Physiology Of The Skin Flashcards
What is the largest organ of the body?
Skin
3.6 Kg
2 m2
3 layers of skin
Epidermis
Dermis
Subcutis
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
Functions of epidermis
Waterproofing
Physical barrier
Immune function
Vitamin D synthesis (Endocrine)
UV protection
Thermoregulation
Dermis
Thermoregulation
Vitamin D synthesis (Endocrine)
Sensory organ
Subcutis
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
A physical barrier
Structure of skin helps resist trauma
Stratified epithelium helps resist abrasive forces
Fat in subcutis acts as shock absorber
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
Vit D3 can be found in oily fish/meat
Skin is 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
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
A barrier to ultraviolet 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
Melanin
Synthesised in melanosomes within melanocytes from tyrosine
Transported via dendrites to adjacent keratinocytes
Pheomelanin (red/yellow)
Eumelanin (brown/black)
Melanocyte density varies between body sites
Red hair contains more pheomelanin
All skin types contain more eumelanin than pheomelanin.
Photoprotective – scatters/filters UV light
Melanin is 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 do we respond to sunlight?
Immediate pigment darkening
Persistent pigment darkening (tanning)
Delayed tanning
Immediate pigment darkening
photooxidation of existing melanin
redistribution of melanosomes
occurs within minutes and lasts hours-days.
Persistent pigment darkening (tanning)
UVA»_space; UVB
Ultraviolet A is better at oxidising melanin than UVB
oxidation of melanin
occurs within hours, lasts 3-5 days
Ultraviolet A (UVA) has a longer wavelength. It is associated with skin aging
Ultraviolet B (UVB) has a shorter wavelength- It is associated with skin burning
UVB stronger than UVA
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)
-β defensins
-S100A7 and S100A8
The skin is an immune organ
Innate and acquired immune functions
Epidermis
-Langerhans cells
Dermis
-Regulatory T cells
-Natural killer cells
-Dendritic cells
-Macrophages
Mast cells
The skin is 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
When skin is challenged
LC migrate to dermis and lymph nodes and activate a T-cell response
Keratinocytes proliferate & secrete cytokines
Leucocytes enter skin from blood
The skin is 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)
Skin helps regulate body temperature
Slide 27-32
