Skin Layers Overview
The skin (integument) has three main layers: epidermis, dermis, and hypodermis (subcutaneous tissue). It is the largest organ of the body (~1.5-2 m² surface area, 4-5 kg).
| Layer | Tissue Type | Key Contents |
|---|---|---|
| Epidermis | Stratified squamous epithelium | Keratinocytes, melanocytes, Langerhans cells, Merkel cells; NO blood vessels |
| Dermis (papillary) | Loose connective tissue | Dermal papillae, capillary loops, Meissner corpuscles (light touch) |
| Dermis (reticular) | Dense irregular connective tissue | Collagen I and III, elastin, hair follicles, glands, Pacinian corpuscles (pressure) |
| Hypodermis | Adipose and loose connective tissue | Fat storage, thermoregulation, Ruffini endings (stretch); NOT part of skin proper |
⭐ Boards PearlEpidermis is avascular — oxygen and nutrients reach it by diffusion from dermal capillaries. This is why thin skin grafts (split-thickness) can survive initially without blood supply.
Epidermis — Layers (Deep to Superficial)
| Layer | Latin Name | Key Features |
|---|---|---|
| Stratum basale | Germinativum | Deepest; mitotically active; contains melanocytes and Merkel cells; attached to basement membrane |
| Stratum spinosum | Spinosum | Prickle cells; desmosomes (strong cell junctions); site of acantholysis in pemphigus vulgaris |
| Stratum granulosum | Granulosum | Keratohyalin granules; lamellar bodies release lipids (waterproof barrier) |
| Stratum lucidum | Lucidum | Only in thick skin (palms, soles); clear, dead cells |
| Stratum corneum | Corneum | Outermost; dead, anucleate, keratin-filled; barrier function; shed (desquamation) ~14 days |
🧐 Mnemonic — Layers deep to surface"Come, Let's Get Sun-burned" = Basale, Spinosum, Granulosum, Lucidum, Corneum (or "Californians Like Girls in String bikinis")
Dermis and Hypodermis
The dermis provides mechanical strength via type I collagen (most abundant) and elasticity via elastin fibers. The dermis-epidermis junction has rete ridges (epidermis projecting down) and dermal papillae (dermis projecting up) — this undulating interface increases adhesion surface area.
- Type I collagen — mature skin, tendons (most abundant collagen)
- Type III collagen — fetal skin, wound healing, blood vessels (reticulin)
- Type IV collagen — basement membrane
- Elastin — provides recoil; degrades with UV exposure (solar elastosis)
🩹 Clinical NoteEhlers-Danlos syndrome: defect in collagen synthesis/processing → hyperextensible skin, joint hypermobility, easy bruising. Type depends on collagen affected.
Skin Cell Types
| Cell | Origin | Location | Function / Relevance |
|---|---|---|---|
| Keratinocytes | Ectoderm | All epidermal layers | 90% of epidermis; produce keratin; barrier function |
| Melanocytes | Neural crest | Stratum basale | Produce melanin (UV protection); 1:10 ratio to keratinocytes; melanoma origin |
| Langerhans cells | Bone marrow (monocyte) | Stratum spinosum | Antigen-presenting cells; destroyed by UV; express CD1a; Birbeck granules on EM |
| Merkel cells | Neural crest | Stratum basale | Slow-adapting mechanoreceptors (fine touch, sustained pressure); Merkel cell carcinoma |
| Fibroblasts | Mesoderm | Dermis | Collagen and elastin synthesis; wound healing |
| Mast cells | Bone marrow | Dermis (near vessels) | IgE-mediated degranulation; histamine release; urticaria, anaphylaxis |
Skin Appendages
| Appendage | Origin | Function | Key Points |
|---|---|---|---|
| Hair follicle | Epidermis (downgrowth) | Hair production, thermoregulation | Arrector pili (sympathetic adrenergic) = goosebumps; stem cells in bulge region |
| Eccrine sweat glands | Epidermis | Thermoregulation (aqueous sweat) | Cholinergic innervation (sympathetic but ACh); all over body; most numerous on palms/soles |
| Apocrine sweat glands | Epidermis | Pheromones, emotional sweat | Axilla, groin, areola; active after puberty; odor from bacterial breakdown |
| Sebaceous glands | Epidermis | Sebum (lipid-rich, waterproofing, antimicrobial) | Attached to hair follicles; androgen-stimulated; acne = plugged pore + P. acnes |
| Nails | Epidermis | Protection, fine manipulation | Nail matrix (under lunula) = nail growth; ~3 mm/month fingernails |
⭐ Boards PearlEccrine glands = cholinergic even though sympathetic. All other sympathetic effectors use norepinephrine. This is a classic pharmacology/autonomic boards question.
Skin Functions
- Protection — physical barrier (keratin, lipids), UV protection (melanin), antimicrobial (acidic pH ~5.5, sebum, defensins)
- Thermoregulation — vasodilation/constriction, eccrine sweating (evaporative cooling)
- Sensation — mechanoreceptors (Meissner, Pacinian, Ruffini, Merkel), free nerve endings (pain, temperature)
- Vitamin D synthesis — UV-B converts 7-dehydrocholesterol → previtamin D3 → vitamin D3 (cholecalciferol); converted to active 1,25-OH vitamin D in kidney
- Immune function — Langerhans cells, mast cells, IgA in sweat
- Fluid barrier — prevents transepidermal water loss (TEWL)
Clinical Pearls
⭐ Burn Classification1st degree (superficial): epidermis only, painful, erythema, no blisters (sunburn). 2nd degree (partial thickness): epidermis + part of dermis, very painful, blisters. 3rd degree (full thickness): all layers destroyed, painless (nerve destruction), leathery. 4th degree: to bone/muscle.
🩹 Wound Healing PhasesHemostasis (immediate): platelet plug, clot. Inflammation (days 1-4): neutrophils then macrophages; debridement. Proliferative (days 4-21): fibroblasts, collagen III, angiogenesis, granulation tissue. Remodeling (weeks-years): collagen III replaced by I; scar matures; tensile strength never exceeds 80% of original.
🧐 ABCDE of MelanomaAsymmetry, Border irregularity, Color variation, Diameter greater than 6mm, Evolution (change over time). Most common in males: back. Most common in females: legs.
⭐ Sensory Receptors Quick ReferenceMeissner corpuscles (papillary dermis) = light touch, two-point discrimination. Pacinian corpuscles (reticular dermis/hypodermis) = pressure, vibration. Ruffini endings (dermis) = skin stretch. Merkel discs (stratum basale) = sustained pressure, fine touch. Free nerve endings = pain, temperature, itch.