Skin Physiology

Question 1

What is the skin like?

Answer 1

Largest organ of the body
–3.6 Kg
–2 m2
•3 layers
–Epidermis
–Dermis
–Subcutis

Question 2

Why is the skin a waterproof barrier?

Answer 2

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

Question 3

What is the epidermis like?

Answer 3

Epidermis
–Waterproofing
–Physical barrier
–Immune function
–Vitamin D synthesis (Endocrine)
–UV protection
–Thermoregulation

Question 4

What is the dermis like?

Answer 4

•Dermis
–Thermoregulation
–Vitamin D synthesis (Endocrine)
- sensory organ

Question 5

What is the subcutis like?

Answer 5

• thermoregulation
• energy reserve
• vitamin D storage
• endocrine organ
• shock absorber

Question 6

Why does skin wrinkle when wet?

Answer 6

Skin on fingers and toes wrinkles if immersed for approx. 5 mins.
•Mediated by sympathetic nervous system
•Due to vasoconstriction in dermis
•Improves grip

Question 7

How does the skin act as a physical barrier?

Answer 7

Skin on fingers and toes wrinkles if immersed for approx. 5 mins.
•Mediated by sympathetic nervous system
•Due to vasoconstriction in dermis
•Improves grip

Question 8

What happens in vitamin D synthesis and storage?

Answer 8

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

Question 9

How does the skin act as an endocrine organ?

Answer 9

Site of hormone action
•Androgens act on follicles and sebaceous glands
•Thyroid hormones act on keratinocytes, follicles, dermal fibroblasts, sebaceous glands, eccrine glands

Question 10

What happens to skin in hypothyroidism?

Answer 10

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

Question 11

What type of hormone synthesis occurs in the skin?

Answer 11

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

Question 12

How is skin a barrier to UV light?

Answer 12

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

Question 13

What is melanin like in skin?

Answer 13

Synthesised in melanosomes within melanocytes from tyrosine
•Transported via dendrites to adjacent keratinocytes
•Pheomelanin (red/yellow)
•Eumelanin (brown/black)

Question 14

What is melanocytes like in skin?

Answer 14

Melanocyte density varies between body sites
•Red hair contains more pheomelanin
•All skin types contain more eumelanin than pheomelanin.
•Photoprotective – scatters/filters UV light

Question 15

How is melanin harmful?

Answer 15

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

Question 16

How does skin respond to sunlight?

Answer 16

Immediate pigment darkening
–photooxidation of existing melanin
–redistribution of melanosomes
–occurs within minutes and lasts hours-days.
•Persistent pigment darkening (tanning)
–UVA&raquo_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

Question 17

How skin act as a barrier to infection?

Answer 17

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

Question 18

How does skin act as an immune organ?

Answer 18

Innate and acquired immune functions
•Epidermis
–Langerhans cells
•Dermis
–Regulatory T cells
–Natural killer cells
–Dendritic cells
–Macrophages
–Mast cells

Question 19

How is the epidermis an immune organ?

Answer 19

Epidermis
Keratinocytes secrete cytokines and chemokines that maintain populations of leucocytes in skin

Langerhans cells are antigen-presenting cells and secrete cytokines

Question 20

How does skin react when challenged?

Answer 20

When challenged:
•LC migrate to dermis and lymph nodes and activate a T-cell response
•Keratinocytes proliferate & secrete cytokines
•Leucocytes enter skin from blood

Question 21

How does skin act as a sensory organ?

Answer 21

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)

Question 22

How does skin help regulate body temperature?

Answer 22

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?]

Question 23

How does skin help regulate body temperature?

Answer 23

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

Question 24

How does skin help regulate body temperature (neurally)?

Answer 24

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

Question 25

How many sweat glands in skin?

Answer 25

1.6-4 million eccrine sweat glands
•1-3 L sweat per hour
•Availability of water is major limiting factor

Question 26

How many sweat glands in skin?

Answer 26

1.6-4 million eccrine sweat glands
•1-3 L sweat per hour
•Availability of water is major limiting factor

Question 27

How do goosebumps work?

Answer 27

Piloerection (Goosebumps)
•Arrector pili muscles innervated by sympathetic α1-adrenergic fibres
•Contraction raises cutaneous hairs
•Likely little significant impact on heat conservation

Question 28

How do goosebumps work?

Answer 28

Piloerection (Goosebumps)
•Arrector pili muscles innervated by sympathetic α1-adrenergic fibres
•Contraction raises cutaneous hairs
•Likely little significant impact on heat conservation

Question 29

How does the skin work as an energy store?

Answer 29

Subcutaneous fat acts as an insulator, a shock absorber and as an energy store
•White adipose connective tissue