs
• Scabies can be treated by applying a cream containing permethrin or lindane
• No treatment today allows the eradication of the papillomavirus. On the other hand, it is possible to destroy warts by various methods: cryotherapy, electrocoagulation, surgical curettage, CO2 laser vaporization or application of a preparation based on salicylic acid.

Atopic dermatitis and psoriasis

There is no treatment to definitively cure these chronic inflammations but therapeutic approaches allow them to be dispelled:

• Medicines applied to the skin (moisturizer, corticosteroids, immunosuppressants, etc.)
• Phototherapy

In severe forms of psoriasis, oral or intravenous drugs may be given (ciclosporin, mycophenolate, methotraxate and acitretin)

The skin is the only organ directly visible to the naked eye, so the patient can self-examine. He must strip himself and be placed in a well-lit room with a large mirror. All parts of the body, even the most intimate, should be inspected. A number of abnormal elements, such as a mole that changes in appearance, should prompt the patient to see a specialist.

The prevention of skin cancer involves measures to limit UV exposure:

• Seek shade, minimize outdoor activities between 10 a.m. and 16 p.m., avoid sunbathing and the use of tanning booths
• Use of protective clothing: long-sleeved shirt, pants and wide-brimmed hat
• Use of sunscreen with a sun protection factor of at least 30