Physiology of the Skin Flashcards
3 layers of skin
Epidermis, dermis and subcutis
Approx. how much does the skin weigh?
3.6 kg
Waterproofing barrier
Tight junction between cells in stratum granulosum, epidermal lipids and keratin in stratum corneum form both an inside-out and outside-in waterproof barrier.
Waterproofing barrier
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
Why does skin wrinkle when wet?
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)
How does the skin help resist abrasive forces?
Stratified squamous epithelium.
Fat in subcutis acts as shock absorber.
Skin synthesis and stores vitamin D
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.
How long would it take for 10,000 IU vitamin D from whole body exposure to UV in white skin?
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)
Where can vitamin D be stored in skin?
It is lipid soluble so is stored in subcutis adipocytes.
Skin is endocrine organ
Androgens acts on follicles and sebaceous glands.
Thyroid hormones act on keratinocytes, follicles, dermal fibroblasts, sebaceous glands, eccrine glands.
Where do androgens (eg testosterone) act?
On follicles and sebaceous glands.
Changes to skin with hypothyroidism
Coarsened thin scaly skin
Myxoedema
Dry brittle and coarse hair
Alopecia
Thin brittle nails
Dry skin
Decreased sweating
Hormone synthesis in skin
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.
What types of UV damages skin?
UV-A and UV-B
Burns, suppress action of Langerhans cells, photo-aging, DNA damage (skin cancers)
Skin colour depends on
Melanin
Carotenoids
Oxy/deoxyhaemoglobin
Vitamin A indirectly damages DNA by
Producing free-oxygen radicals
UVA
Has longer wavelength than UVB so penetrates more deeply and can pass through glass.
Melanin
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.
2 types of melanin
Pheomelanin - red/yellow pigment
Eumelanin - brown/black pigment
Red hair contains more…
pheomelanin
All skin types contain more ____ than pheomelanin
Eumelanin
Role of melanin
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.
Deleterious effect of melanin
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
Immediate response to sunlight
Immediate pigment darkening due to photooxidation of existing melanin.
Redistribution of melanosomes.
Occurs within minutes and lasts hours-days.
Tanning
UVA > UVB
Oxidation of melanin
Occurs within hours, lasts 3-5 days
Delayed tanning
Increased melanin synthesis
Occurs 2-3 days after UV exposure, maximal at 10-28 days
Barrier to Infection
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.
Hairless skin only has noradrenergic innervation.
Epidermis role in immunity
Keratinocytes secrete cytokines and chemokines that maintain populations of leucocytes in skin.
Langerhans cells are antigen-presenting cells and secrete cytokines.
Skin is sensory organ
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)
Skins role in regulating body temp
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.
