Anatomy
Skin Layers
Epidermis, dermis, hypodermis — the body's largest organ, barrier to infection, and thermoregulator
Skin is the body's largest organ — ~1.5-2 m² and ~15% of body weight. Three layers. Epidermis (outermost, avascular, stratified squamous epithelium): keratinocytes turning over every 28-40 days through stratum basale → spinosum → granulosum → corneum, plus melanocytes (pigment), Langerhans cells (immune), and Merkel cells (touch). Dermis (middle, vascular connective tissue): collagen and elastin matrix housing hair follicles, sebaceous and sweat glands, sensory receptors, and blood vessels. Hypodermis (subcutis): adipose tissue providing insulation, energy storage, and cushioning. Functions: barrier against pathogens and UV, thermoregulation, sensation, vitamin D synthesis, fluid balance.
- Surface area~1.5-2 m² in adults
- Epidermal turnover28-40 days (basale to corneum)
- Layer orderEpidermis → dermis → hypodermis
- Thickest skinSoles (~5 mm); thinnest: eyelids (~0.5 mm)
- Sweat glands~2-4 million eccrine glands per body
- Vitamin D synthesisUVB converts 7-dehydrocholesterol → cholecalciferol
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Why skin layers matter
- Barrier function. Stratum corneum prevents water loss and microbial entry; loss causes infection (burns) or dehydration.
- Drug delivery. Transdermal patches (nicotine, fentanyl, estradiol) exploit lipid permeability of stratum corneum.
- Wound assessment. Depth (epidermal vs dermal vs full-thickness) determines healing time and need for grafting.
- Dermatologic disease. Psoriasis, eczema, vitiligo, melanoma each affect specific layers and cell types.
- Cosmetic procedures. Chemical peels, lasers, fillers target specific dermal depths.
- Vitamin D synthesis. UVB on epidermal 7-dehydrocholesterol provides ~80% of body vitamin D in sunny climates.
- Aging and oncology. UV-driven photoaging and skin cancer (basal cell, squamous cell, melanoma) account for most cancer cases worldwide.
Common misconceptions
- Skin is just a covering. It is the largest organ — immune, sensory, endocrine, and metabolic functions.
- Hypodermis is part of the skin proper. Histologically it is subcutaneous tissue beneath true skin (dermis + epidermis).
- Tanning is healthy. Tanning is a DNA damage response; any change in pigmentation reflects UV damage.
- Higher SPF means proportionally more protection. SPF 30 blocks 97%, SPF 50 blocks 98% — diminishing returns.
- Skin pores can open and close. Pores are follicle openings of fixed size; heat doesn't open them.
- Drinking water hydrates skin. Stratum corneum hydration depends on barrier lipids and humectants, not systemic hydration above baseline.
Frequently asked questions
What are the five strata of the epidermis?
From deep to superficial: stratum basale (germinal layer with stem cells dividing on basement membrane), stratum spinosum (keratinocytes connected by desmosomes — "spiny" appearance), stratum granulosum (keratohyalin granules), stratum lucidum (only in thick skin like palms and soles), stratum corneum (15-30 layers of dead, anucleate corneocytes filled with keratin and lipids forming the brick-and-mortar barrier). Total turnover takes 28-40 days; faster in psoriasis (3-7 days).
How does melanin protect skin?
Melanocytes in the basal layer produce eumelanin (brown-black) and pheomelanin (red-yellow) inside melanosomes, transferred to surrounding keratinocytes via dendrites. Melanin absorbs UV photons and dissipates energy as heat, reducing DNA damage. Darker skin has more eumelanin and larger, individually dispersed melanosomes — providing intrinsic SPF ~13 versus ~3 for very fair skin. Melanoma arises from malignant melanocytes; UV is the major carcinogen.
What's the difference between eccrine and apocrine sweat glands?
Eccrine glands are distributed across the entire body (densest on palms, soles, forehead) and secrete watery hypotonic fluid for thermoregulation through sympathetic cholinergic stimulation. Apocrine glands are concentrated in axilla, groin, and areolae; they secrete viscous fluid into hair follicles. Bacterial breakdown of apocrine secretions produces body odor. Apocrine glands activate at puberty under androgen control.
How does skin act as an immune organ?
Multiple layers of defense. Stratum corneum lipids and antimicrobial peptides (defensins, cathelicidins) inhibit microbes. Acid mantle (pH 4.5-5.5) suppresses pathogens. Langerhans cells (epidermal dendritic cells) sample antigens and migrate to lymph nodes to prime T cells. Dermis contains tissue-resident T cells, mast cells, and macrophages. Skin-associated lymphoid tissue (SALT) coordinates local immunity. Topical immune dysfunction underlies atopic dermatitis and contact allergy.
What causes skin aging?
Two processes. Intrinsic aging: collagen synthesis declines ~1% per year after age 20; elastin fragments; dermal thickness decreases. Extrinsic (photoaging): UV radiation generates ROS that activate matrix metalloproteinases degrading collagen — solar elastosis, wrinkles, lentigines. Smoking accelerates both. Tretinoin (topical retinoid) increases collagen production; sunscreen prevents progression. Sun exposure causes ~80% of visible facial aging.
How do burns get classified?
First-degree (superficial): epidermis only; erythema and pain (sunburn); heals in days without scarring. Second-degree partial-thickness: into dermis; blistering, severe pain; heals in 2-3 weeks. Second-degree deep: deeper dermis; pale, less pain due to nerve damage; may require grafting. Third-degree full-thickness: through dermis to subcutis; leathery, painless, no spontaneous healing. Fourth-degree: into muscle/bone. Total body surface area calculated by rule of nines guides fluid resuscitation (Parkland formula).
What are the basic skin appendages?
Hair follicles (with arrector pili muscles, sebaceous glands attached), sebaceous glands (secrete sebum into follicle — lubricates skin), eccrine and apocrine sweat glands, and nails (keratinized epithelium of the nail matrix). Each grows from epidermal invaginations into dermis and is a reservoir of stem cells that re-epithelialize wounds. Loss of follicles in deep burns prevents spontaneous re-epithelialization, requiring grafts.