Skin
High-Yield Summary
- Skin (integument) = largest organ (~16% body weight), derived from ectoderm. 5 functions: protective barrier, immune defense, temperature regulation, sensation, water retention.
- Epidermis: AVASCULAR, 5 strata deep→superficial (basale → spinosum → granulosum → lucidum → corneum). Cell types: keratinocytes (bulk), melanocytes (melanin, UV protection), Langerhans cells (immune, antigen presentation to T cells).
- Dermis: VASCULARIZED, has papillary layer (loose CT, anchoring) + reticular layer (dense CT, strength/elasticity). Houses sweat/sebaceous glands, hair follicles, most sensory receptors.
- Hypodermis (subcutaneous): not technically skin; anchors skin to muscle/bone, stores fat for energy/insulation/cushioning.
- Thermoregulation, heat loss: sweating, vasodilation, hair erector muscles relax. Heat conservation/production: vasoconstriction, piloerection, shivering, brown fat.
- Heat transfer modes: radiation (EM waves), conduction (direct contact), convection (air/fluid movement) — always warm → cool.
Key Terms
- Keratinocytes
- Most abundant epidermal cell; produces keratin, becomes part of the outer protective barrier.
- Melanocytes
- Stratum basale cells that produce melanin, protecting DNA from UV damage.
- Langerhans cells
- Epidermal immune cells that detect antigens and present them to T cells.
- Piloerection
- Hair erector muscles contracting, causing hairs to stand upright and trap insulating air.
- Thermoregulation
- The body's ability to maintain stable internal temperature despite external changes.
Epidermis vs. Dermis
| Epidermis | Dermis |
|---|---|
| Avascular (no blood vessels) | Vascularized (contains blood vessels — why deep cuts bleed) |
| Keratinized epithelial cells; 5 strata | Connective tissue; papillary + reticular layers |
| Primary role: protection | Houses nerves, sweat/sebaceous glands, hair follicles — support, sensation, regulation |
Epidermal Layers (Deep → Superficial)
- 1Stratum basale — new cells generated via active division
- 2Stratum spinosum — cells produce more keratin, strengthen connections
- 3Stratum granulosum — keratin continues, cells lose nuclei/organelles
- 4Stratum lucidum — thin translucent layer, only in thick skin (palms/soles)
- 5Stratum corneum — dead, flattened keratinocytes forming the tough outer barrier
Thermoregulation: Heat Loss vs. Heat Conservation
| High Body Temperature (Lose Heat) | Low Body Temperature (Conserve/Generate Heat) |
|---|---|
| Vasodilation (more surface blood flow) | Vasoconstriction (less surface blood flow) |
| Hair erector muscles relax (hairs flat) | Piloerection (hairs upright, traps air) |
| Sweating active (evaporative cooling) | Shivering + brown fat metabolic heat generation |
Common MCAT Trap
- The epidermis is AVASCULAR — it gets nutrients by diffusion from below. A cut that bleeds has reached the dermis, not just the epidermis.
- Piloerection = hairs stand UP to trap air and conserve heat (cold response) — don't confuse with hair erector muscles relaxing (hot response, hairs lie flat).
- The hypodermis is NOT technically part of the skin — it's a separate subcutaneous layer, though it supports skin function.
Quick Recall
Why doesn't a superficial scrape bleed, but a deep cut does?
What are the 5 epidermal layers, deep to superficial?
What 3 mechanisms does the body use to conserve/generate heat in the cold?
What cell type in the epidermis is responsible for immune surveillance?