Skin

Thick Skin vs Thin Skin

  • Thin skin covers most of the body

  • Thin skin will usually contain hair follicles

  • Thick skin is found on palms and soles of feet

  • Thick skin is hairless

  • Thick skin contains the stratum lucidum while thin skin does not


Three main layers

  • Epidermis - multi layered, mainly composed of keratinocytes, body’s protective layer

  • Dermis - connective tissue layer, rich is collagen and elastic fibres, provides strength and elasticity

  • Hypodermis - anchors skin to underlying tissue, allows skin movement, contains subcutaneous fat (for protection/cushioning, energy storage, and thermal insulation)


Layers of the Epidermis

  1. Stratum Basale - deepest layer, single cell layer + stem cells, site of keratinocyte production

  2. Stratum Spinosum - “prickly” layer, daughter cells from basale layer, desmosomes provide strong connections

  3. Stratum Granulosum - granular layer, keratin formation begins, lipid release = waterproof barrier, cells begin programmed cell death

  4. Stratum Lucidum - only in thick skin, layer of dead flattened cells, transitional later

  5. Stratum Corneum - outermost layer, contains dead cells (corneocytes), packed with keratin, main protection and water-resistant barrier, cells eventually shed from skin surface


Layers of the Dermis

  1. Papillary layer - superficial, loose connective tissue, forms dermal papillae, contains capillaries + sensory nerve endings, interlocks with epidermis, supplies nutrients to avascular epidermis

  2. Reticular layer - deep dermal, dense irregular connective tissue, thick collagen and elastic fibres, contains blood vessels + nerves + hair follicles + glands


Keratinocytes

  • function: main epidermal cell type, produce keratin, provide strength + waterproofing

  • Lifecycle: born in basale, move upward through layers, accumulate keratin, lose nucleus and organelles, become corneocytes, shed from surface

  • Timing: 7 - 10 days to reach top, 2 weeks till shedding


Melanocytes

  • function: produce melanin, mainly in basale, UV protection for DNA

  • process: melanin packaged into melanosomes, transferred to keratinocytes, carry pigment through epidermis migrating upwards


Hair

Bulb = site of cell division

Papilla = contains blood vessels, provides nutrients and growth signals

Process: cells divide, cells move upward, become keratinized, form hair shaft

Structures: Arrector Pili Muscle - smooth muscle, goose bumps; Root Hair Plexus = sensory nerves, detect hair movement

Associated Glands: Sebaceous + Sweat


Types of glands

  1. Sebaceous Glands - associated with hair follicles, holocrine glands (rupture to release contents); sebum = oily substance, lubricates hair + skin

  2. Merocrine (Eccrine) Sweat Glands - most abundant sweat glands, throughout body, secrete via exocytosis; Sweat Composition: 99% water, 1% salts, wastes, metabolites; sweat released onto skin surface, evaporative cooling, important for thermoregulation

  3. Apocrine Sweat Glands - armpits, nipples, pubic region, anal region; empty into hair follicles, produce thicker secretion; bacterial breakdown causes body odour, secretion via exocytosis