Skin Physiology Flashcards
What is the skin like?
Largest organ of the body
–3.6 Kg
–2 m2
•3 layers
–Epidermis
–Dermis
–Subcutis
Why is the skin a waterproof barrier?
Tight junctions between cells in stratum granulosum, epidermal lipids and keratin in stratum corneum form both an inside-out and outside-in barrier to water
•Prevents transepidermal water loss
What is the epidermis like?
Epidermis
–Waterproofing
–Physical barrier
–Immune function
–Vitamin D synthesis (Endocrine)
–UV protection
–Thermoregulation
What is the dermis like?
•Dermis
–Thermoregulation
–Vitamin D synthesis (Endocrine)
- sensory organ
What is the subcutis like?
- thermoregulation
- energy reserve
- vitamin D storage
- endocrine organ
- shock absorber
Why does skin wrinkle when wet?
Skin on fingers and toes wrinkles if immersed for approx. 5 mins.
•Mediated by sympathetic nervous system
•Due to vasoconstriction in dermis
•Improves grip
How does the skin act as a physical barrier?
Skin on fingers and toes wrinkles if immersed for approx. 5 mins.
•Mediated by sympathetic nervous system
•Due to vasoconstriction in dermis
•Improves grip
What happens in vitamin D synthesis and storage?
7-dehydrocholesterol in plasma membranes of epidermal keratinocytes and dermal fibroblasts converted to previtamin D3 by UVB
•15-25 mins whole body exposure produces up to 10,000 IU Vitamin D
•Serum concentrations peak 24-48 hours after exposure
- lipid soluble - can be stored in subcutis adipocytes
How does the skin act as an endocrine organ?
Site of hormone action
•Androgens act on follicles and sebaceous glands
•Thyroid hormones act on keratinocytes, follicles, dermal fibroblasts, sebaceous glands, eccrine glands
What happens to skin in hypothyroidism?
Hypothyroidism
Epidermal changes
Coarsened thin scaly skin
Dermal changes
Myxoedema
Hair and Nail changes
Dry brittle coarse hair
Alopecia
Thin brittle nails
Sweat gland changes
Dry skin
Decreased sweating
What type of hormone synthesis occurs in the skin?
Site of hormone synthesis
•Vitamin D3 – unique site for cholecalciferol synthesis
•17β-hydroxysteroid dehydrogenase in sebocytes and 5α-reductase in dermal adipocytes convert dehydroepiandrosterone (DHEA) and androstenedione to 5α-dihydrotestosterone
•Insulin-like growth factor (IGF) binding protein-3 (IGFBP-3) synthesised by dermal fibroblasts
How is skin a barrier to UV light?
Both UV-A and UV-B damage skin
–Burns
–Suppress action of Langerhans cells
–Photo-aging
–DNA damage (skin cancers)
•Skin colour depends on:
–Melanin
–Carotenoids
–Oxy/deoxyhaemoglobin
What is melanin like in skin?
Synthesised in melanosomes within melanocytes from tyrosine
•Transported via dendrites to adjacent keratinocytes
•Pheomelanin (red/yellow)
•Eumelanin (brown/black)
What is melanocytes like in skin?
Melanocyte density varies between body sites
•Red hair contains more pheomelanin
•All skin types contain more eumelanin than pheomelanin.
•Photoprotective – scatters/filters UV light
How is melanin harmful?
Prone to photodegradation – may generate reactive oxygen species!
•Pheomelanin increases release of histamine
•Lots of melanin = less able to utilize UV light to make vitamin D
How does skin respond to sunlight?
Immediate pigment darkening
–photooxidation of existing melanin
–redistribution of melanosomes
–occurs within minutes and lasts hours-days.
•Persistent pigment darkening (tanning)
–UVA»_space; UVB
–oxidation of melanin
–occurs within hours, lasts 3-5 days
- delayed tanning
- increased melanin synthesis
- occurs in 2-3days after UV exposure, maximal at 10-28 days
How skin act as a barrier to infection?
Skin presents a large surface area to environment
•The properties that render the skin a barrier to water also help prevent infection
•A range of peptides synthesised by granular layer keratinocytes have antimicrobial properties
–Cathelicidin-related antimicrobial peptide (Cramp - called LL37 in humans)
- Beta defensins
- S100A7 and S100A8
How does skin act as an immune organ?
Innate and acquired immune functions
•Epidermis
–Langerhans cells
•Dermis
–Regulatory T cells
–Natural killer cells
–Dendritic cells
–Macrophages
–Mast cells
How is the epidermis an immune organ?
Epidermis
Keratinocytes secrete cytokines and chemokines that maintain populations of leucocytes in skin
Langerhans cells are antigen-presenting cells and secrete cytokines
How does skin react when challenged?
When challenged:
•LC migrate to dermis and lymph nodes and activate a T-cell response
•Keratinocytes proliferate & secrete cytokines
•Leucocytes enter skin from blood
How does skin act as a sensory organ?
Merkle cells - basal epidermis (Light touch)
•Encapsulated mechanoreceptors in dermis
–Pacinian corpuscles (Pressure/Vibration)
–Meissner corpuscles (Touch)
•Myelinated and unmyelinated sensory nerve endings in dermis (pain, itch, temperature)
How does skin help regulate body temperature?
Insulation
•Subcutaneous fat
37 - optimal???
Heat loss
•Cutaneous blood flow
–Deep vascular plexus (lower reticular dermis)
–Superficial vascular plexus (upper reticular dermis)
–Loops of blood vessels from superficial plexus extend to reticular dermis
•Eccrine sweating
•Hair [What might be other functions of human hair?]
How does skin help regulate body temperature?
Humans are endothermic homeotherms
•Heat generated through metabolism
Heat storage = metabolism - work - evaporations +/- radiation +/- conduction +/- convection
•Evaporation depends on:
–Surface area exposed to environment
–Temp and relative humidity of ambient air
–Convective air currents
•Radiation, conduction and convection can add or remove heat
How does skin help regulate body temperature (neurally)?
Autonomic regulation of blood flow in dermal vascular plexuses
–Sympathetic alpha-noradrenergic: vasoconstriction
–Sympathetic cholinergic: vasodilation
–(Both in hairy skin. Hairless skin only has cholinergic innervation)
•Sympathetic cholinergic nerves that govern sweating may be the same as those controlling active vasodilation
•Nitric oxide may play a role in active vasodilation
