Key Takeaways
- The skin has three main layers: epidermis (outer), dermis (middle), and hypodermis/subcutaneous (deepest).
- The epidermis has five layers and contains melanocytes, keratinocytes, and Langerhans cells.
- The dermis contains blood vessels, nerves, sweat glands, sebaceous glands, and hair follicles.
- Primary lesions are initial skin abnormalities; secondary lesions develop from primary lesions.
- Skin renews every 28-30 days in young adults, slowing to 45-50 days in older adults.
Last updated: January 2026
Anatomy & Physiology - Skin
The skin (integumentary system) is the largest organ of the body, covering approximately 22 square feet in adults. Understanding skin anatomy, functions, and disorders is essential for safe cosmetology practice and success on the NIC exam.
Overview of Skin Structure
The skin consists of three main layers:
- Epidermis - Outermost layer (what we see)
- Dermis - Middle layer (contains structures)
- Hypodermis/Subcutaneous - Deepest layer (fat and connective tissue)
Layer 1: The Epidermis
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The epidermis is the outermost, visible layer of skin. It contains no blood vessels and receives nutrients from the dermis below.
Five Layers of the Epidermis
| Layer | Also Called | Description | Key Function |
|---|---|---|---|
| Stratum Corneum | Horny layer | Outermost; dead, keratinized cells | Protection barrier |
| Stratum Lucidum | Clear layer | Only in thick skin (palms, soles) | Additional protection |
| Stratum Granulosum | Granular layer | Cells dying, becoming keratin | Waterproofing |
| Stratum Spinosum | Spiny layer | Contains Langerhans cells | Immune protection |
| Stratum Basale | Basal/Germinativum layer | Deepest; active cell division | New cell production |
Memory Tip: "Come Let's Get Sun Burned"
- Corneum, Lucidum, Granulosum, Spinosum, Basale
Key Cells of the Epidermis
Keratinocytes:
- Make up 95% of epidermal cells
- Produce keratin - the protective protein in skin, hair, and nails
- Form the protective barrier of the stratum corneum
Melanocytes:
- Located in the stratum basale
- Produce melanin - the pigment that gives skin its color
- Protect against UV radiation damage
- Melanin production increases with sun exposure (tanning)
Langerhans Cells:
- Part of the immune system
- Located in the stratum spinosum
- Detect and fight foreign invaders (antigens)
- Help prevent infection
Epidermis Renewal Cycle
- Young adults: Complete cell turnover every 28-30 days
- Older adults: Cell turnover slows to 45-50 days
- New cells form in stratum basale and migrate upward
- By the time cells reach stratum corneum, they are dead and keratinized
Layer 2: The Dermis
The dermis is the middle layer, also called the "true skin." It is much thicker than the epidermis and contains most of the skin's structures.
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Two Layers of the Dermis
| Layer | Location | Composition | Function |
|---|---|---|---|
| Papillary Layer | Upper dermis | Loose connective tissue, capillaries | Nutrients to epidermis, fingerprints |
| Reticular Layer | Lower dermis | Dense connective tissue, collagen, elastin | Strength and elasticity |
Structures Within the Dermis
Blood Vessels:
- Deliver oxygen and nutrients to skin
- Remove waste products
- Help regulate body temperature
Nerve Endings (Sensory Receptors):
- Detect touch, pressure, pain, heat, and cold
- Enable tactile sensation
Sweat Glands (Sudoriferous Glands):
- Eccrine glands: All over body; regulate temperature
- Apocrine glands: Underarms, groin; activated by emotion/stress
Sebaceous Glands (Oil Glands):
- Connected to hair follicles
- Produce sebum - oily substance that lubricates skin and hair
- Overproduction leads to oily skin and acne
Hair Follicles:
- Tube-like structures from which hair grows
- Originate in the dermis
- Associated with sebaceous glands and arrector pili muscles
Collagen and Elastin
- Collagen: Protein providing strength and structure (80% of dermis)
- Elastin: Protein allowing skin to stretch and return to shape
- Both decrease with age, leading to wrinkles and sagging
Layer 3: The Hypodermis (Subcutaneous Layer)
The hypodermis is the deepest layer of skin, composed primarily of adipose (fat) tissue and connective tissue.
Functions of the Hypodermis
| Function | Description |
|---|---|
| Cushioning | Protects muscles and bones from injury |
| Insulation | Fat layer helps maintain body temperature |
| Energy Storage | Stores fat for energy reserves |
| Attachment | Connects skin to underlying muscles and bones |
Functions of the Skin
The skin performs six essential functions, often remembered as SHAPES:
| Function | Description |
|---|---|
| Sensation | Nerve endings detect touch, pain, heat, cold, pressure |
| Heat Regulation | Blood vessels dilate/constrict; sweat glands cool body |
| Absorption | Limited absorption of some substances (medications, products) |
| Protection | Barrier against pathogens, chemicals, UV radiation |
| Excretion | Sweat glands eliminate small amounts of waste |
| Secretion | Sebaceous glands produce sebum to lubricate skin |
Skin Disorders and Diseases
Inflammatory Conditions
| Condition | Description | Characteristics |
|---|---|---|
| Acne Vulgaris | Chronic inflammation of sebaceous glands | Papules, pustules, comedones |
| Dermatitis | Skin inflammation | Redness, itching, sometimes blisters |
| Eczema | Chronic inflammatory condition | Dry, itchy, red, scaly patches |
| Psoriasis | Autoimmune condition | Thick, silvery scales on red patches |
| Rosacea | Chronic redness and flushing | Red face, visible blood vessels, bumps |
Pigmentation Disorders
| Condition | Description |
|---|---|
| Hyperpigmentation | Excess melanin; dark patches (melasma, age spots) |
| Hypopigmentation | Lack of melanin; light patches |
| Vitiligo | Autoimmune loss of melanocytes; white patches |
| Albinism | Genetic absence of melanin production |
Hypertrophic Conditions (Overgrowths)
| Condition | Description |
|---|---|
| Keloids | Raised, thick scars extending beyond original wound |
| Skin Tags | Small, soft, hanging skin growths |
| Moles (Nevi) | Clusters of melanocytes; flat or raised |
| Keratosis | Thickened, rough patches of skin |
Skin Lesions
Lesions are abnormalities or changes in skin tissue. Understanding lesion types helps identify conditions requiring medical referral.
Primary Lesions (Initial Abnormalities)
| Lesion | Description | Example |
|---|---|---|
| Macule | Flat, discolored spot <1cm | Freckle |
| Papule | Small, raised bump <1cm | Pimple, wart |
| Pustule | Raised, pus-filled bump | Acne pustule |
| Vesicle | Small, fluid-filled blister | Herpes simplex |
| Bulla | Large blister >1cm | Burn blister |
| Wheal | Raised, itchy area | Hive, mosquito bite |
| Nodule | Solid, deep bump >1cm | Cyst |
| Tumor | Large mass, may be benign or malignant | Lipoma |
Secondary Lesions (Develop from Primary)
| Lesion | Description |
|---|---|
| Scale | Flaky, dead skin cells (psoriasis) |
| Crust | Dried serum, blood, or pus (scab) |
| Fissure | Crack in the skin |
| Ulcer | Open sore with tissue loss |
| Scar | Fibrous tissue replacing normal skin |
| Excoriation | Scratch or abrasion |
Skin Analysis
Skin Types
| Type | Characteristics | Pore Size |
|---|---|---|
| Normal | Balanced moisture, smooth texture | Small, even |
| Oily | Excess sebum, shiny appearance | Large, visible |
| Dry | Lack of oil, tight feeling, flaky | Small |
| Combination | Oily T-zone, dry cheeks | Mixed |
| Sensitive | Easily irritated, reactive, red | Varies |
Fitzpatrick Skin Type Scale
| Type | Characteristics | Sun Reaction |
|---|---|---|
| Type I | Very fair, light eyes, freckles | Always burns, never tans |
| Type II | Fair skin, light eyes | Usually burns, minimal tan |
| Type III | Medium skin | Sometimes burns, gradual tan |
| Type IV | Olive skin | Rarely burns, tans easily |
| Type V | Brown skin | Very rarely burns |
| Type VI | Dark brown/black skin | Never burns |
Test Your Knowledge
Which layer of the epidermis contains cells that are actively dividing to produce new skin cells?
A
B
C
D
Test Your Knowledge
Sebaceous glands produce:
A
B
C
D
Test Your Knowledge
A small, raised, pus-filled bump on the skin is called a:
A
B
C
D