The Skin (Human Anatomy)

The skin is made up of several layers and each one serves different functions.

The skin is made up of three main layers (from top to bottom), including:

1. Epidermis
   • The outer or top layer of the skin.
   • The majority is composed of keratinocytes; the cells that take part in the skin renewal process.
   • Melanocytes are melanin-producing cells. Melanin gives the skin its color, which varies from person to person. The more the melanin content, the darker the skin color.
2. Dermis
   • The middle layer of the skin.
   • Made up of elastin/collagen (a type of connective tissue); this gives the skin its flexibility and strength.
   • Houses the sweat glands, oil glands (sebaceous glands), hair follicles, muscles, nerve endings, blood vessels, and dendritic cells.
3. Hypodermis
   • The bottom or deepest layer of the skin.
   • Contains mainly fat and other structures like hair follicles, nerve endings, and blood vessels (present in small numbers).

All these three significant layers vary significantly in their anatomy and function.

The epidermis is divided into five layers (from top to bottom), including:

1. Stratum corneum
2. Stratum lucidum
3. Stratum granulosum
4. Stratum spinosum
5. Stratum basale

The thickness of the skin varies in different parts of the body. Skin layers are not present in some regions of the body and other regions, the skin is thicker than it is in the rest of the body. For example, the skin on the palms and soles is the thickest while the eyelids have the thinnest skin.

The skin is the largest organ in the body that covers the entire external surface. It protects the internal organs from germs and thus helps prevent infections.

The main functions of the skin include the following:

1. Protecting from water, microorganisms, mechanical and chemical trauma, and damage from UV light
2. Serving as the first site of immunological defense 
3. Regulating the temperature and water loss (through sweat)
4. Producing vitamin D from exposure to sunlight
5. Secreting oil (by sebaceous glands) that keeps the skin moist and soft and protects it against foreign substances
6. Helping the body sense touch, heat, cold, and pain
7. Insulating the body from external heat and cold

1. Rash: General term for areas of irritation or altered skin texture.
2. Acne: A skin condition that occurs due to plugged skin pores, causing eruptions called comedones.
3. Dermatitis: Inflammation of the skin usually due to an allergy or infection.
4. Eczema: Skin inflammation (dermatitis) accompanied by itching.
5. Psoriasis: Itchy, silver, or reddish scaly plaques.
6. Dandruff: A scaly condition of the scalp caused by a yeast-like organism.
7. Hives: Raised, red and itchy patches on the skin that arise suddenly. Hives usually result from an allergic reaction.
8. Blister: Watery, clear, and fluid-filled area on the skin.
9. Tinea versicolor: Fungal skin infection characterized by pale areas of low pigmentation on the skin.
10. Ringworm: A fungal skin infection (also called tinea) characterized by small, ring-like rashes.
11. Vitiligo: Loss of skin color from certain regions that look like white patchy areas on the skin.
12. Melasma: Darkening of facial skin, particularly around the cheeks.
13. Scabies: An intensely itchy and contagious rash in the webs of fingers, wrists, elbows, and buttocks.
14. Cellulitis: Redness and warmth of the dermis and subcutaneous tissues.
15. Skin abscess (boil or furuncle): A collection of pus under the skin caused by an infection.
16. Rosacea: A chronic skin condition that features a red rash with tiny pimples on the face, particularly on the cheeks and nose.
17. Warts: A small, cauliflower-like growth on the skin caused by a virus.
18. Herpes rash: Periodic blisters or skin irritation around the lips or the genitals caused by the herpes viruses (HSV-1 and HSV-2).
19. Shingles (herpes zoster): This is a viral infection that causes a painful rash on one side of the body.
20. Viral exanthem: Rash or eruption due to a virus. It occurs most commonly in children.
21. Melanoma: Skin cancer that develops from the overgrowth of melanocytes.
22. Basal cell carcinoma: The most common type of skin cancer. It is less dangerous than melanoma because it grows and spreads more slowly.
23. Seborrheic keratosis: A benign and often itchy epidermal growth that resembles a brown wart.
24. Actinic keratosis: A crusty or scaly bump that forms on sun-exposed skin, particularly on fair-skinned elderly people. It can sometimes progress to cancer.
25. Cutaneous squamous cell carcinoma: The second-most common form of skin cancer that usually develops in sun-exposed areas.