Physiology of the Skin

Question 1

3 layers of skin

Answer 1

Epidermis, dermis and subcutis

Question 2

Approx. how much does the skin weigh?

Answer 2

3.6 kg

Question 3

Waterproofing barrier

Answer 3

Tight junction between cells in stratum granulosum, epidermal lipids and keratin in stratum corneum form both an inside-out and outside-in waterproof barrier.

Question 4

Waterproofing barrier

Answer 4

Tight junction 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 5

Why does skin wrinkle when wet?

Answer 5

Mediated by sympathetic nervous system due to vasoconstriction in dermis.

Improves grip.

(eg if median nerve damaged then thumb, index and middle finger won’t wrinkle)

Question 6

How does the skin help resist abrasive forces?

Answer 6

Stratified squamous epithelium.
Fat in subcutis acts as shock absorber.

Question 7

Skin synthesis and stores vitamin D

Answer 7

7-dehydrocholesterol in plasma membranes of epidermal keratinocytes and dermal fibroblasts converted to pre-vitamin D3 (cholecalciferol) by UV light type B.

This is inactive form of Vitamin D, goes to liver and converted.

Question 8

How long would it take for 10,000 IU vitamin D from whole body exposure to UV in white skin?

Answer 8

15-25 minutes
Serum concentrations peak 24-48 hours after exposure

(some studies say need about 6 times longer for black skin other studies say it is the same)

Question 9

Where can vitamin D be stored in skin?

Answer 9

It is lipid soluble so is stored in subcutis adipocytes.

Question 10

Skin is endocrine organ

Answer 10

Androgens acts on follicles and sebaceous glands.

Thyroid hormones act on keratinocytes, follicles, dermal fibroblasts, sebaceous glands, eccrine glands.

Question 11

Where do androgens (eg testosterone) act?

Answer 11

On follicles and sebaceous glands.

Question 12

Changes to skin with hypothyroidism

Answer 12

Coarsened thin scaly skin
Myxoedema
Dry brittle and coarse hair
Alopecia
Thin brittle nails
Dry skin
Decreased sweating

Question 13

Hormone synthesis in skin

Answer 13

Vitamin D3 (cholecalciferol synthesis)

17B-hydroxysteroid dehydrogenase in sebocytes and 5a-reductase in dermal adipocytes convert DHEA and androstenedione to 5a-dihydrotestosterone.

Insulin-like growth factor (IGF) binding IGFBP-3 synthesised by dermal fibroblasts.

Question 14

What types of UV damages skin?

Answer 14

UV-A and UV-B

Burns, suppress action of Langerhans cells, photo-aging, DNA damage (skin cancers)

Question 15

Skin colour depends on

Answer 15

Melanin
Carotenoids
Oxy/deoxyhaemoglobin

Question 16

Vitamin A indirectly damages DNA by

Answer 16

Producing free-oxygen radicals

Question 17

UVA

Answer 17

Has longer wavelength than UVB so penetrates more deeply and can pass through glass.

Question 18

Melanin

Answer 18

Synthesised in melanosomes within melanocytes from tyrosine.

Transported via dendrites to adjacent keratinocytes.

Tends to act as supranuclear cap in cells over the nucleus to protect from UV light.

Question 19

2 types of melanin

Answer 19

Pheomelanin - red/yellow pigment

Eumelanin - brown/black pigment

Question 20

Red hair contains more…

Answer 20

pheomelanin

Question 21

All skin types contain more ____ than pheomelanin

Answer 21

Eumelanin

Question 22

Role of melanin

Answer 22

Not certain.

Photoprotective - scatters/filters UV light.
Melanocyte density caries between body sites.

All ethnicities will have same number of melanocytes but differing levels of melanin.

Question 23

Deleterious effect of melanin

Answer 23

Prone to photodegradation - may generate reactive oxygen species.

Pheomelanin increases release of histamine

Lots of melanin = less able to utilise UV light to make vitamin D

Question 24

Immediate response to sunlight

Answer 24

Immediate pigment darkening due to photooxidation of existing melanin.
Redistribution of melanosomes.
Occurs within minutes and lasts hours-days.

Question 25

Tanning

Answer 25

UVA > UVB
Oxidation of melanin
Occurs within hours, lasts 3-5 days

Question 26

Delayed tanning

Answer 26

Increased melanin synthesis

Occurs 2-3 days after UV exposure, maximal at 10-28 days

Question 27

Barrier to Infection

Answer 27

Water-proofing qualities help prevent infection

Range of peptides synthesised by granular layer keratinocytes have antimicrobial properties.

Innate and acquired functions.

Epidermis - Langerhans cells

Dermis - regulatory t-cells, natural killer cells, dendritic cells, macrophages, mast cells.

Answer 28

Hairless skin only has noradrenergic innervation.

Question 29

Epidermis role in immunity

Answer 29

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

Langerhans cells are antigen-presenting cells and secrete cytokines.

Question 30

Skin is sensory organ

Answer 30

Merkle cells - basal epidermis (light touch)

Encapsulated mechanoreceptors in dermis (Pacinian corpuscles for pressure and vibration and Meissner corpuscles for touch)

Myelinated and unmyelinated sensory nerve endings in dermis (pain, touch, temperature)

Question 31

Skins role in regulating body temp

Answer 31

Autonomic regulation of blood flow in dermal vascular plexuses.

Sympathetic alpha -noraadrenergic: vasoconstriction
Sympathetic cholinergic: vasodilation

Sympathetic cholinergic nerves that govern sweating may be same as those controlling active vasodilation.

NO may play role in active vasodilation.