skin four primary tissue types
- Epithelial tissue – in the epidermis.
- Connective tissue – in the dermis and hypodermis.
- Muscle tissue – smooth muscle in arterioles and attached to hairs (arrector pili).
- Nervous tissue – sensory and motor nerves in all layers
Epidermis
- Outermost epithelial layer
- avascular (no blood vessels).
- Receives oxygen/nutrients by diffusion from dermis.
- Constantly renews itself: keratinocytes die and shed, replaced by new cells from the basal stem layer.
Keratinocytes
– produce keratin (waterproofing protein).
Melanocytes
– produce melanin (protects from UV damage).
Merkel cells
– tactile sensory receptors.
Langerhans cells
– immune cells that engulf pathogens
Dermis
- Connective tissue layer beneath the epidermis.
- Contains fibroblasts (produce collagen & elastin), macrophages, blood vessels, lymphatics, glands, nerves, and hair follicles
- Provides strength, flexibility, and nourishment to the epidermis.
Hypodermis
- Loose connective tissue and adipose (fat) tissue.
Functions as: - insulation
- shock absorption
- anchors skin to underlying structures
Major functions of the skin
- protection
- sensation
- metabolic function
- blood reservoir
- temperature regulation
- excretion
Protection
Chemical barrier:
Physical barrier:
Biological barrier:
Chemical barrier:
- Acidic pH (acid mantle) inhibits bacteria.
- Defensins: destroy pathogens.
- Melanin shields DNA from UV radiation.
Physical barrier:
- Keratin, lipids, and tight junctions prevent water loss and block most substances.
Biological barrier:
Macrophages in dermis destroy microorganisms that penetrate the epidermis.
Sensation
- Nerve endings detect pain, temperature, touch, pressure, and vibration.
- Mechanoreceptors, nociceptors, thermoreceptors relay signals to the CNS.
Metabolic Function
Produces vitamin D precursor in response to sunlight.
Blood Reservoir
Stores ~5% of total blood volume; blood flow adjusted for temperature control.
Excretion
Removes nitrogenous wastes and salts via sweat.
Temperature Regulation
High temperature: Vasodilation & sweating release heat.
Low temperature: Vasoconstriction conserves heat by diverting blood away from skin surface.
Controlled by thermoreceptors and the thermoregulatory centre in the brain
Skin repairs itself
Stage 1 – Inflammation.
Stage 2 – Organisation (Proliferation)
Stage 3 – Maturation (Remodelling)
Stage 1 – Inflammation
- Prepares wound for healing.
- Damaged cells release inflammatory signals → vasodilation and permeability increase.
- Macrophages remove debris and pathogens.
- Clotting proteins form a scab to seal the wound and prevent infection.
- Signs: redness, heat, swelling, pain.
Stage 2 – Organisation (Proliferation)
- Granulation tissue forms to restore blood supply.
- Contains capillaries, fibroblasts (produce collagen), and macrophages.
- Epidermal stem cells proliferate and migrate over granulation tissue to cover the wound
Stage 3 – Maturation (Remodelling)
- Collagen remodelled and contracts to close wound edges.
- Granulation tissue replaced by scar tissue (fibrosis) — strong but lacks elasticity, glands, or hair.
- Regeneration occurs if epidermal stem cells survive → new skin forms and scab falls off.
- Fibrosis only leads to visible scar formation
